1
00:00:00,030 --> 00:00:02,400
The following content is
provided under a Creative

2
00:00:02,400 --> 00:00:03,780
Commons license.

3
00:00:03,780 --> 00:00:06,020
Your support will help
MIT OpenCourseWare

4
00:00:06,020 --> 00:00:10,090
continue to offer high quality
educational resources for free.

5
00:00:10,090 --> 00:00:12,660
To make a donation or to
view additional materials

6
00:00:12,660 --> 00:00:16,405
from hundreds of MIT courses,
visit MIT OpenCourseWare

7
00:00:16,405 --> 00:00:17,030
at ocw.mit.edu.

8
00:00:26,490 --> 00:00:29,020
PROFESSOR: So
collective knowledge

9
00:00:29,020 --> 00:00:32,490
got us a bunch more points.

10
00:00:32,490 --> 00:00:36,230
So the rules for
significant figures

11
00:00:36,230 --> 00:00:39,290
are on the back page of
the handout from Friday.

12
00:00:39,290 --> 00:00:41,430
And so we want to think
about how many are

13
00:00:41,430 --> 00:00:43,692
after the decimal point here.

14
00:00:43,692 --> 00:00:46,150
So there are three after the
decimal point, which gives you

15
00:00:46,150 --> 00:00:49,470
three here in your answer.

16
00:00:49,470 --> 00:00:53,200
Log significant
figure rules-- yes.

17
00:00:53,200 --> 00:00:56,700
So in base, and some in
chemical equilibrium, which

18
00:00:56,700 --> 00:00:58,900
are all kind of
the same thing, you

19
00:00:58,900 --> 00:01:06,080
can have subtraction and
addition, then multiplication

20
00:01:06,080 --> 00:01:09,000
and division, and
then end your answer

21
00:01:09,000 --> 00:01:15,130
with logs-- three significant
figure rolls in one problem.

22
00:01:15,130 --> 00:01:17,450
Yes, it's coming.

23
00:01:17,450 --> 00:01:23,370
So you want to get those
get those rolls down.

24
00:01:23,370 --> 00:01:27,750
OK, so today we're going
to talk about solubility.

25
00:01:27,750 --> 00:01:33,610
And this is really a direct kind
of growth out of thermodynamics

26
00:01:33,610 --> 00:01:35,110
and chemical equilibrium.

27
00:01:35,110 --> 00:01:37,820
You're going to be seeing
a lot of the same ideas

28
00:01:37,820 --> 00:01:40,220
in solubility.

29
00:01:40,220 --> 00:01:41,850
And then at the end
of the class we're

30
00:01:41,850 --> 00:01:42,933
going to start acid-bases.

31
00:01:45,420 --> 00:01:49,740
So far, we have been discussing
mostly pure substances,

32
00:01:49,740 --> 00:01:52,340
and that is all
changing right now.

33
00:01:52,340 --> 00:01:55,200
Because most substances
are actually mixtures.

34
00:01:55,200 --> 00:01:59,710
So most of the time, you have
two things mixed together.

35
00:01:59,710 --> 00:02:02,900
So mixtures-- you have
a homogeneous mixture

36
00:02:02,900 --> 00:02:07,260
of something, something's
dissolved in something else,

37
00:02:07,260 --> 00:02:12,970
and a solution is a
homogeneous mixture.

38
00:02:12,970 --> 00:02:17,260
And so in your solution
you have a solvent,

39
00:02:17,260 --> 00:02:20,040
which is the thing that is
dissolving the other thing,

40
00:02:20,040 --> 00:02:21,770
and the other thing
is the solute.

41
00:02:21,770 --> 00:02:25,670
So the solute is any dissolved
substance in a solution,

42
00:02:25,670 --> 00:02:29,170
and the solvent is the substance
that does the dissolving.

43
00:02:29,170 --> 00:02:31,520
And water is a great solvent.

44
00:02:31,520 --> 00:02:34,160
One of the most common
solvents is water.

45
00:02:34,160 --> 00:02:38,260
So when you have your
solute in your solvent

46
00:02:38,260 --> 00:02:42,040
and it's nicely dissolved,
then you have a solution.

47
00:02:42,040 --> 00:02:43,860
So let's look at
examples of things

48
00:02:43,860 --> 00:02:46,750
that dissolve in other
things, and first we're

49
00:02:46,750 --> 00:02:50,660
going to talk about
NaCl, common table salt,

50
00:02:50,660 --> 00:02:52,720
dissolving in water.

51
00:02:52,720 --> 00:02:55,780
So any NaCl is held
together by an ionic bond

52
00:02:55,780 --> 00:02:57,650
and it forms crystals
where you have

53
00:02:57,650 --> 00:03:01,230
these beautiful arrangement
of our sodium plus

54
00:03:01,230 --> 00:03:03,670
and our Cl minus.

55
00:03:03,670 --> 00:03:08,480
And a polar solvent, like
water, will, what's called,

56
00:03:08,480 --> 00:03:13,060
hydrate the ions and pry
them away from the surface.

57
00:03:13,060 --> 00:03:15,940
So if we look at this picture,
and we have our sodium

58
00:03:15,940 --> 00:03:19,130
and our chloride surrounded
in our salt crystal,

59
00:03:19,130 --> 00:03:21,440
we have water, which
is red with those two

60
00:03:21,440 --> 00:03:25,360
little white dots, which are
the hydrogens on the water.

61
00:03:25,360 --> 00:03:29,620
And so we have our bent geometry
for our polar water molecule.

62
00:03:29,620 --> 00:03:33,060
And the water is coming in,
and it's hydrating the ions.

63
00:03:33,060 --> 00:03:36,570
And so here, you see the
edge of the salt crystal,

64
00:03:36,570 --> 00:03:40,650
and now the water is kind of
pulling one of the ions apart.

65
00:03:40,650 --> 00:03:44,280
And here you see the ions are
coming out of the salt crystal

66
00:03:44,280 --> 00:03:46,790
and dissolving in your solution.

67
00:03:46,790 --> 00:03:50,130
So a polar solvent
like water can just

68
00:03:50,130 --> 00:03:53,830
pry that crystal apart.

69
00:03:53,830 --> 00:03:57,770
So stirring can speed
up this process.

70
00:03:57,770 --> 00:04:01,070
It doesn't often
change the properties.

71
00:04:01,070 --> 00:04:02,610
It doesn't make
things more soluble.

72
00:04:02,610 --> 00:04:04,860
But it will speed it up,
because if you stir it,

73
00:04:04,860 --> 00:04:06,490
you're going to get
more of that water

74
00:04:06,490 --> 00:04:09,830
having access to the edges
of the salt crystals,

75
00:04:09,830 --> 00:04:15,160
and helping to pry apart and
hydrate those ions so that they

76
00:04:15,160 --> 00:04:17,470
solubilize.

77
00:04:17,470 --> 00:04:21,550
We have an equilibrium
expression for solubility,

78
00:04:21,550 --> 00:04:23,230
so we're just
following right along

79
00:04:23,230 --> 00:04:24,840
with chemical equilibrium.

80
00:04:24,840 --> 00:04:30,780
And so we have our solid NaCl
going to our hydrated ions,

81
00:04:30,780 --> 00:04:35,870
our Na plus, that was pulled
out by water, and our CL minus.

82
00:04:35,870 --> 00:04:38,160
And this in here,
it says aqueous,

83
00:04:38,160 --> 00:04:41,690
abbreviation for aqueous, so
we have our dissolved ions.

84
00:04:41,690 --> 00:04:43,580
That's how we would
interpret that.

85
00:04:43,580 --> 00:04:45,980
And S here as for solid.

86
00:04:45,980 --> 00:04:48,290
So we've also not
paid much attention

87
00:04:48,290 --> 00:04:50,540
to phase of things at
this point-- a little bit

88
00:04:50,540 --> 00:04:52,710
when we were talking
about increasing entropy.

89
00:04:52,710 --> 00:04:55,760
We were talking about gases
going to liquids and solids,

90
00:04:55,760 --> 00:04:59,820
and vice versa, and predicting
the change in entropy

91
00:04:59,820 --> 00:05:01,190
for those processes.

92
00:05:01,190 --> 00:05:02,940
But now we're going
to spend a lot of time

93
00:05:02,940 --> 00:05:05,150
paying attention to the phase.

94
00:05:05,150 --> 00:05:09,070
And so solubility, something
solid, going to its aqueous

95
00:05:09,070 --> 00:05:10,470
ions.

96
00:05:10,470 --> 00:05:13,500
So we can write an equilibrium
expression for this.

97
00:05:13,500 --> 00:05:17,270
And it has a special
name-- KSP, SP

98
00:05:17,270 --> 00:05:19,840
stands for Solubility Product.

99
00:05:19,840 --> 00:05:25,520
And so KSP would be equal to
the concentration of sodium ions

100
00:05:25,520 --> 00:05:29,780
and the concentration times the
concentration of chloride ions.

101
00:05:29,780 --> 00:05:35,290
And equilibrium constants
are products over reactants.

102
00:05:35,290 --> 00:05:38,070
We don't have our
reactant in this equation

103
00:05:38,070 --> 00:05:39,400
because it is a solid.

104
00:05:39,400 --> 00:05:43,980
So it doesn't appear in
the equilibrium expression.

105
00:05:43,980 --> 00:05:46,070
So our equilibrium
expression, which

106
00:05:46,070 --> 00:05:49,290
here has the set
special name of KSP,

107
00:05:49,290 --> 00:05:53,280
is just those ions in
solution, the concentration

108
00:05:53,280 --> 00:05:56,220
of each ion times each other.

109
00:05:56,220 --> 00:05:58,290
All right, so let's
have a little practice

110
00:05:58,290 --> 00:06:00,400
with writing KSPs.

111
00:06:00,400 --> 00:06:03,720
And so we have a clicker
question on that.

112
00:06:19,490 --> 00:06:20,800
All right, 10 more seconds.

113
00:06:35,520 --> 00:06:36,480
Great.

114
00:06:36,480 --> 00:06:39,650
Yup, so again, we
don't want to have

115
00:06:39,650 --> 00:06:42,230
the solid in the
expression, and we

116
00:06:42,230 --> 00:06:43,910
need to remember
the stoichiometry,

117
00:06:43,910 --> 00:06:46,580
always need to remember
the stoichiometry.

118
00:06:46,580 --> 00:06:53,110
OK, so they're
solubility product.

119
00:06:53,110 --> 00:06:57,910
OK, so now let's think about
something that is not ionic.

120
00:06:57,910 --> 00:07:00,226
Let's think about glucose.

121
00:07:00,226 --> 00:07:01,600
We've talked a
lot about glucose.

122
00:07:01,600 --> 00:07:04,860
Let's think about glucose
dissolving in water.

123
00:07:04,860 --> 00:07:06,890
And here I have a
picture of two beakers,

124
00:07:06,890 --> 00:07:09,340
a beaker where it's
clear, and a beaker

125
00:07:09,340 --> 00:07:13,060
where there's a lot of
glucose sitting on the bottom.

126
00:07:13,060 --> 00:07:15,530
And so we can think about
the solubility here.

127
00:07:15,530 --> 00:07:17,140
So how can this dissolve?

128
00:07:17,140 --> 00:07:18,460
It can't form ions.

129
00:07:18,460 --> 00:07:20,870
It's not made up of ions
like sodium chloride.

130
00:07:20,870 --> 00:07:22,310
So how does it dissolve?

131
00:07:22,310 --> 00:07:25,840
Well, it can dissolve,
again, water molecules--

132
00:07:25,840 --> 00:07:28,840
water's amazing-- water
molecules form hydrogen

133
00:07:28,840 --> 00:07:32,740
bonds with the glucose and pull
one of the glucose molecules

134
00:07:32,740 --> 00:07:36,360
that is at the edge of the
solid piece of glucose,

135
00:07:36,360 --> 00:07:40,060
pulls it out into solution
by hydrogen bonding to it.

136
00:07:40,060 --> 00:07:42,790
So again, it's
solubilizing the glucose

137
00:07:42,790 --> 00:07:46,500
that's in this solid form by
interacting at the surface

138
00:07:46,500 --> 00:07:49,080
and prying layers
of glucose molecules

139
00:07:49,080 --> 00:07:51,890
off by hydrogen bonding to them.

140
00:07:51,890 --> 00:07:54,310
Luckily, you know all
about hydrogen bonds,

141
00:07:54,310 --> 00:07:58,280
so just to do a little clicker
practice, why don't you tell me

142
00:07:58,280 --> 00:08:00,760
what kind of hydrogen
bonds glucose

143
00:08:00,760 --> 00:08:04,470
is capable of forming if it is,
in fact, capable of forming.

144
00:08:04,470 --> 00:08:06,895
But I kind of gave away
one of the answers here.

145
00:08:32,309 --> 00:08:33,009
10 more seconds.

146
00:08:47,220 --> 00:08:48,980
All right, good.

147
00:08:48,980 --> 00:08:50,320
Zero put four.

148
00:08:50,320 --> 00:08:51,490
That's awesome.

149
00:08:51,490 --> 00:08:54,520
You were listening
to what I was saying.

150
00:08:54,520 --> 00:08:56,640
Yes, so that is correct.

151
00:08:56,640 --> 00:09:01,300
And if we look back at the
molecule for a minute--

152
00:09:01,300 --> 00:09:06,490
so it's a hydrogen bond donor
because of these OH groups

153
00:09:06,490 --> 00:09:08,040
all along here.

154
00:09:08,040 --> 00:09:11,450
And so OH, there's an
electron negativity difference

155
00:09:11,450 --> 00:09:14,990
of greater than 0.4, so it's
capable of being a hydrogen

156
00:09:14,990 --> 00:09:16,190
bond donor.

157
00:09:16,190 --> 00:09:18,490
But this double
bonded O here, it's

158
00:09:18,490 --> 00:09:20,010
not a complete Lewis structure.

159
00:09:20,010 --> 00:09:21,930
It's missing its lone pairs.

160
00:09:21,930 --> 00:09:25,010
But it has lone
pairs on that oxygen,

161
00:09:25,010 --> 00:09:27,760
and that could be a
hydrogen bond acceptor.

162
00:09:27,760 --> 00:09:31,370
So we have hydrogen bond donors
and hydrogen bond acceptors

163
00:09:31,370 --> 00:09:35,460
in this molecule, and water
can also be a hydrogen bond

164
00:09:35,460 --> 00:09:36,820
donor and acceptor.

165
00:09:36,820 --> 00:09:41,340
It has two hydrogens that
can form hydrogen bond.

166
00:09:41,340 --> 00:09:44,620
It also has to lone pairs,
which gives it its bent shape.

167
00:09:44,620 --> 00:09:46,530
So it can be a
donor and acceptor.

168
00:09:46,530 --> 00:09:49,210
So it's capable of
interacting with glucose

169
00:09:49,210 --> 00:09:53,110
and pulling that solid
glucose into solution.

170
00:09:53,110 --> 00:09:55,580
So if we look at
these two pictures,

171
00:09:55,580 --> 00:09:58,030
this could be glucose
[INAUDIBLE] in solution.

172
00:09:58,030 --> 00:10:01,230
You can't see any glucose, but
here you see a level of glucose

173
00:10:01,230 --> 00:10:02,630
at the bottom.

174
00:10:02,630 --> 00:10:05,810
And a solution is
considered saturated

175
00:10:05,810 --> 00:10:09,760
when all of the solvent has
dissolved as much as it can,

176
00:10:09,760 --> 00:10:12,650
but a little bit of
undissolved solute remains.

177
00:10:12,650 --> 00:10:17,080
So this over saturated.

178
00:10:17,080 --> 00:10:20,380
Often it's just saturated when
you have just a little bit more

179
00:10:20,380 --> 00:10:21,430
than can be dissolved.

180
00:10:21,430 --> 00:10:23,370
You've gone past that point.

181
00:10:23,370 --> 00:10:26,800
You can't get any more dissolved
in that volume of water.

182
00:10:26,800 --> 00:10:29,540
This has a pretty large excess.

183
00:10:29,540 --> 00:10:31,900
So sometimes when you
get your solution,

184
00:10:31,900 --> 00:10:35,530
you get your solution clear,
your glucose has dissolved

185
00:10:35,530 --> 00:10:37,820
or whatever it is you're
working on has dissolved,

186
00:10:37,820 --> 00:10:39,980
and then you maybe add
a little bit too much,

187
00:10:39,980 --> 00:10:42,960
and it all crashes
out of solution again,

188
00:10:42,960 --> 00:10:45,220
you have the precipitant
just coming out.

189
00:10:45,220 --> 00:10:48,350
So undissolved
solute, another name

190
00:10:48,350 --> 00:10:51,640
for that, the name that's used
more often, is precipitate.

191
00:10:51,640 --> 00:10:53,980
So it precipitates
out of solution

192
00:10:53,980 --> 00:10:55,930
if it has too much in it.

193
00:10:55,930 --> 00:10:59,490
So if you are not
part of the solution,

194
00:10:59,490 --> 00:11:00,855
you are part of the precipitate.

195
00:11:04,472 --> 00:11:05,222
That's my t-shirt.

196
00:11:07,820 --> 00:11:12,340
OK, so dissolved and
undissolved solute-- they're not

197
00:11:12,340 --> 00:11:13,460
doing nothing.

198
00:11:13,460 --> 00:11:15,010
They're in dynamic equilibrium.

199
00:11:15,010 --> 00:11:18,790
Just like all other kind
of chemical equilibrium,

200
00:11:18,790 --> 00:11:21,672
there is no net change,
but the process is still

201
00:11:21,672 --> 00:11:22,630
going forward and back.

202
00:11:22,630 --> 00:11:24,660
In fact, the rate
going forward is

203
00:11:24,660 --> 00:11:26,390
equal to the rate going back.

204
00:11:26,390 --> 00:11:27,790
The same is true here.

205
00:11:27,790 --> 00:11:30,790
The dissolved and
undissolved solute

206
00:11:30,790 --> 00:11:34,580
are in dynamic equilibrium
with each other.

207
00:11:34,580 --> 00:11:36,640
So how much can
you get in there?

208
00:11:36,640 --> 00:11:41,420
How much solute can you
get into your solvent?

209
00:11:41,420 --> 00:11:44,560
And that depends on the
particular substance

210
00:11:44,560 --> 00:11:47,000
and its molar solubility.

211
00:11:47,000 --> 00:11:50,970
And molar solubility is
defined as S. Either little s

212
00:11:50,970 --> 00:11:54,760
or a big S. I've seen it
both ways in textbooks.

213
00:11:54,760 --> 00:11:59,040
So the molar solubility, S,
is the molar concentration

214
00:11:59,040 --> 00:12:01,280
in that saturated
solution, where

215
00:12:01,280 --> 00:12:04,630
you've put just a
little more in than can

216
00:12:04,630 --> 00:12:06,670
be completely dissolved.

217
00:12:06,670 --> 00:12:10,210
And so that represents
the limit of the ability

218
00:12:10,210 --> 00:12:14,180
of that solvent to dissolve
that solute, that particular

219
00:12:14,180 --> 00:12:15,450
compound.

220
00:12:15,450 --> 00:12:19,840
And its units are, perhaps
unsurprisingly given its name,

221
00:12:19,840 --> 00:12:22,470
is molar, moles per liter.

222
00:12:25,590 --> 00:12:31,220
So molar solubility is
related to our friend KSP,

223
00:12:31,220 --> 00:12:37,310
but it's not the same thing,
or at least most of the time.

224
00:12:37,310 --> 00:12:39,830
I think none of the
time exactly the same.

225
00:12:39,830 --> 00:12:42,830
So when you're talking about the
molar solubility of either ion,

226
00:12:42,830 --> 00:12:46,350
you're talking about that
concentration of sodium plus

227
00:12:46,350 --> 00:12:47,960
or chlorine minus.

228
00:12:47,960 --> 00:12:50,300
So you're talking about
the moles per liter

229
00:12:50,300 --> 00:12:54,120
that can be dissolved in that
particular amount of solvent

230
00:12:54,120 --> 00:12:55,350
or kind of solvent.

231
00:12:55,350 --> 00:12:59,380
And so those concentrations
are the molar solubility.

232
00:12:59,380 --> 00:13:02,210
And again, our
KSP is going to be

233
00:13:02,210 --> 00:13:06,580
equal to the concentration
of the sodium in solution

234
00:13:06,580 --> 00:13:09,850
and the concentration of that
chloride that's dissolved.

235
00:13:09,850 --> 00:13:12,020
So in this case,
the relationship

236
00:13:12,020 --> 00:13:15,610
is that KSP would equal the
molar solubility squared,

237
00:13:15,610 --> 00:13:17,570
the molar solubility
of this times the molar

238
00:13:17,570 --> 00:13:19,420
solubility of that.

239
00:13:19,420 --> 00:13:24,330
So if you know one, you
can derive the other,

240
00:13:24,330 --> 00:13:27,530
and you can do that on a
problem set, in particular.

241
00:13:27,530 --> 00:13:31,610
So those are the
definitions of those terms.

242
00:13:31,610 --> 00:13:33,740
So molar solubility,
how much of something

243
00:13:33,740 --> 00:13:36,480
dissolves in another,
whether it dissolves it all

244
00:13:36,480 --> 00:13:39,350
will depend on, again,
the nature of the material

245
00:13:39,350 --> 00:13:40,800
and the nature of the solvent.

246
00:13:40,800 --> 00:13:42,830
Are they miscible?

247
00:13:42,830 --> 00:13:45,740
Is there a way, like
water, getting in there

248
00:13:45,740 --> 00:13:48,420
and forming hydrogen
bonds, for example.

249
00:13:48,420 --> 00:13:52,890
And this leads to a rule that
a lot of people tend to know,

250
00:13:52,890 --> 00:13:55,560
which is like
dissolves like rule.

251
00:13:55,560 --> 00:13:59,690
So if it's a
non-ionic substance,

252
00:13:59,690 --> 00:14:04,580
then you need to pull it out of
the solid by hydrogen bonding,

253
00:14:04,580 --> 00:14:06,810
if that material has
nothing to hydrogen bond

254
00:14:06,810 --> 00:14:08,970
to, if it's
completely non-polar,

255
00:14:08,970 --> 00:14:11,550
water is not going to be
able to solubilize it.

256
00:14:11,550 --> 00:14:15,700
So polar things, polar
liquids like water,

257
00:14:15,700 --> 00:14:18,890
are generally the best solvents
for things that are ionic,

258
00:14:18,890 --> 00:14:21,950
like salts, or polar compounds.

259
00:14:21,950 --> 00:14:23,670
So it doesn't have
to be a salt. It

260
00:14:23,670 --> 00:14:26,160
can be a compound
made of carbon,

261
00:14:26,160 --> 00:14:28,680
but it needs to be
a polar compound

262
00:14:28,680 --> 00:14:30,200
so that the water
can get in there

263
00:14:30,200 --> 00:14:31,790
and form those hydrogen bonds.

264
00:14:31,790 --> 00:14:34,730
So you can see that knowing what
is capable of forming hydrogen

265
00:14:34,730 --> 00:14:36,750
bonds can be really
useful to think

266
00:14:36,750 --> 00:14:38,260
about what's going
to be dissolved

267
00:14:38,260 --> 00:14:40,890
in what other type of material.

268
00:14:40,890 --> 00:14:46,890
So non-polar liquids,
like hexane, for example,

269
00:14:46,890 --> 00:14:48,650
are better for non-polar.

270
00:14:48,650 --> 00:14:51,250
So non-polar like
dissolves likes.

271
00:14:51,250 --> 00:14:54,240
And so that's why people
go to dry cleaners,

272
00:14:54,240 --> 00:14:59,536
because you don't want to use
water to wash certain things.

273
00:14:59,536 --> 00:15:00,940
You want to be dry.

274
00:15:00,940 --> 00:15:04,930
You want to get your
non-polar stains out.

275
00:15:04,930 --> 00:15:07,410
So this is very important.

276
00:15:07,410 --> 00:15:11,250
There's a number of applications
of these principles.

277
00:15:11,250 --> 00:15:14,890
This is important in the
pharmaceutical industry.

278
00:15:14,890 --> 00:15:18,090
So we talked about some
of the molecules that

279
00:15:18,090 --> 00:15:22,080
are used as
pharmaceutical, in terms

280
00:15:22,080 --> 00:15:25,970
of being polar and non-polar,
and vitamins the same way.

281
00:15:25,970 --> 00:15:27,500
So when you're
designing a molecule

282
00:15:27,500 --> 00:15:30,860
that you want to get into the
body and react with a target,

283
00:15:30,860 --> 00:15:34,500
if you design something
that is really non-polar,

284
00:15:34,500 --> 00:15:37,550
the human body-- there's
a lot of water parts.

285
00:15:37,550 --> 00:15:39,240
And it's not going
to necessarily get

286
00:15:39,240 --> 00:15:41,340
to where you want it to go.

287
00:15:41,340 --> 00:15:46,730
But if it's too polar, it
might just get washed out.

288
00:15:46,730 --> 00:15:49,440
So you want the right
amount of polar non-polar

289
00:15:49,440 --> 00:15:52,020
to stay in the body
long enough, but also

290
00:15:52,020 --> 00:15:53,900
to get to where it needs to go.

291
00:15:53,900 --> 00:15:56,770
So a lot of people who
are designing molecules

292
00:15:56,770 --> 00:15:59,220
to treat disease are
very concerned about,

293
00:15:59,220 --> 00:16:01,560
can we make this
molecule more polar?

294
00:16:01,560 --> 00:16:04,230
Can we make it more soluble?

295
00:16:04,230 --> 00:16:06,120
It hits our target beautifully.

296
00:16:06,120 --> 00:16:08,190
It really destroys that enzyme.

297
00:16:08,190 --> 00:16:11,110
It would be a fantastic
chemotherapeutic agent.

298
00:16:11,110 --> 00:16:14,560
But we need to figure out how
to make it get into the cell

299
00:16:14,560 --> 00:16:16,670
that it needs to target.

300
00:16:16,670 --> 00:16:19,320
And of course, so if you're
going to go into drug designer

301
00:16:19,320 --> 00:16:21,082
medicine, you care about this.

302
00:16:21,082 --> 00:16:22,540
If you're going to
become a parent,

303
00:16:22,540 --> 00:16:24,660
you care a lot about cleaning.

304
00:16:24,660 --> 00:16:27,700
And it's a daily event.

305
00:16:27,700 --> 00:16:31,000
How am I going to get this
stain out of my child's clothes?

306
00:16:31,000 --> 00:16:33,610
Every day this is a question.

307
00:16:33,610 --> 00:16:35,510
So this is important.

308
00:16:35,510 --> 00:16:36,480
All right.

309
00:16:36,480 --> 00:16:39,960
And one other example in the
longs of cleaning-- I just

310
00:16:39,960 --> 00:16:43,410
want to share with you a
short story about a chemist,

311
00:16:43,410 --> 00:16:46,610
Robert H. Black,
and a life altering

312
00:16:46,610 --> 00:16:48,670
event that happened to him.

313
00:16:48,670 --> 00:16:57,270
So one day, Mrs. Black said to
Mr. Black, clean the bath tub.

314
00:16:57,270 --> 00:17:02,280
This was an unprecedented
event in the Black household.

315
00:17:02,280 --> 00:17:06,160
So Robert Black went in
to clean his bath tub,

316
00:17:06,160 --> 00:17:07,150
and it was really hard.

317
00:17:07,150 --> 00:17:08,609
And he scrubbed, and he
scrubbed, and he scrubbed,

318
00:17:08,609 --> 00:17:09,780
and he scrubbed.

319
00:17:09,780 --> 00:17:13,030
And he was a chemist, so he
knew what kind of chemicals

320
00:17:13,030 --> 00:17:15,950
you would need to
clean a bath tub.

321
00:17:15,950 --> 00:17:20,280
You want to have things
to dissolve your likes,

322
00:17:20,280 --> 00:17:22,300
so you want to have your
polar and non-polar.

323
00:17:22,300 --> 00:17:24,810
You want to have
a chelating agent

324
00:17:24,810 --> 00:17:27,950
to pull the heavy metals
out of the tub scum,

325
00:17:27,950 --> 00:17:32,050
a surfactant, to make them bead
up so you can wash them away,

326
00:17:32,050 --> 00:17:35,390
alcohols, to remove sort of
the more greasy kind of stains.

327
00:17:35,390 --> 00:17:36,870
So he knew all of this.

328
00:17:36,870 --> 00:17:40,500
But he realized that if
you didn't apply cleaner

329
00:17:40,500 --> 00:17:45,510
on a regular basis, the tub
scum got so thick and so nasty,

330
00:17:45,510 --> 00:17:48,680
that the cleaner couldn't
penetrate the scum,

331
00:17:48,680 --> 00:17:50,170
and you need surface area.

332
00:17:50,170 --> 00:17:53,060
These things work
by dissolving out.

333
00:17:53,060 --> 00:17:54,130
They touch the surface.

334
00:17:54,130 --> 00:17:55,450
So you need to have a surface.

335
00:17:55,450 --> 00:18:00,610
So he had this idea that instead
of having this tub scum build

336
00:18:00,610 --> 00:18:03,890
up so much that it was a
real problem to clean it,

337
00:18:03,890 --> 00:18:06,090
that you could take
all of these things

338
00:18:06,090 --> 00:18:08,880
that chemists knew
would be useful

339
00:18:08,880 --> 00:18:13,430
and package them in something
that was a daily cleaner.

340
00:18:13,430 --> 00:18:16,220
So he advertised these
things that, every time you

341
00:18:16,220 --> 00:18:19,340
take a shower, you just
spray the shower or the tub,

342
00:18:19,340 --> 00:18:22,967
and then you never have
to scrub your tub again.

343
00:18:22,967 --> 00:18:24,550
Because you do a
little bit every day,

344
00:18:24,550 --> 00:18:26,530
where there's not
a lot of surface,

345
00:18:26,530 --> 00:18:29,230
so it gets it right
off, right away.

346
00:18:29,230 --> 00:18:34,330
And Mr. Black and his wife never
had to clean their tub again,

347
00:18:34,330 --> 00:18:38,870
because this resulted in sales
of about $70 million per year.

348
00:18:38,870 --> 00:18:40,740
And it's important
to point out that he

349
00:18:40,740 --> 00:18:42,740
wasn't using anything new.

350
00:18:42,740 --> 00:18:47,200
He was just packaging his
chemicals for daily use,

351
00:18:47,200 --> 00:18:50,380
rather than, say, for a
weekly or monthly use.

352
00:18:50,380 --> 00:18:52,940
So I think there's a couple
important lessons from this.

353
00:18:52,940 --> 00:18:56,030
One, it's always important
to clean your bathroom.

354
00:18:56,030 --> 00:18:59,770
Two, you never know where
chemistry knowledge is going

355
00:18:59,770 --> 00:19:02,520
to come, what life
altering event

356
00:19:02,520 --> 00:19:06,450
will cause you to realize that
you have in your mind chemistry

357
00:19:06,450 --> 00:19:10,100
knowledge that can make
you $70 million a year.

358
00:19:10,100 --> 00:19:12,600
And when that life
altering event occurs,

359
00:19:12,600 --> 00:19:17,800
remember that I take cash
and checks and stock options.

360
00:19:17,800 --> 00:19:18,790
I added the last one.

361
00:19:18,790 --> 00:19:21,240
I thought, cash and checks--
I should be more flexible.

362
00:19:21,240 --> 00:19:24,230
So I'm also going to
take stock options.

363
00:19:24,230 --> 00:19:27,610
And this is why I teach you all
the fundamentals of chemistry,

364
00:19:27,610 --> 00:19:30,860
cover everything that's really
important for you to know,

365
00:19:30,860 --> 00:19:35,400
because then if you use any of
it, you've learned it from me.

366
00:19:38,010 --> 00:19:39,690
So just keep that in mind.

367
00:19:39,690 --> 00:19:43,416
One never knows what life
event is ahead of you.

368
00:19:45,930 --> 00:19:49,580
So what about gas solubility?

369
00:19:49,580 --> 00:19:51,300
We talked about solids.

370
00:19:51,300 --> 00:19:54,330
Now let's talk about gases.

371
00:19:54,330 --> 00:19:57,520
So whether a gas is
going to be soluble

372
00:19:57,520 --> 00:20:00,090
is going to depend
on Henry's Law,

373
00:20:00,090 --> 00:20:02,320
or we can find out
about it solubility.

374
00:20:02,320 --> 00:20:04,460
So a solubility of
a gas-- we're going

375
00:20:04,460 --> 00:20:06,850
to call that little s,
that's our solubility--

376
00:20:06,850 --> 00:20:09,480
is directly proportional
to the partial pressure

377
00:20:09,480 --> 00:20:13,370
of the gas and a constant
called Henry's Constant.

378
00:20:13,370 --> 00:20:15,580
And that depends on
the nature of the gas

379
00:20:15,580 --> 00:20:18,380
and the solvent and
the temperature.

380
00:20:18,380 --> 00:20:24,240
So let's think about this
rule in this plot in a clicker

381
00:20:24,240 --> 00:20:24,740
question.

382
00:20:42,540 --> 00:20:44,170
All right, just take
10 more seconds.

383
00:20:58,060 --> 00:20:59,530
Awesome.

384
00:20:59,530 --> 00:21:00,336
That is correct.

385
00:21:03,550 --> 00:21:05,870
So let's just take
a look at that.

386
00:21:05,870 --> 00:21:08,370
So one of the clicker
questions, which

387
00:21:08,370 --> 00:21:11,430
is kind of getting
you to read the graph,

388
00:21:11,430 --> 00:21:16,150
and so it was asking,
say at 0.5 atmospheres,

389
00:21:16,150 --> 00:21:18,990
it said oxygen is more
soluble than helium.

390
00:21:18,990 --> 00:21:22,120
This is molar solubility
versus partial pressure.

391
00:21:22,120 --> 00:21:26,070
At point 0.5, sure enough,
oxygen is more soluble.

392
00:21:26,070 --> 00:21:28,350
So that was just
reading the plot.

393
00:21:28,350 --> 00:21:32,840
The other one was just
interpreting Henry's Law again,

394
00:21:32,840 --> 00:21:37,830
that solubility would increase
as partial pressure increases.

395
00:21:37,830 --> 00:21:39,930
You can see that
from the equation.

396
00:21:39,930 --> 00:21:42,960
But you can also think about why
that's true, and why it's true

397
00:21:42,960 --> 00:21:46,430
is the answer to the three,
that the solubility of a gas

398
00:21:46,430 --> 00:21:49,020
is proportional to
its partial pressure.

399
00:21:49,020 --> 00:21:51,330
An increase in
pressure corresponds

400
00:21:51,330 --> 00:21:55,480
to an increase in rate at
which the gas molecules strike

401
00:21:55,480 --> 00:21:56,140
the surface.

402
00:21:56,140 --> 00:21:57,750
And that makes
them more soluble.

403
00:21:57,750 --> 00:21:59,720
So again, solubility
really has to do

404
00:21:59,720 --> 00:22:01,830
with getting at that surface.

405
00:22:01,830 --> 00:22:06,910
If you get at the surface, then
you can dissolve your material.

406
00:22:06,910 --> 00:22:08,290
All right, so why
should you care

407
00:22:08,290 --> 00:22:10,930
about the solubility of gases?

408
00:22:10,930 --> 00:22:14,570
And I'm going to
give you an example.

409
00:22:14,570 --> 00:22:16,100
This is another
in your own words,

410
00:22:16,100 --> 00:22:17,800
so we'll watch the video.

411
00:22:17,800 --> 00:22:20,850
And I think tonight there's
some kind of big climate change

412
00:22:20,850 --> 00:22:21,350
thing.

413
00:22:21,350 --> 00:22:22,740
I've seen fliers around.

414
00:22:22,740 --> 00:22:25,150
I haven't paid enough
attention to what's going on.

415
00:22:25,150 --> 00:22:29,590
But climate change is
certainly a very hot topic,

416
00:22:29,590 --> 00:22:32,721
if you'll excuse that right now.

417
00:22:32,721 --> 00:22:33,720
I didn't really mean it.

418
00:22:33,720 --> 00:22:35,620
It just kind of came out.

419
00:22:35,620 --> 00:22:38,140
But CO2 is a greenhouse gas.

420
00:22:38,140 --> 00:22:39,530
It's a big problem.

421
00:22:39,530 --> 00:22:41,870
And so a number of
researchers here at MIT

422
00:22:41,870 --> 00:22:46,170
are thinking about ways that
you can capture and store it

423
00:22:46,170 --> 00:22:49,070
somewhere, get it out
of our atmosphere.

424
00:22:49,070 --> 00:22:51,010
Other people are
thinking about ways

425
00:22:51,010 --> 00:22:54,660
that you can change industrial
processes to make less CO2.

426
00:22:54,660 --> 00:22:56,480
There's a lot of
research going on.

427
00:22:56,480 --> 00:22:59,080
So today you'll hear from
Hector Hernandez, who

428
00:22:59,080 --> 00:23:01,850
had a Ph.D. in chemistry
from the chemistry department

429
00:23:01,850 --> 00:23:05,640
here at MIT, did his post-doc
here at MIT in chemical

430
00:23:05,640 --> 00:23:09,330
engineering, and is now
a professor of chemical

431
00:23:09,330 --> 00:23:14,610
engineering at Masdar Institute
in the United Arab Emirates.

432
00:23:14,610 --> 00:23:18,020
So this is one of the MIT
satellite universities

433
00:23:18,020 --> 00:23:20,470
that are popping up
all over the world.

434
00:23:20,470 --> 00:23:23,120
So Hector was born,
I think, in Honduras,

435
00:23:23,120 --> 00:23:25,530
and then lived in
Florida, ended up in MIT,

436
00:23:25,530 --> 00:23:27,800
and now he's at the
United Arab Emirates.

437
00:23:27,800 --> 00:23:29,680
So that's just
another example of,

438
00:23:29,680 --> 00:23:32,020
one never knows what's
going to happen to you.

439
00:23:32,020 --> 00:23:35,745
All right, so you can hear
from Hector in his own words.

440
00:23:38,390 --> 00:23:44,710
So Hector's personal
video's online.

441
00:23:44,710 --> 00:23:46,076
I think it's a fun one to watch.

442
00:23:46,076 --> 00:23:47,700
There's a couple
things that are really

443
00:23:47,700 --> 00:23:52,140
interesting about his personal
story, including the fact

444
00:23:52,140 --> 00:23:55,760
that as he mentioned in past
life he did some construction,

445
00:23:55,760 --> 00:23:59,630
he started undergrad
at age 30, I think,

446
00:23:59,630 --> 00:24:03,810
where he was basically
building houses and fixing cars

447
00:24:03,810 --> 00:24:07,820
and he realized that his
back might not survive this

448
00:24:07,820 --> 00:24:09,500
being his permanent career.

449
00:24:09,500 --> 00:24:12,490
So he decided to use his
brain instead of his back.

450
00:24:12,490 --> 00:24:15,280
And so started undergraduate,
now he's a professor.

451
00:24:15,280 --> 00:24:19,550
So it just shows there's a lot
of different paths to success.

452
00:24:19,550 --> 00:24:23,240
All right so C02-- big problem.

453
00:24:23,240 --> 00:24:24,859
We were trying to
use, in that case,

454
00:24:24,859 --> 00:24:26,775
he was interested in
turning it into biofuels,

455
00:24:26,775 --> 00:24:28,880
they were interested
in ways to store it.

456
00:24:28,880 --> 00:24:30,780
You care about
solubility of gases

457
00:24:30,780 --> 00:24:32,850
and you care about Le
Chatelier's principle

458
00:24:32,850 --> 00:24:35,010
of driving things the
direction that you want

459
00:24:35,010 --> 00:24:36,890
to make the products you want.

460
00:24:36,890 --> 00:24:40,680
OK, so let's think more
about these factors that

461
00:24:40,680 --> 00:24:42,610
affect solubility.

462
00:24:42,610 --> 00:24:44,670
And one thing that's
very important,

463
00:24:44,670 --> 00:24:47,450
for sure, is temperature.

464
00:24:47,450 --> 00:24:53,980
So temperature-- most
substances dissolve more quickly

465
00:24:53,980 --> 00:24:56,950
at higher temperature, which
is often what you want.

466
00:24:56,950 --> 00:25:00,890
But it doesn't always mean if
you increase the temperature

467
00:25:00,890 --> 00:25:03,210
that you're going to
make it more soluble.

468
00:25:03,210 --> 00:25:05,460
You may just make
it dissolve faster,

469
00:25:05,460 --> 00:25:08,270
but not actually
change the end product.

470
00:25:08,270 --> 00:25:11,520
Some things are more sensitive
to temperature than others.

471
00:25:11,520 --> 00:25:16,340
So most gases, for example,
are less soluble in warm water

472
00:25:16,340 --> 00:25:19,390
than in cold water,
but with solids, it's

473
00:25:19,390 --> 00:25:23,450
much harder to predict what
effect temperature might have

474
00:25:23,450 --> 00:25:26,670
on their solubility, even
though for many things

475
00:25:26,670 --> 00:25:29,940
it will make it faster.

476
00:25:29,940 --> 00:25:34,070
So let's think now about
how to drive a reaction,

477
00:25:34,070 --> 00:25:37,820
and what factors are
involved, and what's favorable

478
00:25:37,820 --> 00:25:39,410
and what's not favorable.

479
00:25:39,410 --> 00:25:41,690
And when we're talking
about those things,

480
00:25:41,690 --> 00:25:44,200
we're going to be back
to our thermodynamics,

481
00:25:44,200 --> 00:25:49,440
back to enthalpy, entropy,
and Gibb's free energy.

482
00:25:49,440 --> 00:25:52,120
So all these have little
special sub names.

483
00:25:52,120 --> 00:25:55,090
So when we're
talking about delta H

484
00:25:55,090 --> 00:25:58,510
and we're talking about
solutions, things dissolving,

485
00:25:58,510 --> 00:26:02,110
we're talking about
delta H sub sol,

486
00:26:02,110 --> 00:26:07,710
for solution, or if you could
think of it as solubility.

487
00:26:07,710 --> 00:26:09,850
And one point that I'll
make-- a lot of times

488
00:26:09,850 --> 00:26:12,180
when people are learning
a new field, they're like,

489
00:26:12,180 --> 00:26:14,130
oh, there's so much to learn.

490
00:26:14,130 --> 00:26:16,000
But you get to some
point, everything

491
00:26:16,000 --> 00:26:18,750
is just kind of like a subtle
difference in something

492
00:26:18,750 --> 00:26:19,880
you've already learned.

493
00:26:19,880 --> 00:26:21,820
So if you already
learned about delta H,

494
00:26:21,820 --> 00:26:23,680
this is just a
slight modification

495
00:26:23,680 --> 00:26:25,130
of what you learned.

496
00:26:25,130 --> 00:26:27,020
You don't have to learn
a whole new thing.

497
00:26:27,020 --> 00:26:29,210
So that's one of the good
things about the course.

498
00:26:29,210 --> 00:26:32,880
You start recognizing, I
already pretty much know this.

499
00:26:32,880 --> 00:26:36,680
I just need to add a
little sol to the end.

500
00:26:36,680 --> 00:26:40,540
So delta H of solution
can be measured.

501
00:26:40,540 --> 00:26:43,860
You can measure it from
heat released when something

502
00:26:43,860 --> 00:26:47,720
dissolves, or measure
it by the heat absorbed

503
00:26:47,720 --> 00:26:50,550
and at constant pressure.

504
00:26:50,550 --> 00:26:55,390
And so the sign of delta
H sol will tell you

505
00:26:55,390 --> 00:27:00,190
about whether heat is
released or heat is required

506
00:27:00,190 --> 00:27:01,920
when something dissolves.

507
00:27:01,920 --> 00:27:04,760
So a negative
enthalpy of solution

508
00:27:04,760 --> 00:27:08,390
tells us that heat is released
when something dissolves.

509
00:27:08,390 --> 00:27:11,660
And many of you may have
just experienced this.

510
00:27:11,660 --> 00:27:13,382
If you're mixing
something together

511
00:27:13,382 --> 00:27:15,590
and you're holding the
container, and all of a sudden

512
00:27:15,590 --> 00:27:21,760
it heats up, then you know
delta H sol is a negative value.

513
00:27:21,760 --> 00:27:24,590
And of course a
positive value tells you

514
00:27:24,590 --> 00:27:28,930
that that energy is absorbed,
and sometimes things

515
00:27:28,930 --> 00:27:32,890
can get very cold when
they're dissolving.

516
00:27:32,890 --> 00:27:38,400
So you already kind of knew that
that negative delta H tells you

517
00:27:38,400 --> 00:27:41,730
that heat is released, and
positive delta H tells you

518
00:27:41,730 --> 00:27:43,380
that heat is absorbed.

519
00:27:43,380 --> 00:27:47,070
So this is basically the same
thing that you already learned.

520
00:27:47,070 --> 00:27:50,290
It just applies to things
dissolving just the same

521
00:27:50,290 --> 00:27:52,470
as it does to any reaction.

522
00:27:52,470 --> 00:27:52,970
All right.

523
00:27:52,970 --> 00:27:57,530
Now, suppose we want to
know whether the reaction is

524
00:27:57,530 --> 00:28:00,140
going to be spontaneous
at constant pressure

525
00:28:00,140 --> 00:28:01,590
or temperature.

526
00:28:01,590 --> 00:28:07,060
If we want to know whether it
will dissolve spontaneously,

527
00:28:07,060 --> 00:28:08,430
what am I asking about?

528
00:28:08,430 --> 00:28:09,900
What term do I want to know?

529
00:28:14,190 --> 00:28:17,460
I want to know about
delta G. I sure do.

530
00:28:17,460 --> 00:28:21,290
Delta G tells us whether
things are spontaneous or not.

531
00:28:21,290 --> 00:28:24,890
It does with thermodynamics,
it does with solubility.

532
00:28:24,890 --> 00:28:27,320
Delta G is to predictor
of whether something

533
00:28:27,320 --> 00:28:30,060
is going to be spontaneous
or non-spontaneous

534
00:28:30,060 --> 00:28:35,370
by the sine of delta G.

535
00:28:35,370 --> 00:28:38,160
So entropy-- don't want
to leave out entropy.

536
00:28:38,160 --> 00:28:39,840
Never leave out entropy.

537
00:28:39,840 --> 00:28:43,050
My t-shirt's still
unaccounted for,

538
00:28:43,050 --> 00:28:45,800
after they got back
to Massachusetts,

539
00:28:45,800 --> 00:28:48,320
they left for New Jersey
and Cincinnati again.

540
00:28:48,320 --> 00:28:52,000
Last time I checked,
they were still in Ohio.

541
00:28:52,000 --> 00:28:54,780
Entropy.

542
00:28:54,780 --> 00:28:57,980
So since disorder
typically increases

543
00:28:57,980 --> 00:29:01,400
when a solvent dissolves,
typically, entropy

544
00:29:01,400 --> 00:29:02,390
should increase.

545
00:29:02,390 --> 00:29:03,889
And there's a little
star, and we'll

546
00:29:03,889 --> 00:29:06,840
get back to the star, because
it doesn't always increase.

547
00:29:06,840 --> 00:29:10,240
But typically, it
should increase.

548
00:29:10,240 --> 00:29:12,700
So if we're looking at
these pictures here,

549
00:29:12,700 --> 00:29:15,950
over here you have
your nice molecules all

550
00:29:15,950 --> 00:29:20,280
lined up in your solid, and
it's going to then dissolve.

551
00:29:20,280 --> 00:29:22,470
And you can see that
when it's dissolved,

552
00:29:22,470 --> 00:29:24,550
that's a lot more
entropy over there.

553
00:29:24,550 --> 00:29:26,600
There are a lot more
freedom moving around.

554
00:29:26,600 --> 00:29:28,600
This is more constrained.

555
00:29:28,600 --> 00:29:33,650
So typically, entropy
increases when things dissolve.

556
00:29:33,650 --> 00:29:39,020
So if delta H is negative,
and delta S increases

557
00:29:39,020 --> 00:29:41,620
when a solutes
dissolves, then what

558
00:29:41,620 --> 00:29:44,996
do we expect about the
dissolving process?

559
00:29:44,996 --> 00:29:46,245
And that's a clicker question.

560
00:30:03,720 --> 00:30:04,350
Yeah.

561
00:30:04,350 --> 00:30:06,630
So most people have, so
we'll take 10 more seconds.

562
00:30:21,590 --> 00:30:23,910
So let's look at both
of them, because we're

563
00:30:23,910 --> 00:30:27,300
going to do this one
in a few minutes.

564
00:30:27,300 --> 00:30:29,832
So let's just continue
with our notes,

565
00:30:29,832 --> 00:30:31,540
and we're going to
come back to that one.

566
00:30:31,540 --> 00:30:32,081
I don't know.

567
00:30:32,081 --> 00:30:35,384
Can we just leave that one
up, maybe for a minute?

568
00:30:35,384 --> 00:30:36,800
I don't know how
long it was going

569
00:30:36,800 --> 00:30:38,260
to take to me to get there.

570
00:30:38,260 --> 00:30:44,652
All right, so when we're
talking about this--

571
00:30:44,652 --> 00:30:46,110
we'll just keep
things in the notes

572
00:30:46,110 --> 00:30:47,443
and we'll come back to that one.

573
00:30:47,443 --> 00:30:51,810
So if this is
negative and delta S

574
00:30:51,810 --> 00:30:56,480
is increasing-- so if this is
negative and this is positive,

575
00:30:56,480 --> 00:30:59,990
then you're going to have
a spontaneous process here.

576
00:30:59,990 --> 00:31:03,520
So if you can put in your notes
that this should be spontaneous

577
00:31:03,520 --> 00:31:06,490
under these circumstances.

578
00:31:06,490 --> 00:31:10,240
Now let's continue on and
think about getting around

579
00:31:10,240 --> 00:31:13,010
to the next one.

580
00:31:13,010 --> 00:31:18,840
So in some cases-- I don't
know, hopefully people

581
00:31:18,840 --> 00:31:22,330
can see this-- in some
cases, entropy of the system

582
00:31:22,330 --> 00:31:24,860
is actually lowered when
something dissolves.

583
00:31:24,860 --> 00:31:27,760
And this is because of what's
known as this cage effect.

584
00:31:27,760 --> 00:31:30,860
If you have water molecules,
when something dissolves,

585
00:31:30,860 --> 00:31:33,460
sometimes order
around the thing.

586
00:31:33,460 --> 00:31:38,970
And so the water entropy
is determining the process.

587
00:31:38,970 --> 00:31:45,070
And have this new order
because of the water structure.

588
00:31:45,070 --> 00:31:46,380
So this can change.

589
00:31:46,380 --> 00:31:50,430
So it's not always the case that
entropy of the overall system

590
00:31:50,430 --> 00:31:52,780
is going to increase.

591
00:31:52,780 --> 00:31:55,700
So here, even if this
is a negative value,

592
00:31:55,700 --> 00:31:59,110
delta H solution is negative,
delta G might be positive.

593
00:31:59,110 --> 00:32:01,790
It might not be a
spontaneous system.

594
00:32:01,790 --> 00:32:04,860
And so the cage effect
is why some things that

595
00:32:04,860 --> 00:32:07,960
are hydrophobic, just
you're having a hard time

596
00:32:07,960 --> 00:32:12,250
dissolving them, even if they
have a negative enthalpy, even

597
00:32:12,250 --> 00:32:14,210
if you would just look
at the negative enthalpy

598
00:32:14,210 --> 00:32:16,500
and predict that it
should be spontaneous.

599
00:32:16,500 --> 00:32:19,820
Because even if this
is negative, if delta

600
00:32:19,820 --> 00:32:21,880
S is also negative
because you have

601
00:32:21,880 --> 00:32:24,400
more order due to
the water molecules,

602
00:32:24,400 --> 00:32:26,770
then the overall
term can be positive.

603
00:32:26,770 --> 00:32:29,940
It would depend on
the magnitude of these

604
00:32:29,940 --> 00:32:30,962
and on the temperature.

605
00:32:35,366 --> 00:32:37,240
This is actually more
slides than I remember.

606
00:32:37,240 --> 00:32:39,600
Before we get to the next--
oh, this is the last one.

607
00:32:39,600 --> 00:32:45,776
OK, so when gases are
dissolved in a liquid,

608
00:32:45,776 --> 00:32:46,900
there is much less freedom.

609
00:32:46,900 --> 00:32:49,890
Gases not dissolved
can be anywhere.

610
00:32:49,890 --> 00:32:54,320
But when they're in a liquid,
then there's much less freedom.

611
00:32:54,320 --> 00:32:58,140
And so the entropy then
is going to be negative,

612
00:32:58,140 --> 00:33:03,480
and the solubility will decrease
as the temperature rises.

613
00:33:03,480 --> 00:33:08,540
And so that brings us then
to our question about delta

614
00:33:08,540 --> 00:33:12,500
H of solution being positive.

615
00:33:12,500 --> 00:33:14,587
And now we can look
at the answer to that.

616
00:33:18,720 --> 00:33:21,830
Now I guess we can go
maybe to the other slide.

617
00:33:21,830 --> 00:33:24,470
So that was the question
that you had here,

618
00:33:24,470 --> 00:33:27,220
and I've just put
up the answers.

619
00:33:27,220 --> 00:33:30,990
So here, delta H is positive.

620
00:33:30,990 --> 00:33:35,850
And so we have positive
minus T delta S again.

621
00:33:35,850 --> 00:33:39,930
We weren't told anything about
delta S in this particular.

622
00:33:39,930 --> 00:33:43,390
Just ask what if it's positive.

623
00:33:43,390 --> 00:33:46,530
And so for the first one,
it said dissolving is never

624
00:33:46,530 --> 00:33:47,880
spontaneous.

625
00:33:47,880 --> 00:33:51,230
And that isn't necessarily
true, because you

626
00:33:51,230 --> 00:33:53,540
don't know what delta S is.

627
00:33:53,540 --> 00:33:56,750
Dissolving is only
spontaneous if T delta S

628
00:33:56,750 --> 00:34:01,160
is positive and larger
than delta H solution.

629
00:34:01,160 --> 00:34:02,790
And that was the correct answer.

630
00:34:02,790 --> 00:34:06,950
If this is a big term here,
it's larger than that term,

631
00:34:06,950 --> 00:34:09,320
and this is a positive
value, then you

632
00:34:09,320 --> 00:34:11,880
would get a negative
delta G, and it

633
00:34:11,880 --> 00:34:14,969
would be a spontaneous process.

634
00:34:14,969 --> 00:34:19,110
If we look at number three here,
dissolving is only spontaneous

635
00:34:19,110 --> 00:34:24,030
if this is negative-- that
does not help us at all.

636
00:34:24,030 --> 00:34:26,520
And the rate will be slower.

637
00:34:26,520 --> 00:34:29,870
Delta H is not going to tell
us anything about rates.

638
00:34:29,870 --> 00:34:32,790
Rates are determined
by kinetics.

639
00:34:32,790 --> 00:34:34,710
Temperature can
affect the rates,

640
00:34:34,710 --> 00:34:37,489
for sure, stirring can
affect the rates for sure,

641
00:34:37,489 --> 00:34:44,500
but delta H being positive is
not telling us about the rates.

642
00:34:44,500 --> 00:34:48,600
So you'll see in
this particular unit,

643
00:34:48,600 --> 00:34:50,530
when you're thinking
about solubility,

644
00:34:50,530 --> 00:34:53,370
you're thinking about
equilibrium expressions,

645
00:34:53,370 --> 00:34:56,120
especially equilibrium
expression KSP,

646
00:34:56,120 --> 00:35:00,270
you're thinking about
partial pressures again,

647
00:35:00,270 --> 00:35:04,550
you're thinking about effects
on the solubility that

648
00:35:04,550 --> 00:35:06,100
have to do with
temperatures, you're

649
00:35:06,100 --> 00:35:09,810
thinking about delta H,
delta S, and delta G.

650
00:35:09,810 --> 00:35:14,470
So it's really an outgrowth
of thermodynamics and chemical

651
00:35:14,470 --> 00:35:17,100
equilibrium, but you're
just applying, really,

652
00:35:17,100 --> 00:35:19,240
what you've already
learned to thinking

653
00:35:19,240 --> 00:35:22,470
about a solute
dissolving in a solvent.

654
00:35:22,470 --> 00:35:24,500
So that's one of the
things that sets really

655
00:35:24,500 --> 00:35:27,270
great about the course
and the next couple units,

656
00:35:27,270 --> 00:35:29,830
is that everything's
very connected.

657
00:35:29,830 --> 00:35:33,070
So we can just briefly
introduce you to the next unit,

658
00:35:33,070 --> 00:35:36,310
and we're going to be talking
about acids and bases now

659
00:35:36,310 --> 00:35:38,440
for several lectures.

660
00:35:38,440 --> 00:35:42,435
And I have to say that this
is one of the units when

661
00:35:42,435 --> 00:35:44,820
we get to kind of
acid-base titrations

662
00:35:44,820 --> 00:35:47,200
that MIT students
seem to struggle with.

663
00:35:47,200 --> 00:35:51,380
So we're going to slow
this down and really

664
00:35:51,380 --> 00:35:55,343
go over it, and have you be
awesome at acids and bases.

665
00:35:58,570 --> 00:36:02,550
And I'll mention that one of
the reasons why I spent time

666
00:36:02,550 --> 00:36:06,010
on solubility and acids, bases,
because these are topics that

667
00:36:06,010 --> 00:36:09,340
are really important for medical
school, and I know a lot of you

668
00:36:09,340 --> 00:36:11,230
are not pre-med,
but I want everybody

669
00:36:11,230 --> 00:36:14,620
in this class to have that
background that they could

670
00:36:14,620 --> 00:36:16,610
go to med school
if they wanted to,

671
00:36:16,610 --> 00:36:21,240
or they could just advocate
for their own health

672
00:36:21,240 --> 00:36:24,800
with a doctor who doesn't
know about acids and bases.

673
00:36:24,800 --> 00:36:29,230
And I'll tell you a story
about that a little later.

674
00:36:29,230 --> 00:36:33,480
Anyway, OK, so acids
and bases-- we're

675
00:36:33,480 --> 00:36:35,820
just going to some definitions.

676
00:36:35,820 --> 00:36:40,910
So the simplest
definition of this

677
00:36:40,910 --> 00:36:44,960
is that an acid is a substance
that when dissolved in water,

678
00:36:44,960 --> 00:36:50,540
increases the concentration
of hydrogen ions, H plus.

679
00:36:50,540 --> 00:36:53,060
Whereas a base is
a substance that

680
00:36:53,060 --> 00:36:58,800
increases the hydroxide
concentration, OH minus.

681
00:36:58,800 --> 00:37:01,040
So there are acids and
bases that definitely

682
00:37:01,040 --> 00:37:04,940
fit this description, but this
is a very narrow description

683
00:37:04,940 --> 00:37:07,910
of acids and bases.

684
00:37:07,910 --> 00:37:10,600
The next one, the
Bronsted-Lowry,

685
00:37:10,600 --> 00:37:12,910
is a bit broader.

686
00:37:12,910 --> 00:37:15,390
So an acid is
something that donates

687
00:37:15,390 --> 00:37:20,390
a hydrogen ion, H plus,
and a Bronsted-Lowry base

688
00:37:20,390 --> 00:37:23,750
is something that
accepts a hydrogen ion.

689
00:37:23,750 --> 00:37:25,220
And this is really
the definition

690
00:37:25,220 --> 00:37:28,990
we'll be spending the most
time with in the class.

691
00:37:28,990 --> 00:37:33,730
So let's look at some
examples of this.

692
00:37:33,730 --> 00:37:38,620
So here, we have an acid.

693
00:37:38,620 --> 00:37:44,070
And if it's an acid, it's going
to be donating a hydrogen ion,

694
00:37:44,070 --> 00:37:47,090
and it's going to donate
it here to the water.

695
00:37:47,090 --> 00:37:52,360
The water acts as a base in that
it accepts the hydrogen ion.

696
00:37:52,360 --> 00:37:54,400
When it accepts
the hydrogen ion,

697
00:37:54,400 --> 00:38:01,030
it has one extra hydrogen
ion, so it's H3O plus.

698
00:38:01,030 --> 00:38:04,350
And when the acid
loses its hydrogen ion,

699
00:38:04,350 --> 00:38:08,070
it becomes this base over
here, the same molecule

700
00:38:08,070 --> 00:38:11,760
minus the hydrogen ion, so
now it has a negative charge.

701
00:38:11,760 --> 00:38:15,200
And so you'll will
see hydronium ions

702
00:38:15,200 --> 00:38:17,660
quite a bit, which is H3O plus.

703
00:38:17,660 --> 00:38:19,590
That's really the
more accurate form,

704
00:38:19,590 --> 00:38:23,110
rather than just writing
H plus somewhere.

705
00:38:23,110 --> 00:38:25,750
So that's the kind of
true nature of things.

706
00:38:25,750 --> 00:38:28,680
So let me just show you a little
animation of this happening.

707
00:38:28,680 --> 00:38:29,790
I think it's kind of cute.

708
00:38:33,930 --> 00:38:40,155
So here we have our water
molecules and we have our acid.

709
00:38:40,155 --> 00:38:43,000
It donated a hydrogen
ion to these,

710
00:38:43,000 --> 00:38:45,420
and then another water
came and stole it away.

711
00:38:45,420 --> 00:38:48,330
And so there's our H3O plus.

712
00:38:48,330 --> 00:38:51,370
And so in solution,
you have this exchange

713
00:38:51,370 --> 00:38:52,590
of hydrogen atoms.

714
00:38:52,590 --> 00:38:55,040
The acid is donating,
the base is accepting.

715
00:38:55,040 --> 00:38:57,340
And as they donate
and accept, they

716
00:38:57,340 --> 00:38:59,410
become different molecules.

717
00:38:59,410 --> 00:39:04,920
So it gives rise to this idea
of conjugate acid-base pairs.

718
00:39:04,920 --> 00:39:10,440
So here you see that
this acid is paired

719
00:39:10,440 --> 00:39:12,170
with this base over here.

720
00:39:12,170 --> 00:39:15,780
They're the same, except
that one has the hydrogen ion

721
00:39:15,780 --> 00:39:17,690
and one does not.

722
00:39:17,690 --> 00:39:20,600
And then the other pair is here.

723
00:39:20,600 --> 00:39:22,370
It's another acid-base pair.

724
00:39:22,370 --> 00:39:27,300
We have water and hydronium
ion that also differ by H plus.

725
00:39:27,300 --> 00:39:32,250
So every time an acid donates
a hydrogen ion or proton,

726
00:39:32,250 --> 00:39:34,050
it becomes its conjugate base.

727
00:39:34,050 --> 00:39:37,560
Every time a base accepts
a hydrogen ion or proton,

728
00:39:37,560 --> 00:39:41,140
it becomes its conjugate acid.

729
00:39:41,140 --> 00:39:44,140
So the conjugate
base of any acid

730
00:39:44,140 --> 00:39:48,480
is the base that is formed when
the acid has donated a hydrogen

731
00:39:48,480 --> 00:39:50,030
ion or proton.

732
00:39:50,030 --> 00:39:52,440
The conjugate acid
of a base is the acid

733
00:39:52,440 --> 00:39:58,500
that forms when the base accepts
a hydrogen ion or proton.

734
00:39:58,500 --> 00:40:01,780
So let's take a look
at one more example.

735
00:40:01,780 --> 00:40:06,080
If this over here-- is this
acting as an acid or base,

736
00:40:06,080 --> 00:40:07,021
this molecule here?

737
00:40:07,021 --> 00:40:07,770
What do you think?

738
00:40:07,770 --> 00:40:10,550
You can just yell it out.

739
00:40:10,550 --> 00:40:12,650
It's acting as an acid, right?

740
00:40:12,650 --> 00:40:15,430
This is acting,
then, as the base.

741
00:40:15,430 --> 00:40:18,140
When this base accepts
the hydrogen ion,

742
00:40:18,140 --> 00:40:20,400
it forms its conjugate acid.

743
00:40:20,400 --> 00:40:23,380
And this then forms
its conjugate base,

744
00:40:23,380 --> 00:40:27,910
which differs by H plus.

745
00:40:27,910 --> 00:40:32,638
So why don't you try one of
these, then, on your own?

746
00:40:43,710 --> 00:40:44,410
10 more seconds.

747
00:41:02,750 --> 00:41:07,780
OK, so we can go back
to that over there.

748
00:41:07,780 --> 00:41:11,420
So you most people
got this right,

749
00:41:11,420 --> 00:41:14,830
and so you're looking at
what's acting as the base

750
00:41:14,830 --> 00:41:16,920
and what's acting as the acid.

751
00:41:16,920 --> 00:41:20,600
Again, the base is going to
be accepting a hydrogen ion.

752
00:41:20,600 --> 00:41:22,340
So this is acting as the acid.

753
00:41:22,340 --> 00:41:25,240
This forms it's
conjugate it and this.

754
00:41:25,240 --> 00:41:32,880
And quickly, with the idea of
something that's amphoteric,

755
00:41:32,880 --> 00:41:35,920
which is molecule that can
act as an acid or a base.

756
00:41:35,920 --> 00:41:39,030
You just saw some
nice examples of that.

757
00:41:39,030 --> 00:41:45,110
And finally, just one last
definition for acid-bases.

758
00:41:45,110 --> 00:41:52,300
And we'll just put this up
and that's kind of the end.

759
00:41:52,300 --> 00:41:55,060
So the final definition,
the Lewis Base,

760
00:41:55,060 --> 00:41:57,700
donates a lone pair,
whereas a Lewis Acid

761
00:41:57,700 --> 00:42:01,170
accepts such electrons.