2.3: IMF in solution
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Mixing two substances together can always be thought of as a combination of the same three processes:
1. Pull compound A apart -- Always endothermic 2. Pull compound B apart -- Always endothermic 3. Put A and B together -- Always exothermic |
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For something to mix significantly, the ΔG must be negative (as with all things)
The ΔS for mixing is always going to be positive and favorable.
Therefore, the ΔH doesn't have to be favorable...just not too unfavorable.
There are three possible combinations:
1. Polar with polar (or ionic): example H2O and NaCl
Step 1 will be high endo (breaking H-bonds)
Step 2 will be very high endo (pulling apart ions--lattice energy)
Step 3 will be very high exo (making ion-dipole attraction)
On balance, this is usually possible. However, some compounds with very high lattice energy do not dissolve (solubility rules!)
Similarly, you do run into some polar compounds with low enough polarity that step 3 just isn't exo enough.
2. Polar with non-polar: example H2O and C8H18
Step 1 will be high endo (breaking H-bonds)
Step 2 will be low endo (breaking dispersion forces)
Step 3 will be low exo (making dipole-induced dipole attraction)
On balance, this just won't cut it; it's too endothermic for the entropy to make up for it. The compounds won't mix.
IMPORTANT NOTE: The non-polar compound is still more attracted to the polar compound than it is to itself.
The polar compound just doesn't want to come apart to let it in.
3. Non-polar with non-polar: example C4H8 and C8H18
Step 1 will be low endo (breaking dispersion forces)
Step 2 will be low endo (breaking dispersion forces)
Step 3 will be low exo (making dispersion forces)
All the values are low here, so this should typically work out to be possible.
All of this is usually summed up as "LIKE DISSOLVES LIKE"
(polar and ionic stuff dissolves in polar liquids, and non-polar stuff in non-polar liquids)
The ΔS for mixing is always going to be positive and favorable.
Therefore, the ΔH doesn't have to be favorable...just not too unfavorable.
There are three possible combinations:
1. Polar with polar (or ionic): example H2O and NaCl
Step 1 will be high endo (breaking H-bonds)
Step 2 will be very high endo (pulling apart ions--lattice energy)
Step 3 will be very high exo (making ion-dipole attraction)
On balance, this is usually possible. However, some compounds with very high lattice energy do not dissolve (solubility rules!)
Similarly, you do run into some polar compounds with low enough polarity that step 3 just isn't exo enough.
2. Polar with non-polar: example H2O and C8H18
Step 1 will be high endo (breaking H-bonds)
Step 2 will be low endo (breaking dispersion forces)
Step 3 will be low exo (making dipole-induced dipole attraction)
On balance, this just won't cut it; it's too endothermic for the entropy to make up for it. The compounds won't mix.
IMPORTANT NOTE: The non-polar compound is still more attracted to the polar compound than it is to itself.
The polar compound just doesn't want to come apart to let it in.
3. Non-polar with non-polar: example C4H8 and C8H18
Step 1 will be low endo (breaking dispersion forces)
Step 2 will be low endo (breaking dispersion forces)
Step 3 will be low exo (making dispersion forces)
All the values are low here, so this should typically work out to be possible.
All of this is usually summed up as "LIKE DISSOLVES LIKE"
(polar and ionic stuff dissolves in polar liquids, and non-polar stuff in non-polar liquids)
