Dissolving Carbon Dioxide into Water
Date: Fall 2009
Why and how does carbon dioxide dissolve into water, if it
is a gas and non-polar? I thought likes dissolved likes.
First, let us not forget that the statement "like dissolves like" is
just a rule of thumb, a summary statement of a general trend, not a theory.
The idea that like dissolves like can be explained in this way: in
order to form a solution, solute-solute interactions and some
solvent-solvent interactions have to be broken - this requires
energy. Then solute-solvent interactions have to form - this
releases energy. A spontaneous process is usually consequent with a
release of energy or an increase in disorganization (entropy). So,
in order for there to be a resultant release in energy when a solute
dissolves into a solvent, the energy required to break the
solute-solute and solvent-solvent interactions have to be equal to
or less than the energy released by forming the solute-solvent
interactions. There usually is an increase in disorganization
because two separate systems (solute and solvent) is less
disorganized than a combined system (solute mixed with solvent).
This usually happens when the solute-solute interactions are similar
to the solvent-solvent interactions.
So think of water and oil. London Forces interaction (in the
oil/solute) need to be broken - this requires very little energy
since London Forces are weak. Hydrogen bonding forces need to broken
(in water/solvent) and this require a lot of energy since hydrogen
bonding is very strong. Then when the water-oil interaction is
formed, only London Forces can form (limited by what the oil is
capable of) - and this releases only a small amount of energy. So
even though there could be an increase in disorganization if the oil
and water mixed, the high requirement of energy to break the
hydrogen bonding interactions is not compensated for by the increase
in entropy and the weak interactions formed by an oil-water
solution. Thus, solution formation does not happen.
In alcohol-water solutions, both have hydrogen bondings, and the
energy required to break interactions is compensated for by the
energy released by the formed interactions plus the increase in
entropy . . . and we say like dissolve like.
Now on to your question.
1) If we imagine a gas (any gas) above a liquid (any liquid). The
two systems remaining separate is a lower disorganization system
than if the gas were to mix with the liquid. So there is a gain in
entropy, higher disorganization if gases were to mix into liquids
(whatever their intermolecular forces happen to be). So as a general
statement, gases mix into liquids simply because there is an entropy gain.
2) If we look at gases - there is very little interaction between
gas particles, so no solute-solute interaction need be broken. Some
solvent-solvent interaction have to be broken in order to "insert"
gases into the liquid. We should also expect a release of energy
when gas-liquid interactions form. But as you've already noted, CO2
and H2O have different intermolecular forces and so the energy
required to break hydrogen bondings is not compensated by the
release of energy when London Forces are produced when CO2 interacts
3) So we have to conclude that it is the gain in entropy that drives
the solubility of gases in liquids. This is supported by the idea
that when we increase the pressure of the gas above a liquid, more
gas dissolves into the liquid. This is because when pressure
increases for gases, this usually means that there are more gas
particles per unit volume - and this is a more organized state than
when those gas particles are farther apart. So, there is a higher
gain in entropy when a gas dissolves into a liquid when the gas
started from a higher organized state in the first place.
To summarize: gas solubility is a result of an increase in entropy
and not so much from the energy loss as a result of solvation. So
the rule, like dissolves like does not apply.
Greg (Roberto Gregorius)
The old "like dissolves like" rule does not really cover everything
in the world of dissolving. Water will dissolve most molecular
covalent (like carbon dioxide) substances to some extent even if
they are not that polar. Of course the very polar substances tend to
be more soluble, but not always, otherwise all ionic salts would be
soluble in water and there are plenty that are not.
CO2 has no overall dipole due to symmetry but there is the
possibility of fairly strong interactions with water due to each
oxygen's two lone pairs. These can donate electron density to the
positive hydrogens on the water molecule in an analogous way to how
water molecules hydrogen bond to each other.
But the dissolving of CO2 in water is actually more than just
dissolving, it forms an equilibrium with water molecules to form
carbonic acid H2CO3 and this also has equilibria with hydrogen
carbonate HCO3- and carbonate CO3(2-). These ions have strong
attractions to water molecules through hydration spheres, the same
as any soluble ion. It is really this set of equilibria that gives
CO2 its solubility, the actual concentration of CO2(aq) is quite low.
It is worth bearing in mind that the solubility of CO2 is strongly
affected by temperature and pressure, less soluble in high
temperatures and low pressures. You can see this when you open a hot
bottle of coke!
"Likes dissolves Likes" is just a rule of thumb, not a physical law. Carbon
dioxide is especially soluble in water because it reacts with water:
H2O ---> H2CO3 ----> (H+) + (HCO3[-1])
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Update: June 2012