The effect of tendency towards maximum randomness
The gaseous state is more random than the liquid state since the molecules move freely through a much larger space as a gas. Hence randomness decreases as a gas dissolves in a liquid. In this case, unlike solids, the tendency toward maximum randomness favors the gas phase and opposes the dissolving process.
The effect of tendency towards minimum potential energy
In a gas the molecules are far apart and they interact very weakly. As a gas molecule enters a liquid, it comes close to the solvent molecules and they attract each other, lowering the potential energy. Again we find a contrast to the behavior of solids. When a gas dissolves in a liquid, heat is evolved. The tendency toward minimum energy favors the dissolving process.
Thus we see that the equilibrium solubility of a gas again involves a balance between randomness and energy as it does for a solid, but the effects are opposite. For a gas, the tendency toward maximum randomness favors the gas phase, opposing dissolving. The tendency toward minimum energy favors the liquid state, hence favors dissolving.
As an example, consider the solubilities of the two gases, oxygen, O2, and nitrous oxide, N2O, in water. The heats of solution have been measured and are as follows:
O2(g) ⇌ O2(aq) + 12.5 kJ/mole O2 (13)
N2O(g) ⇌ N2O(aq) + 20.0 kJ/mole N2O (14)
Assuming the randomness factor is about the same, the gas with the larger heat effect (favoring dissolving) should have the higher solubility. The measured solubilities at one atmosphere pressure and 20°C of oxygen and nitrous oxide in water are, respectively, O2, 1.4 × 10–3 mole/liter and N2O, 27 × 10–3 mole/liter, consistent with our prediction.
The effect of raising the temperature
Raising the temperature always tends to favor the more random state. This means that less gas will dissolve, since the gas is more random than the liquid. The solubility of a gas decreases as temperature is raised.
The effect of changing the partial pressure of the gas
Raising the partial pressure of a gas in equilibrium with a liquid solution raises the concentration of the gas in the gaseous phase. According to Le Chatelier’s principle, the equilibrium shifts in a direction to decrease the partial pressure, so more gas dissolves. More CO2(g) dissolves in water if the partial pressure of the gas is increased. When a bottle of fizzy drink is opened the partial pressure of CO2(g) on top of the solution is reduced to atmospheric, so the reverse thing happens: gas starts to leave the liquid; i.e. the solubility of the gas decreases.
From the heat of solution of chlorine in water, –29 kJ/mol (heat evolved), how do you expect the solubility of chlorine at one atmosphere pressure and 20°C to compare with that of oxygen and of nitrous oxide, N2O? (Refer to equations 13 & 14)?
O2(g) ⇌ O2(aq) + 12.5 kJ/mol O2 (13)
N2O(g) ⇌ N2O(aq) + 20.0 kJ/mol N2O (14)
The gas with more heat released when dissolved in water should have the
solubility.
Write the correct answer using the words: higher or lower.
From the heat of solution of chlorine in water, –29 kJ/mol (heat evolved), how do you expect the solubility of chlorine at one atmosphere pressure and 20°C to compare with that of oxygen and of nitrous oxide, N2O? (Refer to equations 13 & 14)?
O2(g) ⇌ O2(aq) + 12.5 kJ/mol O2 (13)
N2O(g) ⇌ N2O(aq) + 20.0 kJ/mol N2O (14)
N2O(g) ⇌ N2O(aq) + 20.0 kJ/mol N2O (14)
Chlorine in water, at one atmosphere pressure and 20°C, will have
solubility than oxygen and nitrous oxide.
Write the correct answer using the words: higher or lower.
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