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In a previous post, I discussed why it is not appropriate to assume the molar concentrations of solids and liquids to be one in equilibrium constant expressions.

Before I attempt to answer this question, let's review the Equilibrium Law:

For a general chemical reaction aA + bB ⇌ cC + dD, we can define an equilibrium constant, K_c, where

Let's see how this law is applied to different equilibria.

This is where all species are present in the same phase, and the application of the Equilibrium Law is straight-forward: the concentrations of all species present in the equilibrium are included in the K_c expression.

For example,

N2(g)+ 3H2(g) ⇌ 2NH3(g)
CH3CO2H(l) + CH3CH2OH(l) ⇌ CH3CO2CH2CH3(l) + H2O(l)

This is where the species are present in different phases. These include reactions that involve solids and gases, or solids and liquids. For these cases, the concentrations of solids are omitted from their K_c expressions.

For example,

I     C(s)+ H2O(g) ⇌ CO(g) + H2(g)
II    Mg(OH)2(s) ⇌ Mg2+(aq) + 2OH−(aq)

 

It may seem so, but the answer is no. If we apply the Equilibrium Law directly to Reactions I and II, their K_c expressions should include the concentrations of the solid reactants:

I     C(s)+ H2O(g) ⇌ CO(g) + H2(g)
II    Mg(OH)2(s) ⇌ Mg2+(aq) + 2OH−(aq)

However these solid concentrations are meaningless in K_c expressions. As long as there is some solid present in the equilibrium, whether it is present in a small or large quantity does not affect equilibrium. We can therefore re-define a new K_c expression without the solid concentrations:

For Reaction I: C(s) + H₂O(g) ⇌ CO(g) + H₂(g)

where [C(s)] is a constant that can be calculated from the density of carbon used in the reaction

Similarly, for Reaction II: Mg(OH)₂(s) ⇌ Mg²⁺(aq) + 2OH⁻(aq)

where [Mg(OH)₂(s)] is a constant that can be calculated from the density of magnesium hydroxide

1. The molar concentration of a solid may be calculated from its density as follows:
   
   
   
   
   
2. The K_c^/ for Reaction II is known as the solubility product of magnesium hydroxide, and is represented by the symbol, K_sp.

This is where all species are dissolved in water. By convention, only concentrations of the species involved in the equilibrium are included in the K_c expression; the concentration of the solvent, i.e. water, is excluded, even if the latter takes part in the equilibrium as a reactant or a product.

For example, ethanoic acid reacts with water as follows:

III    CH₃CO₂H(aq) + H₂O(l) ⇌ CH₃CO₂⁻(aq) + H₃O⁺(aq)

As long as the solution is dilute, the amount of water present in the solution is hardly affected by the reaction with ethanoic acid.

Because the concentration of water remains essentially the same before and after the equilibrium is reached, we can re-define the K_c expression for Reaction III as follows:

1. The concentration of pure water was calculated using the density of water (1.00 g cm⁻³) as follows:
   
   
   
   
   
2. The K_c^/ for Reaction III is known as the acid dissociation constant for ethanoic acid, and is represented by the symbol K_a.
3. We can similarly define the base dissociation constant, K_b, for the reaction between a weak base and water:
   
   
   B(aq) + H₂O(l) ⇌ BH⁺ + OH⁻(aq)
   
   
   
4. While the concentration of water is generally omitted from K_c expressions for aqueous equilibria involving water (either as a reactant or a product), you should not apply this omission across the board without first considering what the equilibrium constant is used for. I shall illustrate some exceptions with examples in my next post.