Equilibrium constants for heterogeneous/aqueous reactions
The equilibrium constant for a reaction at a particular temperature is related to pressures or concentrations of reactants and products through the reaction quotient expression.
The expressions for both Kc are shown for the general equation below.
The reaction quotient expression includes reactants and products for which the equilibrium concentration/pressure of one depends on the concentration/pressure of the others.
Reactants and products that are solids or pure liquids are assigned a value of 1 in the reaction quotient expression.
Why? It is assumed that solvents are present in large excess and their concentration does not change as a consequence of reaction.
The expressions for both Kc are shown for the general equation below.
aA + bB 
cC + dD
A, B, C and D are all solutes (dissolved) or gases for Kc
a, b, c and d are the numbers required to balance the equation
cC + dDA, B, C and D are all solutes (dissolved) or gases for Kc
a, b, c and d are the numbers required to balance the equation
| Kc = | [C]c[D]d |
| [A]a[B]b | |
| reaction quotient |
The reaction quotient expression includes reactants and products for which the equilibrium concentration/pressure of one depends on the concentration/pressure of the others.
Because K is constant, a change in the concentration/pressure of one component results in a corresponding change in the concentration/pressure of other components.
Reactants and products that are solids or pure liquids are assigned a value of 1 in the reaction quotient expression.
Why?
While all reactants and/or products must be present for the system to be at equilibrium, the concentrations or pressures of the other components do not depend on the amount of a pure liquid or solid that is present.
Example 1: The equilibrium system in which CuSO4(s) is dissolved in water to give a solution that is blue due to Cu2+(aq) is represented by the equation below.
CuSO4(s)
Cu2+(aq) + SO42–(aq)
As seen in the images, the intensity of the blue colour does not depend on the amount of solid CuSO4.
Example 2: The equilibrium of bromine gas with bromine liquid can be represented as shown below.
Br2(l)Br2(g)
The colour due to Br2 gas (thus its concentration) does not depend on the amount of Br2(l) present in the flask.
Reactants and/or products that are the solvent are assigned a value of 1 in the reaction quotient expression (for example H2O for reactions in aqueous solution).While all reactants and/or products must be present for the system to be at equilibrium, the concentrations or pressures of the other components do not depend on the amount of a pure liquid or solid that is present.
 |  |
Example 1: The equilibrium system in which CuSO4(s) is dissolved in water to give a solution that is blue due to Cu2+(aq) is represented by the equation below.
CuSO4(s)
Cu2+(aq) + SO42–(aq)As seen in the images, the intensity of the blue colour does not depend on the amount of solid CuSO4.
Br2(l)
Br2(g)The colour due to Br2 gas (thus its concentration) does not depend on the amount of Br2(l) present in the flask.
Why? It is assumed that solvents are present in large excess and their concentration does not change as a consequence of reaction.