The Equilibrium Constant (DP IB Chemistry): Revision Note
The Equilibrium Constant
The equilibirium constant, K
The size of the equilibrium constant, K, tells us how the equilibrium mixture is made up with respect to reactants and products
Interpreting the magnitude of K
If K << 1
The reaction hardly proceeds
The equilibrium lies far to the left
The mixture contains mostly reactants
If K<1:
The reaction favours the reactants
The equilibrium lies to the left-hand side
If K=1:
There are significant amounts of both reactants and products
The equilibrium is balanced between the two sides
If K>1:
The reaction favours the products
The equilibrium lies to the right-hand side
If K >> 1:
The reaction goes almost to completion
The equilibrium lies far to the right with mostly products present
K is a constant at a specified temperature
Since temperature can affect the position of equilibrium, it follows that K is dependent on temperature
Worked Example
When the following reactions reach equilibrium, state whether the equilibrium mixture contains mostly reactants or products. Assume the value of K corresponds to the temperature of the reaction mixture
Ag+ (aq) + Fe2+ (aq) ⇌ Ag (s) + Fe3+ (aq) K = 7.3 x 10-26
N2 (g) + 3H2 (g) ⇌ 2NH3 (g) K = 2.6 x 10-18
2SO2 (g) + O2 (g) ⇌ 2SO3 (g) K = 5.0 x 1013
Answer:
Reactions 1 and 2:
K is very much smaller than 1
So, the denominator in the equilibrium constant expression must be much larger than the numerator
This means that the concentration of the reactants is much larger than the concentration of products
Therefore, the equilibrium lies far to the left and the equilibrium mixture contains mostly reactants
Reaction 3:
K is very much larger than 1
So, the numerator in the equilibrium constant expression must be much larger than the denominator
This means that the concentration of the products is much larger than the concentration of reactants
Therefore, the equilibrium lies to the right-hand side and the reaction mixture contains mostly products
Examiner Tips and Tricks
Stronger acids dissociate more than weaker acids in solution, meaning that equilibrium lies towards the products
So, stronger acids will have a higher value of K than weaker acids.
The relationship between K values for reactions that are the reverse of each other
The equilibrium constant expression is dependent on a specific reaction
For example, take the reaction between nitrogen and hydrogen to make ammonia:
N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
The equilibrium constant expression for this reaction is:
If we reverse the equation:
2NH3 (g) ⇌ N2 (g) + 3H2 (g)
The equilibrium constant expression for the reverse of this reaction, K', is:
What is the relationship between these two K values? At the same temperature, K' becomes the reciprocal of the original K value:
or
Worked Example
The equilibrium constant for the following reaction is 7.1 × 1032.
2NO2 (g) + F2 (g) ⇌ 2NO2F (g)
What is the equilibrium constant for the reverse at the same temperature?
Answer:
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