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Equilibrium Constants

Key Concepts

Equilibrium Constants (K):

  • are derived from experimental data

  • are temperature dependent

  • remain the same value for a constant temperature even if the reactant and/or product concentrations are altered

  • do NOT change in the presence of a catalyst (a catalyst changes the time taken to reach equilibrium but does not alter the equilibrium position or equilibrium constant)

  • For the chemical reaction:
    aA + bB cC + dD

    where A, B, C and D are the chemical species and
    a, b, c and d are the stoichiometric coefficients,

    the law of chemical equilibrium is expressed as an equation for the equilibrium constant, K, for the reaction:
    Kc = [C]c[D]d

  • If the concentration of species in the reaction is used to calculate the equilibrium constant then it can be given the symbol Kc. If the equilibrium constant has no subscript, that is it is given as K, this refers to Kc and the concentration of each species in the reaction is used to calculate the equilibrium constant.

  • If a solvent takes part in a reaction, its concentration is said to remain constant and is incorporated into the value of K.

  • If a solid is present in a reaction, its concentration is said to remain constant and is incorporated into the value of K.


Consider the chemical reaction in which a pale yellow aqueous solution of iron(III) ions (Fe3+(aq)) reacts with a colourless aqueous solution of thiocyantate ions (SCN-(aq)) to produce bright red Fe(SCN)2+(aq):

Fe3+(aq) + SCN-(aq) Fe(SCN)2+(aq)

The concentration of the species can be determined by measuring the absorption of light in the sample.

Three separate experiments are performed at the same temperature.
In each experiment the initial concentrations of Fe3+(aq) and SCN-(aq) are different.
The reaction is allowed to proceed to equilibrium.

The concentration of each species at equilibrium is measured and recorded as shown below:

Experiment Initial Concentrations
mol L-1
Equilibrium Concentrations
mol L-1
[Fe3+(aq)] [SCN-(aq)] [Fe(SCN)2+(aq)] [Fe3+(aq)] [SCN-(aq)] [Fe(SCN)2+(aq)]
1. 0.110 0.110 0.000 0.0100 0.0100 0.100
2. 0.600 0.505 0.000 0.100 0.00500 0.500
3. 0.275 0.260 0.000 0.0250 0.0100 0.2500

Can you see a pattern in the data?

We can see that the equilibrium concentrations of the reactants [Fe3+(aq)] and [SCN-(aq)] are lower than their initial concentrations.
And we can see that the equilibrium concentration of the product [Fe(SCN)2+(aq)] is higher than its initial concentration.
But can you see another pattern?
Remember that we can write a mass-action expression for this reaction:

Q =     [Fe(SCN)2+(aq)]    

Let's use the values for the concentration of each species at equilibrium to calculate the value of the mass-action expression, Q

Experiment Equilibrium Concentrations
mol L-1
Q =     [Fe(SCN)2+(aq)]    
[Fe3+(aq)] [SCN-(aq)] [Fe(SCN)2+(aq)]
1. 0.0100 0.0100 0.100
= 1000
2. 0.100 0.00500 0.500
= 1000
3. 0.0250 0.0100 0.2500
= 1000

At equilibrium, the value of the mass-action expression, Q, is a constant.
This constant is known as the equilibrium constant and is given the symbol K.
At equilibrium, Q = K and is known as the equilibrium condition.
The value of the equilibrium constant for a given reaction at a specified temperature does not change if the concentration of a reactant or product changes.

Want to see another worked example?


Some Special Special Cases

The equilibrium constant for some types of reactions is given a special name, and a special symbol, because of the importance of these reactions.
Some examples are given in the table below:

Reaction Name for Equilibrium Constant Symbol for
Equilibrium Constant
Self-dissociation of water Ion Product for Water Kw
Dissociation of acids Acid Dissociation Constants Ka
Dissociation of bases Base Dissociation Constants Kb
Dissolving an ionic compound Solubility Products Ksp
gaseous reactions (calculated using partial pressures not concentrations) KP

Want to see a worked example for calculating K using concentrations?


Calculating the Equilibrium Concentration of a Species Given the Value of K

(Based on the StoPGoPS method for problem solving)

Question: Methanol can be made by reacting carbon monoxide gas with hydrogen gas:

CO(g) + 2H2(g) CH3OH(g)

The value of the equilibrium constant, K, for this reaction at 100oC was found to be 0.600.
The concentration of CO(g) at equilibrium was determined to be 3.75 x 10-3 mol L-1, while the equilibrium concentration of CH3OH(g) was found to be 4.25 x 10-8 mol L-1.
Calculate the concentration of H2(g) in mol L-1 at equilibrium.

  1. What have you been asked to do?

    Calculate concentration of H2(g) in mol L-1 at equilibrium:

    [H2(g)] = ? mol L-1

  2. What data have you been given in the question?

    (i) CO(g) + 2H2(g) CH3OH(g)

    (ii) K = 0.600

    (iii) at equilibrium, [CO(g)] = 3.75 x 10-3 mol L-1

    (iv) at equilibrium, [CH3OH(g)] = 4.25 x 10-8 mol L-1

    What is the relationship between what you have been given and what you need to find?

    K =   [CH3OH]  

    Rearrange the equation:

    [H2]2 =   [CH3OH]  


    [H2] = √( [CH3OH]

  3. Substitute in the values and solve:

    [H2] = √( [CH3OH]
          = √(     [4.25 x 10-8]    
    0.600[3.75 x 10-3]
          = √( 4.25 x 10-8
    2.25 x 10-3
      = √(1.889 x 10-5)  
      = 4.35 x 10-3 mol L-1  

  4. Is your answer plausible?

    Use the equilibrium concentrations given, and your calculated value of [H2(g)] to see if you get the same value for the equilibrium constant, K, as that given in the question:

    K =   [CH3OH]  
      =         [4.25 x 10-8]        
    [3.75 x 10-3][4.35 x 10-3]2
      =         [4.25 x 10-8]        
    [3.75x 10-3]1.89 x 10-5
      = 4.25 x 10-8
    7.09 x 10-8
      = 0.600

    By using our calculated value for [H2] we calculate the same value for K as was given in the question, so we are confident our answer is correct.

  5. State the solution to the problem:

    The concentration of hydrogen gas at equilibrium is 4.35 x 10-3 mol L-1

Ready to try some exam questions?


Calculating the Value of the Equilibrium Constant, K, Given Initial Concentration Data

This requires you to calculate the equilibrium concentration of each species before you can calculate the value of the equilibrium constant.
You will find a full discussion of this process in the R.I.C.E. Tables and Calculating K Tutorial

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