IB Chemistry home > Syllabus 2016 > Equilibrium > The equilibrium constant

Syllabus ref: 7.1

More about the constant, Kc, obtained by dividing the concentrations of the products raised to the power of their coefficients by the concentrations of the reactants raised to the power of their coefficients.

The equilibrium constant

The equilibrium law tells us that for any reaction that attains equilibrium, the equilibrium constant depends on the concentration of products and reactants at equilibrium.

For the reaction:
wA + xB yC + zD
The equilibrium expression:

If a reaction proceeds nearly to completion, then the concentrations of the products is much larger than the concentration of the reactants and the value of the equilibrium constant is very large (much greater than 1).

If the reaction only makes a small quantity of the products at equilibrium, then the value of the equilibrium constant is very small (much less than 1)

Value of Kc position of equilibrium reaction
<< 1 almost entirely reactants almost none
>> 1 almost entirely products proceeds virtually to completion
≈ 1 roughly in the centre both reactants and products present

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Factors affecting Kc

The equililbrium constant is ONLY affected by the temperature at which equilibrium is established.

An equilibrium will change its position towards the side of endothermic change if the temperature is increased. This is in accordance with Le Chatelier, as the system is responding to remove the extra energy that has been supplied. It does so by converting heat energy to chemical energy (thus reducing the heat).

Any change in Kc depends on the direction of endothermic change. If the equilibrium is endothermic in the forward direction, then increasing the temperature increases the products and the value of Kc. The opposite is true for an exothermic forward reaction.

Example: How does an increase in the temperature of the following equilibrium affect the value of the equilibrium constant?

A(g) + B(g) C(g)    Δ H = -150 kJ

The reaction is exothermic (negative enthalpy change) in the forward direction, therefore increasing the temperature drives the reaction in the reverse direction, increasing the reactant concentration and reducing the product concentration.

The vaue of Kc decreases correspondingly.


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The units of Kc

By units, we mean the dimensions of the equilibrium constant. The value of the equilibrium constant depends on the specific equation to which it applies and so do its units.

To find the units of any quantity that is derived from an equation it is simply a process of introducing the units for each of the equations components and then can cancelling down to arrive at the smallest possible unit .

Example: Find the units of the equilibrium constant for the equilibrium:

A + B C + D

The equilibrium constant is given by the equilibrium law equation   

Substitute in the units for each of the components in the equation:

It shoiuld be pretty clear that the units on the top of the sum are equal to the units on the bottom and so they cancel out:

leaving the value of Kc dimensionless, i.e. it has no units.


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