In this lesson, we will learn:
- To write an expression for the equilibrium constant Keq.
- How to interpret the value of Keq and describe the reaction using this value.
- How to use the equilibrium expression with equilibrium concentrations to solve for Keq (and vice versa).
- We now know the definition of equilibrium; a chemical process where the forward reaction rate is equal to the reverse rate. Be careful – this tells us nothing about how much product or reactant is there! To find that, we need to use the equilibrium constant expression.
- Using measurements of reactant and product concentrations, it is possible to find what is called the equilibrium constant, Keq, of a given reaction at equilibrium. This is done using the expression:
For the reaction at equilibrium:
Be clear with your language:
- The whole equation is the equilibrium expression.
- Keq is the equilibrium constant.
- Keq is the general term – if the equilibrium is measuring concentration (in mol dm-3) it might be called Kc. Kp would be used if it was partial pressures (for gases).
- The equilibrium constant is called a constant because it is not affected by changes in some conditions. Changes to concentration of reactants or products and changes in pressure do not affect the equilibrium constant!
- Remember Le Chatelier’s principle: the system will counteract any change made. If you add reactant, more product will be made to counteract the change. This keeps Keq constant in the long run.
- Changing temperature WILL affect Keq, depending on whether the reaction is endothermic or exothermic.
Because of this, ALWAYS quote Keq with a temperature.
- The equilibrium expression looks complicated, so breaking it down using simple math can help.
- It is a fraction. It has terms for amount of product and reactant.
- You can write fractions as ratios. So…
- It is a ratio of products to reactants in the reaction. Written as a decimal, the value of Keq tells us something important:
- Keq is smaller than 1: There is less product than reactant in the reaction mixture. The smaller the value of Keq, the less product there is.
- Keq is approximately 1: There is roughly the same amount of product as reactant in the reaction mixture.
- Keq is larger than 1: There is more product than reactant in the reaction mixture. The larger Keq is, the more product compared to reactant.
- When writing Keq for heterogeneous systems, where the substances are not all in the same phase, ignore any substances in the solid state. Solid reagents do not affect the equilibrium constant; everything else in the Keq expression is written as normal.
For example, let’s look at the thermal decomposition of calcium carbonate:
CaCO3 (s) CaO (s) + CO2 (g)
- If CO2 escapes the reaction vessel because it’s open, equilibrium cannot and will not be established. CO2 alone dictates whether the equilibrium happens, so that is all the Keq expression contains.
Keq = [ CO2 (g)]