Equilibrium Constant for Homogenous Systems - Gas Equilibria and Kp (A-Level Chemistry)
Gas Equilibria and Kp
Reversible Reactions
Key Terms
A reversible reaction is one that can go in either direction, depending on the conditions of the reaction.
Many reversible reactions take place in the gaseous phase. In the example of the Haber process above, all reactants and products are in the gas state.
A closed system means that in a reaction no product or reactant can leave the reaction vessel.
A dynamic equilibrium occurs when a reversible reaction takes place in a closed system so that the rate of the forward reaction is equal to the rate of the reverse reaction. The temperature, concentrations of reactants and products and pressure are all constant at equilibrium.
A homogeneous equilibrium is a reversible reaction where the products and reactants are all in the same physical state.
A heterogenous equilibrium is a reversible reaction where the products and reactants are all in different physical states.
Finding Kp
Kp is like the equilibrium constant, Kc ,but using partial pressures instead of concentrations.
Kp is the equilibrium constant only for a reversible reaction with all reactants and products in the gaseous state.
Let’s find the expression for Kp for the reaction:
Where p = partial pressure.
You can work out the value of Kp by substituting the numbers for partial pressure into the formula you construct using your chemical equation. You then work out your units like you did for Kc.
Finding Kp for heterogeneous equilibria
The expression for Kp for heterogeneous equilibria will exclude anything that are not gases.
For example, the Kp expression for the reversible reaction NH4HS (s) ⇌ NH3 (g) + H2S(g) is:
NH4HS is not included because it is a solid.
Worked example: The following reaction reaches equilibrium,
Work out the equilibrium constant (Kp) for the reaction above and give its units (2).
Answer:
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