Rates and equilibrium for Edexcel A-level Chemistry

This page covers the following topics:

1. Calculating Kc
2. Calculating Kp
3. Rate equations
4. The rate constant

At equilibrium, the concentrations of the products and reactants will be constant but not equal. Kc is the equilibrium constant, expressing these equilibrium concentrations combined.

aA + bB ⇌ cC + dD

Equilibrium constants can be explained in terms of partial pressures of gases, Kp. It can be expressed from the equation for a reversible reaction.

aA + bB ⇌ cC + dD

Rate equations describe the rate of reaction in terms of concentration and order of the reaction with respect to each reactant. The rate constant k varies depending on the reaction.

The rate constant for a reaction various with temperature and activation energy. This is shown by the Arrhenius equation.

1

Calculate Kc when 2.43 x 10⁻³ mol l⁻¹ H₂ and 2.43 x 10⁻³ mol l⁻¹ Br₂ react to form 3.59 mol l⁻¹ HBr.

Kc = 2.18 x 10⁶

2

Give the expression for Kc.

image

3

Calculate Kp given a an equilibrium system where partial pressures of reactant A, product B and product C are 37.5 atm, 10.3 atm and 5.60 atm respectively.

1.54 atm

4

Given the rate equation rate = k[B]², what does this suggest about reactant A?

The reaction is 0 order with respect to A. The concentration of A doesn't affect the reaction rate.

5

Calculate the rate constant of a combustion reaction at 330 K with activation energy 45 kJ mol⁻¹. A = 6.2 x 10³ mol⁻¹ dm³ s⁻¹ and R = 8.31 J K⁻¹ mol⁻¹.

4.6 x 10⁻⁷ mol⁻¹ dm³ s⁻¹

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