3.1.9.1 Rate equations (A-level only)

Solutions of two compounds, W and X, react together in the presence of a soluble catalyst, Y, as shown in the equation

2W + X → Z

When the concentrations of W, X and Y are all doubled, the rate of reaction increases by a factor of four.

Which is a possible rate equation for this reaction?

• rate = k [W]² [X]

• rate = k [W]² [Y]

• rate = k [X] [Y]

• rate = k [X] [Z]

A rate investigation was carried out on a reaction involving three reactants, X, Y and Z.

The concentrations of the reactants were varied and the relative rate for each mixture determined.

The reaction is zero order with respect to Y.

What is the overall order of reaction?

• 0

• 1

• 2

• 3

The rate equation for the hydrogenation of ethene

C₂H₄(g) + H₂(g) --> C₂H₆(g)

is Rate = k[C₂H₄][H₂]

At a fixed temperature, the reaction mixture is compressed to triple the original pressure.

What is the factor by which the rate of reaction changes?

• 6

• 9

• 12

• 27

This question is about the reaction between propanone and an excess of ethane-1,2-diol, the equation for which is given in the image.

In a typical procedure, a mixture of 1.00 g of propanone, 5.00 g of ethane-1,2-diol and 0.100 g of benzenesulphonic acid, C₆H₅SO₃H, is heated under reflux in an inert solvent. Benzenesulphonic acid is a strong acid.

When the concentration of benzenesulphonic acid is doubled, the rate of the reaction doubles. It can be deduced that

• the reaction is first order overall.

• the reaction is third order overall.

• the reaction is acid-catalysed.

• units for the rate constant, k, are mol⁻² dm⁶ s⁻¹.

The equation and rate law for the reaction of substance P with substance Q are given below.

2P + Q → R + S

rate = k[P]²[H⁺]

Under which one of the following conditions, all at the same temperature, would the rate of reaction be slowest?

• [P] / mol dm⁻³: 0.1

  pH: 0

• [P] / mol dm⁻³: 1

  pH: 2

• [P] / mol dm⁻³: 3

  pH: 3

• [P] / mol dm⁻³: 10

  pH: 4

Rate = k [A]2 [B]

Correct units for the rate constant in the rate equation above are

• mol dm⁻³ s⁻¹

• mol⁻¹ dm⁻³ s⁻¹

• mol² dm⁻⁶ s⁻¹

• mol⁻² dm⁶ s⁻¹