A Level Kinetics Full Topic

• +38.8 kJ mol^-1

• +38.8 J mol^-1 K^-1

• + 562 J mol^-1

• + 562 J mol^-1 K^-1

Answer explanation

B uses the incorrect units for activation energy, as does D.

C has divided the gradient by R (as has D)

A is the only possible answer.

What can be deduced from the position of the activation energy on a Maxwell-Boltzmann distribution curve?

• Number of particles in the rate determining step

• Number of successful collisions per second

• Order of reaction

• Proportion of particles with sufficient energy to react

Two ways of writing an equation for the reaction between nitrogen monoxide and oxygen are shown.

2NO + O₂ --> 2NO₂

NO + 1/2 O₂ --> NO₂

Which pair of rate equations could be correct for this reaction?

• A

• B

• C

• D

Answer explanation

Correct answer is A

rate = k[NO]²[O₂]

Answer can't be B because rate equation doesn't depend on the stoichiometric equation.

Answer can't be C or D because the rate can't only depend on the concentration of the products.

Bromine oxidises methanoic acid to carbon dioxide. The equation for the reaction is:

HCOOH (aq) + Br₂ (aq) --> 2H⁺ (aq) + 2Br^- (aq) + CO₂ (g)

Which of the following methods would not be suitable for measuring the progress of this reaction?

• colorimetry

• measuring electrical conductivity

• quenching and titrating with acid

• measuring the volume of gas

Answer explanation

Colorimetry is incorrect because bromine is coloured.

Measuring electrical conductivity is incorrect because the reaction produces ions.

Measuring gas volume is incorrect because the reaction produces carbon dioxide gas.

Answer must be quenching and titrating with acid.

Which methods can be used for continuous monitoring of the progress of the reaction between nitrogen monoxide and oxygen to produce nitrogen dioxide?

• colorimetry

• titration

• volume change

• mass change

Answer explanation

Titration can' be used for continuous monitoring of a reaction, and the mass of the system does not change.

The volume will change as the number of moles of gas changes.

Nitrogen dioxide is a brown gas so the colour of the system will change.

A student carried out an experiment to determine the equilibrium constant for the reaction to form ethyl ethanoate from ethanol and ethanoic acid.

CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O

Different amounts of each substance were added to conical flasks, each containing 2.0 cm³ of 1.0 mol dm⁻³ hydrochloric acid.

The flasks were then stoppered and left for a week to reach equilibrium. Each mixture was then titrated with 1.0 mol dm⁻³ sodium hydroxide.

In which flask(s) was the equilibrium approached from the right-hand side of the equation?

• Flask 1 only

• Flask 4 only

• Flask 1 and 4 only

• Flask 2 and 3 only

Answer explanation

The only correct answer is C (flask 1 and 4 only).

A is incorrect as flask 4 also contains only substances from the RHS of the equilibrium (HCl (aq) includes some water).

B is incorrect as flask 1 also contains only substances from the RHS of the equilibrium.

D is incorrect as flasks 2 and 3 do not contain any ester.

A student carried out an experiment to determine the equilibrium constant for the reaction to form ethyl ethanoate from ethanol and ethanoic acid.

CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O

Different amounts of each substance were added to conical flasks, each containing 2.0 cm³ of 1.0 mol dm⁻³ hydrochloric acid.

The flasks were then stoppered and left for a week to reach equilibrium. Each mixture was then titrated with 1.0 mol dm⁻³ sodium hydroxide.

Which of these statements explains why it is possible to titrate the reaction mixture directly to find the equilibrium concentrations?

• The equilibrium reaction is slow.

• Sodium hydroxide quickly hydrolyses the ester.

• All the reactant concentrations remain constant during the titration.

• A buffer solution forms in the reaction.

Answer explanation

The only correct answer is that the equilibrium reaction is slow.

B is incorrect as rapid hydrolysis would affect the position of the equilibrium.

C is incorrect as the acid would be neutralised.

D is incorrect as although a buffer may form, it would not affect the position of the equilibrium.

The concentration of Br⁻ ions is determined at different times. The concentrations of BrO₃^- ions and H⁺ ions are in large excess and effectively constant.

The graph of concentration of Br⁻ ions against time is shown.

Determine the order of the reaction with respect to bromide ions.

Answer explanation

The reaction is first order because the half-lives are the same each time.

5Br^- (aq) + BrO₃^- (aq) + 6H⁺ (aq) --> 3Br₂ (aq) + 3H₂O (l)

The initial concentrations of BrO₃^- ions and H⁺ ions are changed and the initial rate of reaction is determined. The initial concentration of Br⁻ ions is constant and in large excess.

Determine the order of reaction with respect to the bromate(V) ions.

In an exam you will be required to show all of your working.

Answer explanation

First order with respect to BrO₃^- ions because in runs 1 and 2, as the [BrO₃^-] doubles (and [H⁺] is constant), the rate doubles.

5Br^- (aq) + BrO₃^- (aq) + 6H⁺ (aq) --> 3Br₂ (aq) + 3H₂O (l)

The initial concentrations of BrO₃^- ions and H⁺ ions are changed and the initial rate of reaction is determined. The initial concentration of Br⁻ ions is constant and in large excess.

Determine the order of reaction with respect to the hydrogen ions.

In an exam you will be required to show all of your working.

Answer explanation

In runs 1 and 3, as [BrO₃^-] triples, the rate should triple as well (to 1.08 x 10^-2).

[H⁺] also doubles and the rate increases by a factor of 4.

So reaction is second order with respect to H⁺ ions.