Kinetics II (Edexcel A Level Chemistry): Exam Questions

Gentian violet is a purple crystalline solid used as an antifungal treatment. 

It can be synthesised from dimethylphenylamine, C₆H₅N(CH₃)₂.

The dimethylphenylamine used in the synthesis can be made by the stepwise reaction of phenylamine with chloromethane.  

Step 1  

Step 2   

The reaction mechanism for Step 1 between phenylamine and chloromethane is the same as that in the reaction between ammonia and chloromethane.  

i) What is the reaction type and mechanism in Step 1? 

(1)

ii) Draw the mechanism for the reaction in Step 1. Include curly arrows, and relevant lone pairs and dipoles. 

(4)

iii) Describe, in outline, how a sample of a solid, such as gentian violet, is purified by recrystallisation. Specific details of the solvent used are not required.  

(4)

The rate constant for the reaction between a solution of gentian violet and aqueous sodium hydroxide was determined at different temperatures.

i) Complete the data in the table. 

(1)

ii) Plot a graph and use it to determine the activation energy for the reaction in kJ mol^-1.

You should include the value and units of the gradient of the line.

The Arrhenius equation can be shown as  

(6)

Gradient  

..................................................................................................

Activation energy

..................................................................................................

This question is about the effect of temperature on the rate of decomposition of nitrogen(V) oxide.

2N₂O₅ (g) → 2N₂O₄ (g) + O₂ (g)

The diagram shows the Maxwell-Boltzmann distribution of molecular energies for nitrogen(V) oxide at a temperature T₁. E_a is the activation energy of this reaction.

i) Give the label for the vertical axis.

(1)

ii) Draw a second curve on the same set of axes for the same gas at a lower temperature, T₂.

(2)

iii) Explain, in terms of collisions and energy, why lowering the temperature decreases the rate of reaction.

(2)

iv) A catalyst is added to the gas. Label the diagram above with the symbol E_cat to show a possible activation energy for the reaction in the presence of a catalyst.

(1)

The rate constant for the decomposition of nitrogen(V) oxide was determined at two temperatures.

Calculate the activation energy for this reaction.

Include units and give your answer to an appropriate number of significant figures.

You should not attempt to use any graphical method to answer this question.

The Arrhenius equation relating two rate constants, k₁ and k₂ , at two different temperatures, T₁ and T₂ , can be expressed as

This question is about the reaction kinetics of an ‘iodine clock’ reaction.

One example of an ‘iodine clock’ reaction that involves the iodate(V) ions and iodide ions in acidic solution is

State why the order of reaction with respect to iodide ions cannot be five, even though 5 mol of iodide ions are shown in the equation.

A series of experiments was carried out by a student to determine the order of reaction with respect to iodate(V) ions. The concentrations of the iodide ions and the acid were in large excess and the volume of the iodate(V) solution was varied.

The total volume of the reaction mixture was kept constant by the addition of suitable volumes of deionised water. The following results were obtained:

i) In experiment 6, the student forgot to add deionised water to keep the total volume the same for each experiment. State why the total volume should be kept the same.

(1)

ii) Complete the table and use the results from experiments 1, 2, 3, 4 and 5 to plot a graph of 1000/t against volume of iodate(V) ions.

(4)

iii) Deduce the order of reaction with respect to the iodate(V) ions. Justify your answer.

(2)

A different version of the ‘iodine clock’ reaction involves mixing hydrogen peroxide with aqueous solutions of potassium iodide, sodium thiosulfate and starch. The main reaction is

H₂O₂ (aq) + 2I^-( aq) + 2H⁺ (aq) → I₂ (aq) + 2H₂O (l)

The reaction is first order with respect to hydrogen peroxide and iodide ions but zero order with respect to hydrogen ions.

i) In one experiment, the following data were obtained:

Initial rate = 1.24 × 10^-3 mol dm^-3 s^-1

Write the rate equation and hence deduce the value of the rate constant, k, from these data. Include units and give your answer to an appropriate number of significant figures.

(2)

ii) Explain the purpose of the starch present in the reaction mixture when starch is neither in the rate equation, nor in the reaction equation.

(2)

Another ‘iodine clock’ reaction produced data that enabled the following graph of ln k against 1/T to be drawn.

i) The Arrhenius equation can be expressed as

In 

From the gradient of the graph, determine the activation energy, E_a , for this reaction.

Include a sign and units in your answer.

(3)

ii) Give a reason for the point at ln k = −7 not being included in the line drawn on the graph.

(1)

A series of experiments was carried out to determine the kinetics of the reaction between a chloroalkane, RCl, and potassium hydroxide in aqueous solution. A large excess of the chloroalkane was used.

The data obtained are shown.

Plot a graph of the concentration of the hydroxide ions against time.

State the order with respect to hydroxide ions. Justify your answer by reference to your graph in (a).

Deduce the type of mechanism occurring. Justify your answer.

Give the classification of the chloroalkane in this reaction.

A bromoalkane, RBr, reacts with aqueous hydroxide ions in a nucleophilic substitution reaction.

RBr + OH⁻ → R━OH + Br⁻

This reaction is first order with respect to the bromoalkane and the rate equation is

rate = k[RBr]¹ [OH⁻]x

where x is the order of the reaction with respect to hydroxide ions. In an experiment, a sample of the bromoalkane was added to a large excess of aqueous sodium hydroxide and the concentration of the bromoalkane was determined at regular time intervals.

Results

This experiment is carried out using the bromoalkane dissolved in ethanol and the hydroxide ions dissolved in water.

Give a reason why a solution of hydroxide ions dissolved in pure ethanol should not be used.

Plot a graph of [RBr] against time.

Explain how the graph shows that the reaction is first order with respect to RBr. Include the values of two consecutive half-lives.

You must show your working for the half-lives on the graph.

The experiment was repeated using equal concentrations of RBr and varying the concentration of hydroxide ions.

A graph was plotted of the results.

i) Deduce the value of x in the rate equation.

rate = k[RBr]¹[OH−]^x

⁽¹⁾

ii) Give the mechanism for the reaction that is consistent with the orders of reaction with respect to R━Br and hydroxide ions. Include curly arrows and relevant lone pairs.

(3)

2-bromobutane can react with aqueous hydroxide ions by an S_N1 mechanism.

Explain why the butan-2-ol produced from a single optical isomer of 2-bromobutane, using this mechanism, is not optically active.