Energetics I (Edexcel A Level Chemistry): Exam Questions

This question is about enthalpy changes.

i) State what is meant by the term ‘standard enthalpy change of combustion’.

(2)

ii) Write the equation, including state symbols, for the reaction that occurs when the standard enthalpy change of combustion of octane, C₈H₁₈ (l), is measured.

(2)

iii) The standard enthalpy change of combustion of octane is –5 470 kJ mol^–1.

Complete the reaction profile diagram for the combustion of octane.

Include labels showing the standard enthalpy change of combustion, Δ_c and the activation energy, E_a.

(2)

Enthalpy changes of reactions which cannot be measured directly can be calculated using standard enthalpy changes of combustion.

The table shows some of these values.

Complete the Hess cycle and use it to calculate the standard enthalpy change for the following reaction.

The equations for the combination of gaseous atoms of carbon and hydrogen to form methane, CH₄, and propane, C₃H₈, are

C(g) + 4H(g) → CH₄(g) ΔH = −1652 kJ mol⁻¹
3C(g) + 8H(g) → C₃H₈(g) ΔH = −3998 kJ mol⁻¹

Calculate:

i) the mean bond enthalpy of a C—H bond.

(1)

ii) the mean bond enthalpy of a C—C bond.

(2)

The standard molar enthalpy change of neutralisation is the enthalpy change when an acid and an alkali react under standard conditions to form one mole of water.

An experiment was carried out to determine the enthalpy change of neutralisation for the reaction between propanoic acid and sodium hydroxide. The equation for this reaction is

CH₃CH₂COOH (aq) + NaOH (aq) → CH₃CH₂COO^-Na⁺ (aq) + H₂O (l)

50.0 cm³ of sodium hydroxide solution, of concentration 1.00 mol dm^-3, was placed in a polystyrene cup. The initial temperature was measured.

i) Which piece of equipment has the smallest measurement uncertainty for the measurement of 50.0 cm³ of sodium hydroxide solution?

(1)

ii) 50.0 cm³ of propanoic acid solution, of concentration 1.00 mol dm^-3, was added and thoroughly mixed with the sodium hydroxide solution in the polystyrene cup.

The maximum temperature rise was 6.5 °C.

Calculate the enthalpy change of neutralisation for propanoic acid, in kJ mol ^-1, giving your answer to the nearest whole number. 

[Assume density of the mixture = 1.00 g cm^-3, specific heat capacity of the mixture = 4.18 J g ^–1 °C ^–1]

(3)

Another experiment was carried out with a solution of ethanoic acid and sodium hydroxide solution of the same concentration.

i) Which graph shows the correct way that the maximum temperature rise should be determined?

(1)

ii) Explain why the data book value for the standard enthalpy change of neutralisation of ethanoic acid with sodium hydroxide is –55.2 kJ mol ^–1 but the value for hydrochloric acid is –57.1 kJ mol ^–1.

(2)

Nitric acid reacts with sodium hydroxide solution in a neutralisation reaction.

HNO₃ (aq) + NaOH (aq) → NaNO₃ (aq) + H₂O (l)

In an experiment to determine the enthalpy change of neutralisation, the following results were obtained.

Volume of 1.00 mol dm⁻³ HNO₃ = 25.0 cm³

Volume of 1.05 mol dm⁻³ NaOH = 25.0 cm³

Temperature rise = 6.8°C

Give a reason why excess sodium hydroxide was used.

Calculate the enthalpy change of neutralisation for the reaction between nitric acid and sodium hydroxide solution, using the results of the experiment.

Give your answer to an appropriate number of significant figures.

When solid calcium sulfate dihydrate, CaSO₄·2H₂O, is heated in a crucible, it forms solid calcium sulfate hemihydrate, CaSO₄·1⁄2H₂O.

Write an equation, including state symbols, for this reaction.

Which two terms could be used to describe this reaction?

When water is added to calcium sulfate hemihydrate, there is a rise in temperature.

A student decided to investigate this reaction using the following procedure:

Results

Other data

i) Calculate the minimum volume of water needed to convert 10.00g of CaSO₄·1⁄2H₂O into CaSO₄·2H₂O.

(2)

ii) Calculate the enthalpy change, in kJmol^–1, for this reaction.

Include a sign in your answer and give your answer to an appropriate number of significant figures.

Assume that the liquid has a mass of 10.00 g and a specific heat capacity of 4.18 J g^–1 °C^–1.

(4)

iii) Deduce which measurement has the greatest uncertainty in this experiment. Justify your answer by calculating the percentage uncertainty of this piece of apparatus.

(2)

Methanol, CH₃OH, is a liquid fuel. An experiment was carried out to determine the enthalpy change of combustion of liquid methanol.

The energy obtained from burning 2.08 g of methanol was used to heat 75.0 g of water.

The temperature of the water rose from 25.0 °C to 91.0 °C.

[Specific heat capacity of water = 4.18 J g⁻¹ °C⁻¹]

Use the data to calculate a value for the enthalpy change of combustion of one mole of methanol.

Give your answer to an appropriate number of significant figures and include a sign and units.

Methanol can be synthesised from methane and steam by a process that occurs in two steps.

Step 1          CH₄ (g) + H₂O (g) ⇌ 3H₂ (g) + CO (g)             ΔH = +206 kJ mol⁻¹

Step 2           CO (g) + 2H₂ (g)  ⇌ CH₃OH (g)                        ΔH = −91 kJ mol⁻¹

i) Explain the effects of increasing the pressure on the yield of the products and on the rate of the reaction in Step 1.

(4)

ii) Step 2 is carried out at a compromise temperature of 500 K.

Explain why 500 K is considered to be a compromise for Step 2 by considering what would happen at higher and lower temperatures.

(3)

Calculate a value for the standard enthalpy change of combustion of gaseous methanol using the enthalpy change for Step 2 and the standard enthalpy change of combustion of gaseous carbon monoxide and of hydrogen.

This question is about 2-methylpropan-2-ol.

Draw the fully displayed formula of 2-methylpropan-2-ol.

The mass spectrum of 2-methylpropan-2-ol is shown.

i) The relative molecular mass of 2-methylpropan-2-ol is 74. Give a possible reason why there is no molecular ion peak in the mass spectrum of 2-methylpropan-2-ol.

(1)

ii) Write the formula for a species that could be responsible for the peak at m / z = 59.

(1)

The equation for the complete combustion of 2-methylpropan-2-ol is

 C₄H₁₀O (l) + 6O₂ (g) → 4CO₂ (g) + 5H₂O (l)

i) Using the bond enthalpies shown in the table, calculate a value for the enthalpy change, in kJ mol⁻¹, for the complete combustion of 2-methylpropan-2-ol.

(4)

ii) 2-methylpropan-2-ol burns in air with a smoky flame. Explain how burning with a smoky flame affects the value of the experimentally determined enthalpy change of combustion.

(2)

iii) A Data Book value for the enthalpy change of combustion of 2-methylpropan-2-ol is −2643.8 kJ mol⁻¹. Give the main reason for the difference between this value and your answer to part 8(c)(i).

(1)

Which observation would be expected when 2-methylpropan-2-ol is heated with potassium dichromate(VI) and dilute sulfuric acid?

An equation for the formation of ammonia using the Haber process is shown.

N₂ (g) + 3H₂ (g)  ⇌  2NH₃ (g)

i) Calculate the enthalpy change for the forward reaction shown in the equation, selecting from the bond enthalpies in the table. Include a sign in your answer.

(3)

ii) A data book gives the standard enthalpy change of formation of ammonia as –46.1 kJ mol^–1.

Give two reasons for the difference between this value and the value that you calculated in (a)(i).

(2)

iii) What is the percentage atom economy, by mass, for ammonia in the forward reaction?

N₂ (g) + 3H₂ (g)  ⇌  2NH₃ (g)

(1)

iv) What is the equilibrium expression for K_c?

(1)

In the chemical industry, many processes involve reversible reactions. The product is often removed before equilibrium is attained.

Give three reasons why the product may be removed before its maximum concentration is achieved.

Ammonia is stable in air but can be oxidised on the surface of a copper catalyst. An equation for this reaction is

4NH₃ (g) + 5O₂ (g) → 6H₂O (g) + 4NO (g)                   Δ_rH = –905.2 kJmol^–1

The catalyst is usually warmed to approximately 300 °C to start the reaction, but after a short reaction time the copper catalyst often melts.

i) Give a reason why the catalyst is warmed and a reason why the catalyst may melt.

(2)

ii) Complete the reaction profile for this catalysed oxidation of ammonia, showing the enthalpy change, Δ_rH.

(2)

iii) Describe the processes that occur on the surface of a heterogeneous catalyst during the oxidation of ammonia in air.

(3)