Enthalpy Change, ΔH — Structured Questions (Cambridge International AS Chemistry): Exam Questions

Standard enthalpy changes include enthalpy of formation, enthalpy of combustion and enthalpy of neutralisation.

i) Define the term standard enthalpy change of formation.

ii) Construct an equation that represents the standard enthalpy change of formation of ammonia, NH₃. Include state symbols.

State what is meant by standard conditions in enthalpy measurements.

The equation for the reaction between dilute sulfuric acid and aqueous sodium hydroxide is shown.

H₂SO₄ (aq) + 2NaOH (aq) → Na₂SO₄ (aq) + 2H₂O (l)

Explain why the enthalpy change for this reaction does not represent the standard enthalpy change of neutralisation.

Freon 21 is a hydrochlorofluorocarbon that was used as a propellant and refrigerant but later withdrawn from use because of its effect on ozone depletion. The formula is CHCl₂F.

Freon 21 is produced by reacting hydrogen fluoride with trichloromethane in the following reaction:

CHCl₃ (g) + HF (g) → CHCl₂F (g) + HCl (g)

Use the data in Table 2.1 to calculate the standard enthalpy change of reaction, ΔH°_r, for this reaction. Show your working.

Table 2.1

Extract:

Freon 21 is a hydrochlorofluorocarbon that was used as a propellant and refrigerant but later withdrawn from use because of its effect on ozone depletion. The formula is CHCl₂F.

Freon 21 is produced by reacting hydrogen fluoride with trichloromethane in the following reaction:

CHCl₃ (g) + HF (g) → CHCl₂F (g) + HCl (g)

Use the data in Table 2.1 to calculate the standard enthalpy change of reaction, ΔH°r, for this reaction.

On Fig. 2.1, sketch a reaction pathway diagram for this reaction. Label the diagram to show the enthalpy change of reaction, ΔH°_r, and the activation energy, E_a.

Fig. 2.1

Define the term activation energy.

Define the term bond energy.

Extract:

The definition of bond energy is:

• The energy required to break one mole of a specified bond
• In a gaseous molecule, averaged over a range of similar compounds

Suggest why the value of ΔH_r calculated using bond energies may differ from the value obtained by experiment.

Use the data given in Table 3.1 to calculate the enthalpy change for the hydration of ethene to form ethanol. Show your working.

CH₂=CH₂ (g) + H₂O (g) → CH₃CH₂OH (l)

Table 3.1

Ethane reacts with bromine to form 1,2-dibromoethane and hydrogen bromide.

Construct an equation for this reaction. Hence, use the data in Table 3.2 to calculate the enthalpy change for this reaction. Show your working.

Table 3.2

Propane reacts with chlorine to form chloropropane.

C₃H₈ (g) + Cl₂ (g) → C₃H₇Cl (g) + HCl (g)

i) Use bond energies from Table 1.1 to calculate the enthalpy change for this reaction.

Show your working.

Table 1.1

Include a sign in your answer.

enthalpy change = ................................. kJ mol^–1

ii) State the conditions needed for this reaction to occur.

Extract:

Propane reacts with chlorine to form chloropropane.

C₃H₈ (g) + Cl₂ (g) → C₃H₇Cl (g) + HCl (g)

Using Fig. 1.1, sketch a labelled reaction pathway diagram for the reaction in part (a) including the activation energy.

Fig. 1.1

Extract:

Propane reacts with chlorine to form chloropropane.

C₃H₈ (g) + Cl₂ (g) → C₃H₇Cl (g) + HCl (g)

i) Use bond energies from Table 1.1 to calculate the enthalpy change for this reaction.

Table 1.1

ii) State the conditions needed for this reaction to occur.

Ethane and chlorine will also react together under the same conditions.

State and explain the difference in enthalpy change for the two reactions

Extract:

Propane reacts with chlorine to form chloropropane.

C₃H₈ (g) + Cl₂ (g) → C₃H₇Cl (g) + HCl (g)

Table 1.1

Propane will react with bromine in a similar reaction to part (a).

Suggest, with a reason, whether the sum of the bonds broken would be larger or smaller compared to the reaction between propane and chlorine.

The apparatus shown in Fig. 2.1 can be used to determine the enthalpy of combustion of butan-1-ol, C₄H₉OH (M_r = 74.12 g mol^-1).

Fig. 2.1

i) Construct an equation to represent the standard enthalpy change of combustion of butan-1-ol.

Include state symbols.

ii) Suggest the purpose of the copper spiral and small electric heater in the apparatus.

An experiment was carried out and the following measurements were recorded in Table 2.1

Table 2.1

(Specific heat capacity of water = 4.18 J g^-1 K^-1)

Use the data to calculate the energy change, q, for the reaction.

Show your working.

Give your answer to two significant figures.

q = ........................................... kJ

Extract:

The apparatus shown in Fig. 2.1 can be used to determine the enthalpy of combustion of butan-1-ol, C4H9OH (Mr = 74.12 g mol^-1^).

Table 2.1

(Specific heat capacity of water = 4.18 J g^-1^ K^-1^)

Determine the enthalpy of combustion of butan-1-ol using Table 2.1 and your answer to part (b).

Show your working.

Give your answer to three significant figures.

ΔH = ..................... kJ mol^-1

Two common oxides of nitrogen are nitrogen monoxide, NO, and nitrogen dioxide, NO₂.

i) Complete Table 3.1 to show the oxidation number of nitrogen in each compound.

Table 3.1

ii) Construct equations for the formation of NO₂ by:

• The reaction of N₂ with O₂

• The reaction of NO with O₂

Molecules of NO₂ can be formed by the reaction between N₂ and O₂. The bond between the N and O atoms in NO₂ is a double covalent bond.

The enthalpy change of reaction for this reaction is +497 kJ mol^–1. Calculate the bond energy, in kJ mol^–1, of the N=O bond.

Show your working.

Use relevant data from Table 3.2.

Table 3.2

Bond energy of the N=O bond = ............................. kJ mol^–1

The boiling points of carbon dioxide and nitrogen dioxide are -78.5 ^oC and 21 ^oC respectively.

Suggest a reason for the difference.

Construct an equation that represents the standard enthalpy change of formation of nitrogen monoxide, NO.

Include state symbols.

Propane gas is used widely as a fuel which can be used in camping gas stoves. The enthalpy of combustion for propane is −2219.2 kJ mol^-1.

Calculate the minimum mass of fuel needed to bring 150 cm³ of water from 25.0°C to its boiling point of 100.0°C.

Show your working.

................... g

Extract:

Propane gas is used widely as a fuel which can be used in camping gas stoves. The enthalpy of combustion for propane is −2219.2 kJ mol⁻¹.

Calculate the minimum mass of fuel needed to bring 150 cm³ of water from 25.0°C to its boiling point of 100.0°C.

Suggest why more propane may be required than calculated in part (a).

Extract:

Propane gas is used widely as a fuel which can be used in camping gas stoves. The enthalpy of combustion for propane is −2219.2 kJ mol^-1^.

Using Fig. 5.1, Sketch a labelled reaction pathway diagram for the reaction in part (a) including the activation energy.

Fig. 5.1

The enthalpy change of solution for ammonium chloride can be measured using calorimetry. 12.04 g of ammonium chloride is dissolved in 125.0 cm³ of water at 19.5 °C.

The standard enthalpy change of solution of ammonium chloride is +15.1 kJ mol^-1. Calculate the energy change, q, in J, when 12.04 g of ammonium chloride is dissolved in 125.0 cm³ of water.

Show your working.

q = ................................. J

Extract:

The enthalpy change of solution for ammonium chloride can be measured using calorimetry. 12.04 g of ammonium chloride is dissolved in 125.0 cm^3^ of water at 19.5 °C.

The standard enthalpy change of solution of ammonium chloride is +15.1 kJ mol^-1^.

Use your answer to part (a) to calculate the final temperature, in °C, of the solution.

The specific heat capacity of the solution is 4.18 J g^-1 K^-1.

Show your working.

Final temperature = ................................. °C

Use the data in Table 2.1 to calculate the enthalpy change of reaction for the hydrogenation of propene.

Show your working.

Table 2.1

Enthalpy of hydrogenation, ΔH_r = ................................. kJ mol^–1

Extract:

Table 2.1

Use the data in Table 2.1 to suggest, with a reason, whether the polymerisation of propene is exothermic or endothermic

Carbon, hydrogen and ethene each undergo complete combustion in oxygen.

Use the data to calculate the standard enthalpy change of formation, ΔH^θ_f, in kJ mol^–1, of ethene at 298 K.

Show your working.

ΔH^θ_f = ................................. kJ mol^–1

Compound A is a liquid containing carbon, hydrogen and oxygen. Quantitative analysis shows that A has the empirical formula CH₂O.

A sample of A with a mass of 0.240 g is vaporised. At 80.0 °C and 101 kPa, the vapour occupies a volume of 116 cm³.

Calculate the relative molecular mass, M_r, of A.

Show your working.

M_r = .............

Methyl methanoate, HCOOCH₃, is a liquid ester.

i) Define the term standard enthalpy change of formation.

ii) The standard enthalpy changes of combustion, ΔH^θ_c, in Table 2.1 are for complete combustion to CO₂ (g) and H₂O (l) under standard conditions.

Table 2.1

The enthalpy of formation of methyl methanoate refers to the reaction:

2C (s) + 2H₂ (g) + O₂ (g) → HCOOCH₃ (l)

Use the data in Table 2.1 to calculate the standard enthalpy change of formation, ΔH^θ_f, of liquid methyl methanoate.

Show your working.

ΔH^θ_f = ........................ kJ mol^-1

The equation for the formation of gaseous methyl methanoate from gaseous atoms is shown.

2C (g) + 4H (g) + 2O (g) → HCOOCH₃ (g)

Use bond energy data from the Data Booklet to calculate the enthalpy change for this reaction.

Show your working.

A student investigates the rate of reaction of methyl methanoate with dilute hydrochloric acid.

i) A Boltzmann distribution curve for the reactant molecules at temperature T *is shown.

On the same axes, sketch the Boltzmann distribution curve for the reactant molecules at a higher temperature, T_high.

Label the curve T_high and mark the activation energy, E_a, on the x-axis.

ii) Use your Boltzmann distribution curves to explain why the rate of reaction is greater at T_high.