Hess’s Law — Structured Questions (Cambridge International A-Level Chemistry): Exam Questions

Complete the following Hess's law energy cycle relating butane to its standard enthalpy change of formation, ΔH^ϴ_f, and standard enthalpy change of combustion, ΔH^ϴ_c.

On your diagram:

• include the relevant species in the two empty boxes
• label each enthalpy change with its appropriate symbol
• complete the remaining two arrows to show the correct direction of enthalpy change

Use the data in Table 1.1 to calculate the standard enthalpy change of combustion of butane, ΔH^ϴ_c. Show your working.

Table 1.1

ΔH^ϴ_c (C₄H₁₀) = .........................................kJ mol^-1

Use the bond energies given in Table 1.2 to calculate the standard enthalpy change of combustion of butane. Show your working.

Table 1.2

ΔH^ϴ_c (C₄H₁₀) = .........................................kJ mol^-1

Extract:

Use the data in Table 1.1 to calculate the standard enthalpy change of combustion of butane, ΔH^ϴ^c. Show your working.

Table 1.1

Use the bond energies given in Table 1.2 to calculate the standard enthalpy change of combustion of butane. Show your working.

Table 1.2

Suggest, with a reason, which of the two values for the standard enthalpy change of combustion from parts (b) and (c) is more accurate.

Propane gas is commonly used as a fuel for outdoor cooking. It can be produced in a number of ways, including from the addition reaction of propyne gas with hydrogen. Propyne has the formula CHCCH₃ and includes a triple bond.

Construct an equation for the formation of propane from propyne. Include state symbols.

Extract:

Propane gas is commonly used as a fuel for outdoor cooking. It can be produced in a number of ways, including from the addition reaction of propyne gas with hydrogen. Propyne has the formula CHCCH3 and includes a triple bond.

The equation for the formation of propane from propyne is:

CHCCH3 (g) + 2H2 (g) → CH3CH2CH3 (g)

Table 2.1 lists the relevant standard enthalpy change of combustion data for the formation of propane from propyne.

Table 2.1

Calculate the standard enthalpy change of reaction, ΔH^ϴ_r, for the formation of propane from propyne. Show your working.

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

Suggest why a calculated bond energy value is often different to the Data Booklet value.

Enthalpy changes of combustion can be determined using calorimetry or calculated using Hess cycles.

Fig. 3.1 shows the equipment used to determine the standard enthalpy change of combustion, ΔH^ϴ_c, of 2-methylpropan-2-ol, C₄H₉OH, using a calorimeter containing 75.0 g of water.

Fig. 3.1

Table 3.1 shows the results of the experiment.

Table 3.1

Calculate the standard enthalpy change of combustion, ΔH^ϴ_c, of 2-methylpropan-2-ol. Include the sign and units. Show your working.

The standard enthalpy change of combustion, ΔH^ϴ_c, of 2-methylpropan-2-ol can be calculated using the data shown in Table 3.2.

Table 3.2

i) State why the standard enthalpy change of formation of oxygen gas, O₂ (g), is zero.

ii) Complete Fig. 3.2, showing the Hess's law energy cycle for the combustion of 2-methylpropan-2-ol.

Fig. 3.2

iii) Use the data in Table 3.2 to calculate the standard enthalpy change of combustion of 2-methylpropan-2-ol. Show your working.

Extract:

Enthalpy changes of combustion can be determined using calorimetry or calculated using Hess cycles.

Fig. 3.1 shows the equipment used to determine the standard enthalpy change of combustion, ΔH°c, of 2-methylpropan-2-ol, C4H9OH, using a calorimeter containing 75.0 g of water.

Table 3.1 shows the results of the experiment.

Table 3.1

The value for ΔH^ϴ_c obtained in part (a) is much less exothermic than ΔH^ϴ_c calculated in (b)(iii).

Suggest two reasons for this, other than non-standard conditions.

Define the term standard enthalpy of combustion, ΔH^ϴ_c.

Construct an equation for the complete combustion of propan-1-ol, CH₃CH₂CH₂OH (l). Include state symbols.

Extract:

Construct an equation for the complete combustion of propan-1-ol, CH3CH2CH2OH (l). Include state symbols.

The equation for the complete combustion of propan-1-ol is:

2CH3CH2CH2OH (l) + 9O2 (g) → 6CO2 (g) + 8H2O (l)

OR

CH3CH2CH2OH (l) + 4.5O2 (g) → 3CO2 (g) + 4H2O (l)

Construct a Hess's law cycle for the complete combustion of propan-1-ol.

Extract:

Construct an equation for the complete combustion of propan-1-ol, CH3CH2CH2OH (l). Include state symbols.

The equation for the complete combustion of propan-1-ol is:

CH3CH2CH2OH (l) + 4.5O2 (g) → 3CO2 (g) + 4H2O (l)

OR

2CH3CH2CH2OH (l) + 9O2 (g) → 6CO2 (g) + 8H2O (l)

Use the enthalpy of formation data given in Table 4.1 to calculate the standard enthalpy change of combustion of propan-1-ol, ΔH^ϴ_c. Show your working.

Table 4.1

Urea, CO(NH₂)₂, is a naturally occurring substance which can be hydrolysed with water to form ammonia and carbon dioxide.

The standard enthalpy changes of formation of relevant species are given in Table 1.1.

Table 1.1

i) Construct an equation for the hydrolysis of aqueous urea. Include state symbols.

ii) Construct a Hess's law energy cycle for the hydrolysis of aqueous urea.

Extract:

Urea, CO(NH2)2, is a naturally occurring substance which can be hydrolysed with water to form ammonia and carbon dioxide.

The standard enthalpy changes of formation of relevant species are given in Table 1.1.

Table 1.1

Use the data in Table 1.1 to calculate the standard enthalpy change of reaction, ΔH^ϴ_r, for the hydrolysis of aqueous urea. Show your working.

Extract:

Urea, CO(NH2)2, is a naturally occurring substance which can be hydrolysed with water to form ammonia and carbon dioxide.

The standard enthalpy changes of formation of relevant species are given in Table 1.1.

Table 1.1

Use the data in Table 1.1 to calculate the standard enthalpy change of combustion, ΔH^ϴ_c, of solid urea, given the following equation. Show your working.

2CO(NH₂)₂ (s) + 3O₂ (g) → 2CO₂ (g) + 2N₂ (g) + 4H₂O (l)