CAIE A-Level Chemistry 5.2 Hess’s Law

Practise constructing energy cycles and combining formation, combustion or bond-energy data to obtain indirect ΔH values.

Exam points

state that enthalpy change is independent of pathway because enthalpy is a state function
align equations or cycle arrows with the target reaction and reverse signs when reversing a route
multiply enthalpy values by stoichiometric coefficients before summing the complete path

Covered syllabus points

5.2.1—Hess’s law to construct simple energy cycles 5.2.2—Calculations: cycles and relevant energy terms

[Maximum number: 4]

For many compounds the enthalpy change of formation cannot be calculated directly. An indirect method based on enthalpy changes of combustion can be used.

The enthalpy change of combustion can be found by a calorimetry experiment in which the heat energy given off during combustion is used to heat a known mass of water and the temperature change recorded.

(a)

The table gives some enthalpy change of combustion values.

Table for Question (a) — CAIE A-Level Chemistry AS

[ 4 ]

(i)

Construct a labelled energy cycle to show how these values could be used to calculate the enthalpy change of formation of $\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{OH}(\mathrm{I}), \Delta H_{\mathrm{f}}$ .

3 \mathrm{C}(\mathrm{~s})+4 \mathrm{H}_{2}(\mathrm{~g})+\frac{1}{2} \mathrm{O}_{2}(\mathrm{~g}) \xrightarrow{\Delta H_{\mathrm{f}}} \mathrm{C}_{3} \mathrm{H}_{7} \mathrm{OH}(\mathrm{l})

[ 3 ]

(ii)

Calculate the enthalpy change of formation, $\Delta H_{\mathrm{f}}$ , of $\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{OH}(\mathrm{I})$ .

\Delta H_{\mathrm{f}}=\ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots . . \mathrm{kJ} \mathrm{~mol}^{-1}

[ 1 ]

$\begin{aligned} \Delta H_{f}+(-2021.0)=3(-393.5)+4(-285.8) \Delta H_{f}=-302.7\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right) \end{aligned}$

12 1

Total:

[Maximum number: 2]

The Group 17 elements are oxidising agents.

$\mathrm{Cl}_{2}$ and $\mathrm{Br}_{2}$ can each react with $\mathrm{NH}_{3}$ to give $\mathrm{N}_{2}$ and a hydrogen halide, HX .

3 \mathrm{X}_{2}+2 \mathrm{NH}_{3} \rightarrow \mathrm{~N}_{2}+6 \mathrm{HX}

X = Cl or Br

The relative bond strengths of X-X and H-X determine the difference in enthalpy change of the two reactions.

Table 1.1 gives data relevant to the reaction of $\mathrm{Cl}_{2}(\mathrm{~g})$ with $\mathrm{NH}_{3}(\mathrm{~g})$ .

Use the data in Table 1.1 to calculate the enthalpy change of the reaction of $\mathrm{Cl}_{2}(\mathrm{~g})$ with $\mathrm{NH}_{3}(\mathrm{~g})$ .

Table 1.1

M1 $(6 \times -92) - (2 \times -46)=$

M2 $-460\,\mathrm{kJ\,mol}^{-1}$

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Covered syllabus points

5.2 Hess’s law question 1

Question (a)

Question (i)

Question (ii)

5.2 Hess’s law question 2