Edexcel IAL Chemistry 6.6 Hess's Law and enthalpy cycles

Balance the target equation, place common cycle species correctly, multiply enthalpies by coefficients and reverse signs whenever an arrow or reaction is reversed.

Exam points

Complete Hess cycles with correct species, state symbols, arrows and coefficients.
Calculate reaction enthalpy from formation, combustion or experimental values.
Reverse signs and apply stoichiometric multiples when combining cycle routes.

[Maximum number: 3]

Ammonium nitrate, $\mathrm{NH}_{4} \mathrm{NO}_{3}$ , is used in the manufacture of fertilisers and explosives. It is produced on a large scale using only methane, water and air.
The process has four stages.

(a)

In Stage 4, a solution of $\mathrm{NH}_{4} \mathrm{NO}_{3}$ is produced by reacting $\mathrm{NH}_{3}$ (from Stage 2) with $\mathrm{HNO}_{3}$ (from Stage 3).

\mathrm{NH}_{3}(\mathrm{~g})+\mathrm{HNO}_{3}(\mathrm{aq}) \rightarrow \mathrm{NH}_{4} \mathrm{NO}_{3}(\mathrm{aq})

Data

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

[ 3 ]

(i)

Complete the enthalpy cycle.

Enthalpy cycle

Figure for Question (i) — Edexcel A-Level Chemistry AS

[ 2 ]

(ii)

Calculate the enthalpy change, $\Delta_{\mathrm{r}} H$ , in $\mathrm{kJ} \mathrm{mol}^{-1}$ , for the reaction of $\mathrm{NH}_{3}(\mathrm{~g})$ with $\mathrm{HNO}_{3}(\mathrm{aq})$ .

[ 1 ]

An answer that makes reference to the following point:

- calculation of $\Delta_{\mathrm{r}} H$

Example of calculation:
$\begin{aligned} \Delta_{\mathrm{r}} H & =-(-32.6)-(-220.2)+(-365.6)+25.6 & =-87.2 /-87\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right) \end{aligned}$

Allow omission of units

Allow kJ

TE on cycle in (g)(i)

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6.6—Hess’s Law question 1

Question (a)

Question (i)

Question (ii)