To determine the enthalpy change for a student uses the following steps

CHEMISTRY · Single correct · A Level · Chemistry · Energetics · Energetics

Problem

To determine the enthalpy change for

2\text{NO}(g) + \text{O}_2(g) \rightarrow 2\text{NO}_2(g)

a student uses the following steps. Step 1:

\text{N}_2(g) + \text{O}_2(g) \rightarrow 2\text{NO}(g) \qquad \Delta H_1 = +182\,\text{kJ}

Step 2:

2\text{NO}_2(g) \rightarrow \text{N}_2(g) + 2\text{O}_2(g) \qquad \Delta H_2 = +114\,\text{kJ}

The student adds Steps 1 and 2 to obtain the target equation and calculates

\Delta H_{\text{calc}} = +296\,\text{kJ}

. Which of the following statements correctly describes the student’s error and the corrected value of

\Delta H

? (a)

\Delta H = +296\,\text{kJ}

because the sign of Step 2 was not changed after reversal. (b) Step 2 was written in the wrong direction; reversing it changes its sign, giving

\Delta H = -296\,\text{kJ}

. (c)

\Delta H = -68\,\text{kJ}

, the value obtained when Step 1 is reversed instead of Step 2. (d) Doubling Step 1 to match the

2\text{NO}

in the target gives

\Delta H = +478\,\text{kJ}