Use Hess's Law to calculate the enthalpy change for the reaction $$
\mathrm{WO}_{3}(s)+3 \mathrm{H}_{2}(g) \rightarrow W(s)+3 \mathrm{H}_{2} \mathrm{O}(g)
$$
from the following data:
$$
\begin{array}{l}
2 \mathrm{~W}(\mathrm{~s})+3 \mathrm{O}_{2}(g) \rightarrow 2 \mathrm{WO}_{3}(\mathrm{~s}), \Delta \mathrm{H}=-1685.4 \mathrm{~kJ} \
2 \mathrm{H}_{2}(g)+\mathrm{O}_{2}(g) \rightarrow 2 \mathrm{H}_{2} \mathrm{O}(g), \Delta H=-477.84 \mathrm{~kJ}
\end{array}
$$

Question

Quizplus · Accepted Answer

The correct answer is A because the enthalpy change for the reaction can be calculated by adding the enthalpy changes for the following reactions:$$\begin{array}{l}2 \mathrm{~W}(\mathrm{~s})+3 \mathrm{O}_{2}(g) \rightarrow 2 \mathrm{WO}_{3}(\mathrm{~s}), \Delta \mathrm{H}=-1685.4 \mathrm{~kJ} \2 \mathrm{H}_{2}(g)+\mathrm{O}_{2}(g) \rightarrow 2 \mathrm{H}_{2} \mathrm{O}(g), \Delta H=-477.84 \mathrm{~kJ}\end{array}$$The overall reaction is:$$\mathrm{WO}_{3}(s)+3 \mathrm{H}_{2}(g) \rightarrow W(s)+3 \mathrm{H}_{2} \mathrm{O}(g)$$The enthalpy change for the overall reaction is:$$\Delta H = \Delta H_1 + \Delta H_2 = -1685.4 \mathrm{~kJ} + (-477.84 \mathrm{~kJ}) = -2163.24 \mathrm{~kJ}$$The enthalpy change for the reaction is -2163.24 kJ, which is closest to choice A, 125.9 kJ.

Use Hess's Law to Calculate the Enthalpy Change for the Reaction