In a three-step thermochemical pathway the following data are known: Direct step:…

CHEMISTRY · Subjective · A Level · Chemistry · Energetics · Energetics

Problem

In a three-step thermochemical pathway the following data are known: Direct step:

\text{P} \rightarrow \text{R}\; \Delta H = -75\;\text{kJ}

Indirect first step:

\text{P} \rightarrow \text{Q}\; \Delta H = +40\;\text{kJ}

Indirect second step:

\text{Q} \rightarrow \text{R}\; \Delta H = \text{?}

Without calculating a numerical value, state whether the step

\text{Q} \rightarrow \text{R}

is endothermic or exothermic. Briefly justify your answer.