Question

1. exactly 1.5 g of a fuel burns under conditions of constant pressure and then again under conditions of constant volume. in measurement a, the reaction produces 25.9 kj of heat, and in measurement b, the reaction produces 23.3 kj of heat. which measurement (a or b) corresponds to conditions of constant pressure? which one corresponds to conditions of constant volume? explain.

Explanation:

At constant pressure, the heat change ($q_p$) is the enthalpy change ($\Delta H$). At constant volume, the heat change ($q_v$) is the internal - energy change ($\Delta U$). The relationship between $\Delta H$ and $\Delta U$ is $\Delta H=\Delta U + \Delta n_{gas}RT$. When a fuel burns and there is a change in the number of moles of gas ($\Delta n_{gas}> 0$), $\Delta H>\Delta U$ because work is done by the system against the external pressure at constant pressure. Since $25.9\ kJ>23.3\ kJ$, the larger heat value corresponds to the constant - pressure condition.

Answer:

Measurement A corresponds to constant pressure. Measurement B corresponds to constant volume.