A cup of coffee is heated in a microwave oven.It absorbs 40 kJ of energy from the microwave,and the volume slightly increases.Which one of the following statements correctly describes the relationship between the change in enthalpy $\Delta$H and the change in internal energy $\Delta$E of the coffee?

Question

Quizplus · Accepted Answer

The change in enthalpy (ΔH) is equal to the amount of heat absorbed or released in a process at a constant pressure, while the change in internal energy (ΔE) is equal to the amount of heat absorbed or released in a process at a constant volume. In this case, the cup of coffee absorbs 40 kJ of energy from the microwave, which corresponds to the change in enthalpy of the system. However, since the volume of the coffee slightly increases, it means that some of the absorbed energy is used to do work against the external pressure (i.e. atmospheric pressure), which is equivalent to the change in internal energy. Therefore, the change in internal energy is less than 40 kJ, so we can conclude that:

ΔH = 40 kJ 
ΔE < 40 kJ

The only choice that reflects this relationship is A.

A Cup of Coffee Is Heated in a Microwave Oven Δ\DeltaΔ

A Cup of Coffee Is Heated in a Microwave Oven $\Delta$