Question

(a) Why is the change in enthalpy usually easier to measure than the change in internal energy? (b) For a given process at constant pressure, \(\Delta H\) is negative. Is the process endothermic or exothermic?

Short answer

(a) Measuring change in enthalpy (ΔH) is easier than change in internal energy (ΔE) because enthalpy change is mainly the heat transferred at constant pressure, which is common in lab experiments, while internal energy change depends on both heat and work done, making it harder to measure accurately. (b) If the change in enthalpy (ΔH) at constant pressure is negative, the process is exothermic, as heat is being released by the system to the surroundings. If ΔH were positive, the process would be endothermic.

Step-by-step solution

Answer to (a) - Explanation for measuring ease

The change in enthalpy (ΔH) and the change in internal energy (ΔE) are both measures of the heat absorbed or released by a system during a process. However, enthalpy change is easier to measure experimentally, as it is mainly the heat transferred at constant pressure, whereas the change in internal energy also depends on the work done by or on the system. In most chemistry laboratory experiments, the processes occur at constant pressure, which makes it easier to measure the heat transferred directly as ΔH. Measuring ΔE, on the other hand, would need the precise measurement of heat and work done, which might not be feasible due to inaccuracies in work done calculations and the instruments available.

Answer to (b) - Classifying the process

For a given process at constant pressure, if the change in enthalpy (ΔH) is negative, it means that the heat is being released by the system to the surroundings. When heat is released in a process, that process is called exothermic. So, the process is exothermic. Conversely, if the change in enthalpy were positive, indicating heat being absorbed by the system, the process would be endothermic.