Free Enthalpy MCQs with Answers

1. For a reaction carried out at constant pressure, the heat exchanged is equal to the change in

• A. internal energy
• B. enthalpy
• C. entropy
• D. free energy

Explanation: Enthalpy was defined precisely so that its change measures the heat of a process at constant pressure, which is the condition of almost every reaction done in an open vessel. At constant volume, as in a bomb calorimeter, the heat measured is the change in internal energy instead, since no expansion work is done. This is why the two quantities differ slightly for reactions involving gases.

2. The standard enthalpy of formation of an element in its most stable form is

• A. always negative
• B. always positive
• C. zero by definition
• D. equal to its atomic mass

Explanation: Forming an element from itself involves no change, so the value is defined as zero and this provides the reference point from which all other enthalpies of formation are measured. The convention applies only to the most stable allotrope, so graphite is zero while diamond is not. Choosing an arbitrary zero is necessary because absolute enthalpies cannot be measured.

3. The standard enthalpy of combustion is the enthalpy change when

• A. one mole of a substance is burnt completely in excess oxygen under standard conditions
• B. one gram of a substance is burnt
• C. one mole of a substance is formed from its elements
• D. one mole of a substance is dissolved in water

Explanation: The definition specifies one mole, complete combustion and excess oxygen, and combustion values are always negative because burning releases energy. Incomplete combustion would give a different and smaller value, which is why excess oxygen is stipulated. Formation from elements is a separate quantity that may be positive or negative.