Which of the following best describes the heat of combustion of ethane (C_2H_6)?

• A. It is the amount of heat released when one gram of ethane is burned in oxygen at constant volume.
• B. It is the amount of heat required to raise the temperature of one mole of ethane by 1 K.
• C. It is the amount of heat released when one mole of ethane is completely burned in oxygen at constant pressure.
• D. It is the amount of heat absorbed when ethane is dissolved in water.

Answer: (C)

Verified step by step guidance

1. Understand that the heat of combustion refers to the energy change when a substance undergoes complete combustion, typically reacting with oxygen to form CO_2 and H_2O.
2. Recognize that heat of combustion is usually expressed per mole of substance burned, not per gram, and it involves complete burning under standard conditions.
3. Note that the process occurs at constant pressure, which is important because heat measured at constant pressure corresponds to enthalpy change (\$\Delta H\$).
4. Distinguish heat of combustion from other concepts such as specific heat capacity (heat required to raise temperature) and heat of solution (heat absorbed or released when a substance dissolves).
5. Conclude that the best description is: the amount of heat released when one mole of ethane is completely burned in oxygen at constant pressure.