Question

In a process carried out at constant volume, the heat absorbed (Q\(_{\text{v}}\)) is equal to the change in:

• A. Gibbs free energy
• B. Enthalpy
• C. Internal energy
• D. Work done

Hint:

Remember these thermodynamic definitions:
- Constant Volume: \( Q_v = \Delta U \) (Internal Energy).
- Constant Pressure: \( Q_p = \Delta H \) (Enthalpy).
This is a core concept tested frequently in chemical and materials thermodynamics.

Answer 1

The Correct Option is C

Picture a rigid, sealed container so its volume cannot change no matter how much heat flows in. Any work a system does on its surroundings during expansion is \( P\,dV \) work, and since the volume is fixed, \( dV \) is zero, meaning no boundary work can leave the system at all. With nowhere else for the added heat to go, every joule that enters simply raises the internal energy of the molecules inside. This is exactly why a bomb calorimeter, which is a rigid sealed vessel, directly measures the change in internal energy of a reaction rather than enthalpy. Enthalpy change equals heat only when pressure, not volume, is held constant. So the correct choice is option (C).