Question

The change in enthalpy of a system is related to the heat absorbed at a:

• A. Constant volume
• B. Constant temperature
• C. Constant mole fraction
• D. Constant pressure

Hint:

Remember: $\Delta H = Q$ at constant pressure, and $\Delta U = Q$ at constant volume — an important distinction in thermodynamics.

Answer 1

The Correct Option is D

In thermodynamics, the enthalpy ($H$) of a system is defined as:
\[ H = U + PV \]
where $U$ is the internal energy, $P$ is pressure, and $V$ is volume.
The differential form of enthalpy is:
\[ dH = dU + PdV + VdP \]
From the First Law of Thermodynamics, the change in internal energy is:
\[ dU = \delta Q - PdV \]
Substituting this into the equation for $dH$ gives:
\[ dH = \delta Q - PdV + PdV + VdP = \delta Q + VdP \]
Now, if the process occurs at constant pressure ($dP = 0$), then:
\[ dH = \delta Q_P \]
This means that at constant pressure, the change in enthalpy $\Delta H$ is equal to the heat absorbed or released by the system.
Let’s review the options:
- (1) Constant volume — at constant volume, heat is related to change in internal energy ($\Delta U$), not enthalpy.
- (2) Constant temperature — this is relevant for isothermal processes, not necessarily related to $\Delta H$.
- (3) Constant mole fraction — does not influence how enthalpy relates to heat.
Therefore, the correct answer is: constant pressure.