Question

The First Law of Thermodynamics states that heat is a form of energy, and thermodynamic processes are therefore subject to the principle of conservation of energy. This means that heat energy cannot be created or destroyed. It can, however, be transferred from one location to another and converted to and from other forms of energy.

The equation for the first law of thermodynamics is given as;

$\Delta U=q+W$

Where,

Two moles of an ideal gas expanded isothermally and reversibly from $1\text{L}\text{to}10\text{L}$ at $300\text{K}$. What is the enthalpy change?

• A. $4.98\text{kJ}$
• B. $11.47\text{kJ}$
• C. $11.47\text{kJ}$
• D. $0\text{kJ}$

Answer 1

The correct option is D $0\text{kJ}$

We know,
$H=U+PV$
$\Rightarrow \Delta H=\Delta U+\Delta \left(PV\right)$
For an ideal gas,
$PV=nRT$
$\Rightarrow \Delta H=\Delta U+\Delta \left(nRT\right)$
$\Rightarrow \Delta H=\Delta U+nR\Delta T$
Since the process is isothermal, $\Rightarrow \Delta T=0$
$\text{For an ideal gas if}\Delta T=0\Rightarrow \Delta U=0$
$\Rightarrow \Delta H=0$
Hence, enthalpy change for an ideal gas in an isothermal reversible process is zero.