Topic: First law: work, heat, internal energy, enthalpy· Chemical Thermodynamics

Under isothermal condition, a gas at 300 K expands from 0.1 L to 0.25 L against a constant external pressure of 2 bar. The work done by the gas is :[Given that 1 L bar = 100 J]

1. \(-30\) J

2. 5 kJ

3. 25 J

4. 30 J

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Answer: 4. 30 J

Key idea: for expansion against a constant external pressure, the magnitude of work is \(p_{ext}\Delta V\), and the question asks for the work done by the gas.

\(\Delta V = 0.25 - 0.1 = 0.15\ \mathrm{L}\)

Work done by the gas \(= p_{ext}\Delta V = 2\times0.15 = 0.30\ \mathrm{L\ bar} = 0.30\times100 = 30\ \mathrm{J}\).

In the IUPAC sign convention the work done on the gas would be \(w = -30\ \mathrm{J}\), which is option (1), but the question asks for work done by the gas, so the answer is +30 J, option (4).

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Under isothermal condition, a gas at 300 K expands from 0.1 L to 0.25 L against a constant external pressure of 2 bar. The work done by the gas is :[Given that 1 L bar = 100 J]

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