Chemical Thermodynamics — Chemistry STD 12 Science — Question
Maharashtra BoardEnglish MediumSTD 12 ScienceChemistryChemical Thermodynamics3 Marks
Question
Explain sign convention of work during expansion and compression. ### Explain $+ W$ and $- W$.
✓
Answer
(A) Expansion of a gas: (1) When a gas expands against a constant pressure, $P _{ ex }$ changing the volume from initial volume $V _1$ to final volume $V _2$, Change in volume, $\Delta V = V _2- V _1$ The mechanical work $= W =- P _{ ex } \times \Delta V$ =$-P_{e x}\left(V_2-V_1\right)$ (2) During expansion $V_2>V_1$. The work is said to be performed by the system on the surroundings. This results in the decrease in the (work) energy of the system. Hence the work is negative, i.e. $W$ is -ve. (B) Compression of a gas : During compression, $V _2< V _1$. The work is said to be performed on the system by the surroundings. This results in the increase in the (work) energy of the system. Hence the work is positive, i.e. $W$ is + ve.
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