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Question Bank [2022] — Chemistry STD 12 Science — Question

Maharashtra BoardEnglish MediumSTD 12 ScienceChemistryQuestion Bank [2022]3 Marks
Question
Derive the expression for work done in chemical reaction. Write the relationship between $\triangle H$ and $\triangle U$ for an isochoric process.
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Answer

Expression for work done in a chemical reaction:
The work done by a system at constant temperature and pressure is given by
$W=-P_{\text {ext }} \Delta V . . . . .(1)$
Assuming $P _{\text {ext }}= P$,
$W=-P \Delta V$
$=-P\left(V_2-V_1\right) W$
$=-P V_2+P V_1 \ldots \ldots$
If the gases were ideal, at constant temperature and pressure.,
$P V_1=n_1 R T \text { and } P V_2=n_2 R T \ldots(3)$
Substitution of equation (3) into equation (2) yields
$W=-n_2 R T+n_1 R T$
$=-\left(n_2-n_1\right) R T$
$=-\Delta n_g R T \ldots(4)$
Equation (4) gives the work done by the system in chemical reactions.
Relationship between $\Delta H$ and $\Delta U$ for an isochoric process:
For an isochoric process, $\Delta V =0$.
$\Delta H=\Delta U+P \Delta V=\Delta U+0=\Delta U$
$\therefore \Delta H=\Delta U$

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