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Thermodynamics — Chemistry STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 ScienceChemistryThermodynamics3 Marks
Question
Although heat is a path function but heat absorbed by the system under certain specific conditions is independent of path. What are those conditions? Explain.
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Answer

Heat is independent of path under two conditions :
  1. At constant volume
By first law of thermodynamics,

$\text{q}=\Delta\text{U}+(-\text{w})$

and, $(-\text{w})=\text{P}\Delta\text{V}$

Therefore, $\text{q}=\Delta\text{U}+\text{P}\Delta\text{V}$

By first law of thermodynamics $\Delta\text{V}=0$

or, $\text{qV}=\Delta\text{U}+0$

or, $\text{qV}=\Delta\text{U}=$ change in internal energy
  1. At constant pressure
$\text{q}_\text{p}=\Delta\text{U}+\text{P}\Delta\text{V}$

But, $\Delta\text{U}+\text{P}\Delta\text{V}=\Delta\text{H}$

Therefore, $\text{q}_\text{p}=\Delta\text{H}=$ change in enthalpy

So, at constant volume and at constant pressure heat change is a state function because it is equal to change in internal energy and change in enthalpy respectively which are state functions.

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