Ncert Exercises & Solutions

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.

The two conditions under which heat becomes independent of path are

(i) when volume remains constant

(ii) when pressure remains constant

Explanation

(i) At constant volume By first law of thermodynamics,

∆ U = q + W

q = ∆ U - W

. But

W = - p ∆ V

. Hence,

q = ∆ U + p ∆ V

. But as volume remains constant

∆ V = 0

∴ q_{v} = ∆ V but ∆ U

is a state function.

Hence, q_v is a state function.

(ii) At constant pressure As we know,

q_{p} = ∆ U + p ∆ V

. But

∆ U + p ∆ V = ∆ H

∴ q_{p} = ∆ H

∆ H

is a state function therefore, q_p is a state function.

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