JEE MAIN - Chemistry (2019 - 8th April Morning Slot - No. 10)

Which one of the following equations does not correctly represent the first law of thermodynamics for the given processes involving an ideal gas? (Assume non-expansion work is zero)
Adiabatic process : $$\Delta $$U= – w
Cyclic process : q = –w
Isochoric process : $$\Delta $$U= q
Isothermal process : q = – w

Explanation

From 1st law of thermodynamics we know,

$$\Delta $$U = q + W

Option A :

In adiabatic process exchage of heat = 0

$$ \therefore $$ q = 0

$$ \therefore $$ From 1st law of thermodynaics, $$\Delta $$U = W

So option A is wrong.

Option B :

U is a state function. In cyclic process, initial state and final state both are same. So change in all the state function in cyclic process will be zero.

$$ \therefore $$ $$\Delta $$U = 0

$$ \therefore $$ From 1st law of thermodynaics,

q + W = 0 $$ \Rightarrow $$ q = -W

So option B is correct..

Option C :

In isochoric process volume (V) is constant. So dV = 0.

We know, W = $$ - \int {{P_{ex}}} dV$$

$$ \therefore $$ W = 0

$$ \therefore $$ From 1st law of thermodynaics,

$$\Delta $$U = q

So option C is correct.

Option D :

In isothermal process temerature (T) is constant. So dT = 0.

We know, $$\Delta $$U = nCvdT

$$ \therefore $$ $$\Delta $$U = 0

$$ \therefore $$ From 1st law of thermodynaics,

q + W = 0 $$ \Rightarrow $$ q = -W

So option D is correct.

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