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.
$$\Delta $$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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