1
MCQ (Single Correct Answer)

JEE Main 2019 (Online) 10th January Evening Slot

The process with negative entropy change is
A
Dissociation of CaSO4 (s) to CaO(s) and SO3(g)
B
Dissolution of iodine in water
C
Synthesis of ammonia from N2 and H2
D
Sublimation of dry ice

Explanation

N2(g) + 3H2(g) $$\rightleftharpoons$$ 2NH3(g); $$\Delta $$ng < 0
2
MCQ (Single Correct Answer)

JEE Main 2019 (Online) 10th January Evening Slot

An ideal gas undergoes isothermal compression from 5m3 to 1 m3 against a constant external pressure of 4 Nm–2. Heat released in this process is used to increase the temperature of 1 mole of Al. If molar heat capacity of Al is 24 J mol–1 K–1, the temperature of Al increases by
A
$${2 \over 3}K$$
B
$${3 \over 2}K$$
C
1 K
D
2 K

Explanation

Work done on isothermal irreversible for ideal gas

= $$-$$Pext (V2 $$-$$ V1)

= $$-$$4 N/m2 (1 m3 $$-$$ 5m3)

= 16 Nm

Isothermal process for ideal gas

$$\Delta $$U = 0

q = $$-$$ w

= $$-$$ 16 Nm

= $$-$$ 16 J

Heat used to increase temperature of

A$$\ell $$ q = n Cm $$\Delta $$T

16 J = 1 $$ \times $$ 24 $${J \over {mol.K}}$$ $$ \times $$ $$\Delta $$T

$$\Delta $$T = $${2 \over 3}$$K
3
MCQ (Single Correct Answer)

JEE Main 2019 (Online) 11th January Morning Slot

For the chemical reaction X $$\rightleftharpoons$$ Y, the standard reaction Gibbs energy depends on temperature T (in K) as $$\Delta $$rGo (in kJ mol–1) = 120 $$ - {3 \over 8}$$ T.

The major component of the reaction mixture at T is :
A
Y if T = 300 K
B
Y if T = 280 K
C
X if T = 350 K
D
X if T = 315 K

Explanation

X $$\rightleftharpoons$$ Y

Keq = $${{\left[ Y \right]} \over {\left[ X \right]}}$$

If Keq > 1 $$ \Rightarrow $$ [Y] > [X]

and Keq < 1 $$ \Rightarrow $$ [Y] < [X]

We know, $$\Delta $$Go = -RT ln(Keq)

So when Keq > 1 then $$\Delta $$Go < 0 and Y is major.

And when Keq < 1 then $$\Delta $$Go > 0 and X is major.

Temperature at which $$Go = 0 is

120 $$ - {3 \over 8}$$ T = 0

$$ \Rightarrow $$ T = 320 K

For T > 320 K

$$\Delta $$Go < 0 and Y is major.

For T < 320 K

$$\Delta $$Go > 0 and X is major.
4
MCQ (Single Correct Answer)

JEE Main 2019 (Online) 11th January Morning Slot

Two blocks of the same metal having same mass and at temperature T1 and T2, respectively, are brought in contact with each other and allowed to attain thermal equilibrium at constant pressure. The change in entropy, $$\Delta $$S, for this process is :
A
2Cp In $$\left[ {{{{{\left( {{T_1} + {T_2}} \right)}^{{1 \over 2}}}} \over {{T_1}{T_2}}}} \right]$$
B
2Cp In $$\left[ {{{\left( {{T_1} + {T_2}} \right)} \over {2{T_1}{T_2}}}} \right]$$
C
Cp In $$\left[ {{{{{\left( {{T_1} + {T_2}} \right)}^2}} \over {4{T_1}{T_2}}}} \right]$$
D
2Cp In $$\left[ {{{\left( {{T_1} + {T_2}} \right)} \over {4{T_1}{T_2}}}} \right]$$

Explanation

When two blocks comes in contact with each other and attain thermal equilibrium then

final temperature of the blocks,

Tf = $${{{T_1} + {T_2}} \over 2}$$

$$\Delta $$S = $$\Delta $$S1 + $$\Delta $$S2

= Cp $$\ln \left( {{{{T_f}} \over {{T_1}}}} \right)$$ + Cp $$\ln \left( {{{{T_f}} \over {{T_2}}}} \right)$$

= Cp $$\ln \left( {{{{T_1} + {T_2}} \over {2{T_1}}}} \right)$$ + Cp $$\ln \left( {{{{T_1} + {T_2}} \over {2{T_2}}}} \right)$$

= Cp $$\ln \left( {{{{{\left( {{T_1} + {T_2}} \right)}^2}} \over {4{T_1}{T_2}}}} \right)$$

Questions Asked from Thermodynamics

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