1

### JEE Main 2019 (Online) 9th January Evening Slot

The entropy change associated with the conversion of 1 kg of ice at 273 K to water vapours at 383 K is :

(Specific heat of water liquid and water vapour are 4.2 kJ K$-$1 kg$-$1 and 2.0 kJ K$-$1 kg$-$1; heat of liquid fusion and vapourisation of water are 334 kJ$-$1 and 2491 kJ kg$-$1, respectively). (log 273 = 2.436, log 373 = 2.572, log 383 = 2.583)
A
7.90 kJ kg$-$1 K$-$1
B
2.64 kJ kg$-$1 K$-$1
C
8.49 kJ kg$-$1 K$-$1
D
9.26 kJ kg$-$1 K$-$1

## Explanation $\Delta {S_1} = {{\Delta {H_{fusion}}} \over {273}} = {{334} \over {273}} = 1.22$

$\Delta {S_2} = 4.2\ell N\left( {{{363} \over {273}}} \right) = 1.31$

$\Delta {S_3} = {{\Delta {H_{vap}}} \over {373}} = {{2491} \over {373}} = 6.67$

$\Delta {S_4} = 2.0\ell n\left( {{{383} \over {373}}} \right) = 0.05$

$\Delta {S_{total}} = 9.26\,$ kJ Kg$-$1 K$-$1
2

### JEE Main 2019 (Online) 10th January Morning Slot

A process has $\Delta$H = 200 J mol–1 and $\Delta$S = 40 JK–1 mol–1. Out of the values given below, choose the minimum temperature above which the process will be spontaneous:
A
4 K
B
20 K
C
5 K
D
12 K

## Explanation

$\Delta$G = $\Delta$H $-$ T$\Delta$S

T = ${{\Delta H} \over {\Delta S}} = {{200} \over {40}} = 5K$
3

### JEE Main 2019 (Online) 10th January Morning Slot

The values of Kp/Kc for the following reactions at 300 K are, respectively : (At 300 K, RT = 24.62 dm3 atm mol–1)

N2(g) + O2(g)  $\rightleftharpoons$ 2 NO(g)

N2O4(g)  $\rightleftharpoons$   2 NO(g)

N2(g) + 3H2(g) $\rightleftharpoons$ 2 NH3(g)
A
24.62 dm3 atm mol–1, 606.0 dm6 atm2 mol–2 1.65 $\times$ 10–3 dm–6 atm–2 mol2
B
1,4.1 $\times$ 10–2 dm–3 atm–1 mol, 606 dm6 atm2 mol–2
C
1,24.62 dm3 atm mol–1 , 1.65 $\times$ 10–3 dm–6 atm –2 mol2
D
1, 24.62 dm3 atm mol–1, 606.0 dm6 atm2 mol–2

## Explanation 4

### 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