NEET

Which of the following pairs of processes is certain to occur in a spontaneous chemical reaction?

Exothermic and increasing disorder
Exothermic and decreasing disorder
Endothermic and increasing disorder
Endothermic and decreasing disorder

Solution

Measure of disorder of a system is nothing but entropy.For a spontaneous reaction, ?G < 0. As per Gibbs Helmholtz equation, ?G =?H – T?S Thus ?G is –ve only When ?H = –ve (exothermic) and ?S = +ve (increasing disorder)

a
b
c
d

Solution

According to second law of thermodynamics heat is partly converted into useful work and part of it :

becomes electrical energy
always wasted
increases the weight of the body
becomes K.E.

Solution

Follow II law of thermodynamics.

a
b
c
d

Solution

For conversion C (graphite) ⟶ C(diamond) the ?S is

zero
positive
negative
cannot be predicted

Solution

Conversion of graphite into diamond is an endothermic reaction. But ΔS would be negative for the conversion of graphite into diamond. Diamond has more compact structure so the volume is smaller. However, the atoms in graphite are able to move with in their lattice so the entropy is higher.

Entropy of vaporisation of water at 100°C, if molar heat of vaporisation is 9710 cals mol^-1, will be

28 cal mol^-1 K^-1
26 cal mol^-1 K^-1
24 cal mol^-1 K^-1
20 cal mol^-1 K^-1

Solution

Given ?H_v= 9710 cals mol^-1 T = 100°C = 373 K

Using ?S = \(\frac{\bigtriangleup H_{v}}{T}\) = \(\frac{9710}{373}\) = cal mol^-1 K^-1 = = 26.032 cal mol^-1 K^-1

a
b
c
d

Solution

For a spontaneous process,total ?S_total is always positive.

2 mole of an ideal gas at 27ºC temperature is expanded reversibly from 2 lit to 20 lit. Find the entropy change(R =2 cal/mol K)

92.1
4
9.2

Solution

The entropy change in the fusion of one mole of a solid melting at 27ºC (Latent heat of fusion, 2930 J mol^-1) is :

9.77 J K^-1 mol^-1
10.73 J K^-1 mol^-1
2930 J K^-1 mol^-1
108.5 J K^-1 mol^-1

Solution

The latent heat of vaporisation of water at 100° C is 540 cal g–1. Calculate the entropy increase when one mole of water at 100°C is evaporated?

1.82 cal K^-1 mol^-1
540 × 18 cal K^-1 mol^-1
26 cal K^-1 mol^-1
367 cal K^-1 mol^-1

Solution

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