1
MCQ (Single Correct Answer)

AIPMT 2005

Which of the following pairs of a chemical reaction is certain to result in a spontaneous reaction ?
A
Exothermic and increasing disorder
B
Exothermic and decreasing disorder
C
Endothermic and increasing disorder
D
Endothermic and decreasing disorder

Explanation

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

For a reaction to become spontaneous, $$\Delta $$G must be negative.

For case (a) exothermic and increasing disorder

For exothermic reaction,

$$\Delta $$H = –ve and increasing disorder,

$$\Delta $$S = +ve

$$\Delta $$G = –ve –T(+ve)

Thus, $$\Delta $$G is negative for all temperature range

For case (b) exothermic and decreasing disorder For exothermic reaction, $$\Delta $$H = –ve and for decreasing disorder,

$$\Delta $$S = –ve

$$ \Rightarrow $$ $$\Delta $$G = –ve – T(–ve)

Thus, $$\Delta $$G is not negative for all temperature range.

For case (c) endothermic and increasing disorder. For endothermic reaction, $$\Delta $$H = +ve and increasing disorder,

$$\Delta $$S = +ve

Thus, $$\Delta $$G is not negative for all temperature range.

For case (d) endothermic and decreasing disorder

For endothermic reaction,

$$\Delta $$H = +ve and decreasing disorder,

$$\Delta $$S = –ve

$$\Delta $$G = +ve – T (–ve)

Thus, $$\Delta $$G is positive for all temperature range.
2
MCQ (Single Correct Answer)

AIPMT 2005

A reaction occurs spontaneously if
A
T$$\Delta $$S < $$\Delta $$H and both $$\Delta $$H and $$\Delta $$S are +ve
B
T$$\Delta $$S > $$\Delta $$H and $$\Delta $$H is +ve and $$\Delta $$S are $$-$$ve
C
T$$\Delta $$S > $$\Delta $$H and both $$\Delta $$H and $$\Delta $$S are +ve
D
T$$\Delta $$S = $$\Delta $$H and both $$\Delta $$H and $$\Delta $$S are +ve

Explanation

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

For spontaneous reaction, $$\Delta $$G has to be negative.

Among the given options, it is positive only when T$$\Delta $$S > $$\Delta $$H and both $$\Delta $$H and $$\Delta $$S are +ve .
3
MCQ (Single Correct Answer)

AIPMT 2004

Considering entropy (S) as a thermodynamic parameter, the criterion for the spontaneity of any process is
A
$$\Delta {S_{system}} + \Delta {S_{surroundings}} > 0$$
B
$$\Delta {S_{system}} - \Delta {S_{surroundings}} > 0$$
C
$$\Delta {S_{system}} > 0\,\,\,$$only
D
$$\Delta {S_{surroundings}} > 0$$ only

Explanation

For the reaction to be spontaneous, the total entropy of system and universe increases i.e.,

$$\Delta {S_{system}} + \Delta {S_{surroundings}} > 0$$
4
MCQ (Single Correct Answer)

AIPMT 2004

Standard enthalpy and standard entropy changes for the oxidation of ammonia at 298 K are $$-$$ 382.64 kJ mol$$-$$1 and $$-$$ 145.6 kJ mol$$-$$1, respectively. Standard Gibb's energy change for the same reaction at 298 K is
A
$$-$$ 221.1 kJ mol$$-$$1
B
$$-$$339.3 kJ mol$$-$$1
C
$$-$$ 439.3 kJ mol$$-$$1
D
$$-$$ 523.2 kJ mol$$-$$1

Explanation

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

$$\Delta $$G = –382.64 × 103 J mol–1 – (298K) (–145.6 JK–1 mol–1)

= –382640 + 43388.8

= – 339251.2 J mol–1 = – 339.3 kJ mol–1

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