1

### 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

### 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

### 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

### 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