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1

### 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 .
2

### 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.
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

If the bond energies of H $$-$$ H, Br $$-$$ Br, and H $$-$$ Br are 433, 192 and 364 kJ mol$$-$$1 respectively, the $$\Delta$$Ho for the reaction

H2(g) + Br2(g) $$\to$$ 2HBr(g) is
A
$$-$$ 261 kJ
B
+103 kJ
C
+261 kJ
D
$$-$$103 kJ

## Explanation

H2(g) + Br2(g) $$\to$$ 2HBr(g),   $$\Delta$$Hof = ?

$$\Delta$$Hof = $$\Sigma$$(B.E.)reactants – $$\Sigma$$(B.E.)products

= (B.E.)H–H + (B.E.)Br–Br –2(B.E)H-Br

= [433 + 192] – 2(364) kJ mol–1

= (625 – 728) kJ mol–1 = –103 kJ mol–1

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