1

### 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.,

$\Delta {S_{system}} + \Delta {S_{surroundings}} > 0$
2

### 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
3

### 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
4

### AIPMT 2003

What is the entropy change (in J K$-$1 mol$-$1) when one mole of ice is converted into water at 0oC? (The enthalpy change for the conversion of ice to liquid water is 6.0 kJ mol$-$1 at 0oC).
A
20.13
B
2.013
C
2.198
D
21.98

## Explanation

H2O(s) → H2O(l)

$\Delta$H = 6.0 kJ mol–1

T = 0 + 273 K = 273 K; $\Delta$S = ?

$\Delta$S = ${{\Delta H} \over T}$ = ${{6000} \over {273}}$ = 21.98 JK–1 mol–1