1

### 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
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

The work done during the expansion of a gas from a volume of 4 dm3 to 6 dm3 against a constant external pressure of 3 atm is (1 L atm = 101.32 J)
A
$-$ 6 J
B
$-$ 608 J
C
+ 304 J
D
$-$ 304 J

## Explanation

Work done during the expansion, W = – pdV

W = –3 atm (6 dm3 – 4 dm3)

= – 3 atm ( 2 dm3 ) (1 dm3 = 1 L)

= – 3 atm × 2 L

= – 6 L atm

As, 1 L atm = 101.32 J

$\therefore$ W = – 6 × 101.32 J = – 607.92 J ≈ – 608 J
4

### AIPMT 2003

The molar heat capacity of water at constant pressure, C, is 75 J K$-$1 mol$-$1. When 1.0 kJ of heat is supplied to 100 g of water which is free to expand, the increase in temperature of water is
A
1.2 K
B
2.4 K
C
4.8 K
D
6.6 K

## Explanation

As we know, q = nC$\Delta$T

q = 1.0 kJ = 1000 J

C = 75 JK–1 mol–1

m = 100 g ⇒ Number of moles = ${{100} \over {18}}$ g mol–1

1000 = ${{100} \over {18}}$ $\times$ 75 $\times$ $\Delta$T

$\Rightarrow$ $\Delta$T = ${{10 \times 18} \over {75}}$ K

$\Rightarrow$ $\Delta$T = 2.4 K