1

### AIPMT 2009

The values of $\Delta$H and $\Delta$S for the reaction,

C(graphite) + CO2(g) $\to$  2CO(g)

are 170 kJ and 170 J K$-$1, respectively. This reaction will be spontaneous at
A
910 K
B
1110 K
C
510 K
D
710 K

## Explanation

We know, $\Delta$G = $\Delta$H – T$\Delta$S

For reaction to be spontaneous, $\Delta$G < 0

$\Rightarrow$ $\Delta$H – T$\Delta$S < 0

$\Rightarrow$ 170 $\times$ 103 - T(170) < 0

$\Rightarrow$ T > 1000 K

Among the given temperatures, only 1110 K is greater than 1000 K thus, at this temperate the reaction will be spontaneous.
2

### AIPMT 2008

For the gas phase reaction,

PCl5(g)  $\rightleftharpoons$ PCl3(g) + Cl2(g)

which of the following conditions are correct ?
A
$\Delta$H < 0 and $\Delta$S < 0
B
$\Delta$H > 0 and $\Delta$S < 0
C
$\Delta$H = 0 and $\Delta$S < 0
D
$\Delta$H > 0 and $\Delta$S > 0

## Explanation

$\Delta$H = $\Delta$E + $\Delta$ngRT

$\Delta$ng = (1 + 1) – (1) = 1

$\Rightarrow$ $\Delta$H = $\Delta$E + RT

Thus, $\Delta$H is a positive quantity i.e., ∆H > 0. Now one mole of gaseous reactant dissociate into two moles of gaseous products thus, entropy increases i.e., $\Delta$S > 0.
3

### AIPMT 2008

Bond dissociation enthalpy of H2, Cl2 and HCl are 434, 242 and 431 kJ mol$-$1 respectively. Enthalpy of formation of HCl is
A
$-$ 93 kJ mol$-$1
B
245 kJ mol$-$1
C
93 kJ mol$-$1
D
$-$ 245 kJ mol$-$1

## Explanation

${1 \over 2}$H2 + ${1 \over 2}$Cl2 $\to$ HCl, $\Delta$Hf = ?

$\Delta$Hreaction = [ ${1 \over 2}$(B.E)H2 + ${1 \over 2}$(B.E)Cl2] - (B.E)HCl

= [217 + 121] – 431

= 338 – 431 = – 93 kJ mol–1
4

### AIPMT 2008

Which of the following are not state functions ?
(I) q + w     (II) q
(III) w        (IV) H $-$ TS
A
(I), (II) and (III)
B
(II) and (III)
C
(I) and (IV)
D
(II), (III) and (IV)

## Explanation

Enthalpy (H = q + W) and Gibbs energy, (G = H –TS) are state functions which depend only on the initial and final states of system.

While, heat (q) and work done (W) are the path function which depends on the path followed in bringing the change between two states of the system.