1

### AIPMT 2006

Assume each reaction is carried out in an open container. For which reaction will $\Delta$H = $\Delta$E ?
A
2CO(g) + O2(g) $\to$  2CO2(g)
B
H2(g) + Br2(g) $\to$ 2HBr(g)
C
C(s) + 2H2O(g) $\to$ 2H2(g) + CO2(g)
D
PCl5(g) $\to$ PCl3(g) + Cl2(g)

## Explanation

We know that

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

In the reaction, H2(g) + Br2(g) $\to$ 2HBr(g)

$\Delta$ng = 2 - (1 + 1) = 0

So, $\Delta$H = $\Delta$E for this reaction.
2

### AIPMT 2006

The enthalpy of hydrogenation of cyclohexene is is $-$ 119.5 kJ mol$-$1. If resonance energy of benzene is $-$ 150.4 kJ mol$-$1, its enthalpy of hydrogenation would be
A
$-$ 358.5 kJ mol$-$1
B
$-$ 508.9 kJ mol$-$1
C
$-$ 208.1 kJ mol$-$1
D
$-$ 269.9 kJ mol$-$1

## Explanation

The resonance energy provides extra stability to the benzene molecule so it has to over come for hydrogenation to take place.

So $\Delta$H = – 358.5 – (–150.4) = –208.1 kJ
3

### AIPMT 2006

The enthalpy and entropy change for the reaction:
Br2(l) + Cl2(g) $\to$ 2BrCl(g)
are 30 kJ mol$-$1 and 105 J K$-$1 mol$-$1 respectively.
The temperature at which the reaction will be in equilibrium is
A
300 K
B
285.7 K
C
273 K
D
450 K

## Explanation

$\Delta$G = $\Delta$H – T$\Delta$S

Now, at equilibrium $\Delta$G = 0

0 = $\Delta$H – T$\Delta$S

$\Rightarrow$ 0 = 30000 –T (105)

$\Rightarrow$ T = ${{30000} \over {105}}$ = 285.7 K
4

### AIPMT 2006

Identify the correct statement for change of Gibb's energy for a system ($\Delta$Gsystem) at constant temperature and pressure.
A
If $\Delta$Gsystem < 0, the process is not spontaneous.
B
If $\Delta$Gsystem > 0, the process is spontaneous.
C
If $\Delta$Gsystem = 0, the system has attained equilibrium.
D
If $\Delta$Gsystem = 0, the system is till moving in a particular direction.

## Explanation

$\Delta$Gsystem < 0, process is spontaneous.

$\Delta$Gsystem = 0, process is in equilibrium.

$\Delta$Gsystem > 0, process is not spontaneous.