1

### 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
2

### 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
3

### 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.
4

### AIPMT 2005

The absolute enthalpy of neutralisation of the reaction :

Mg(O)(s) + 2HCl(aq) $\to$ MgCl2(aq) + H2O(l) will be
A
$-$57.33 kJ mol$-$1
B
greater than $-$ 57.33 kJ mol$-$1
C
less than $-$ 57.33 kJ mol$-$1
D
57.33 kJ mol$-$1

## Explanation

We know that enthalpy of neutralization of a strong acid and a strong base is –57.33 kJ mol–1.

Here, MgO is the weak base and HCl is a strong acid thus, a small amount of energy is used in the ionization of MgO thus, the heat of neutralization decreases. Therefore, enthalpy of neutralization is less than – 57.33 kJ mol–1