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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.

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