1

### 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.
2

### 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
3

### AIPMT 2005

Which of the following pairs of a chemical reaction is certain to result in a spontaneous reaction ?
A
Exothermic and increasing disorder
B
Exothermic and decreasing disorder
C
Endothermic and increasing disorder
D
Endothermic and decreasing disorder

## Explanation

$\Delta$G = $\Delta$H – T$\Delta$S

For a reaction to become spontaneous, $\Delta$G must be negative.

For case (a) exothermic and increasing disorder

For exothermic reaction,

$\Delta$H = –ve and increasing disorder,

$\Delta$S = +ve

$\Delta$G = –ve –T(+ve)

Thus, $\Delta$G is negative for all temperature range

For case (b) exothermic and decreasing disorder For exothermic reaction, $\Delta$H = –ve and for decreasing disorder,

$\Delta$S = –ve

$\Rightarrow$ $\Delta$G = –ve – T(–ve)

Thus, $\Delta$G is not negative for all temperature range.

For case (c) endothermic and increasing disorder. For endothermic reaction, $\Delta$H = +ve and increasing disorder,

$\Delta$S = +ve

Thus, $\Delta$G is not negative for all temperature range.

For case (d) endothermic and decreasing disorder

For endothermic reaction,

$\Delta$H = +ve and decreasing disorder,

$\Delta$S = –ve

$\Delta$G = +ve – T (–ve)

Thus, $\Delta$G is positive for all temperature range.
4

### AIPMT 2005

A reaction occurs spontaneously if
A
T$\Delta$S < $\Delta$H and both $\Delta$H and $\Delta$S are +ve
B
T$\Delta$S > $\Delta$H and $\Delta$H is +ve and $\Delta$S are $-$ve
C
T$\Delta$S > $\Delta$H and both $\Delta$H and $\Delta$S are +ve
D
T$\Delta$S = $\Delta$H and both $\Delta$H and $\Delta$S are +ve

## Explanation

$\Delta$G = $\Delta$H – T$\Delta$S

For spontaneous reaction, $\Delta$G has to be negative.

Among the given options, it is positive only when T$\Delta$S > $\Delta$H and both $\Delta$H and $\Delta$S are +ve .

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