1

### JEE Main 2019 (Online) 11th January Evening Slot

The reaction, MgO(s) + C(s) $\to$ Mg(s) + CO(g), for which $\Delta$rHo + 491.1 kJ mol–1 and $\Delta$rSo = 198.0 JK–1 mol–1, is not feasible at 298 K. Temperature above which reaciton will be feasible is :
A
2480.3 K
B
2040.5 K
C
2380.5 K
D
1890.0 K

## Explanation

We know,

$\Delta$Go = $\Delta$Ho - T$\Delta$So

For a reaction to be spontaneous $\Delta$Go must be negative i.e.,
T$\Delta$So > $\Delta$Ho

$\Rightarrow$ T > ${{\Delta H^\circ } \over {\Delta S^\circ }}$

$\Rightarrow$ T > ${{491.1 \times 1000} \over {198}}$

$\Rightarrow$ T > 2480.3 K
2

### JEE Main 2019 (Online) 11th January Evening Slot

The standard reaction Gibbs energy for a chemical reaction at an absolute temperature T is given by $\Delta$rGo = A – BT

Where A and B are non-zero constants. Which of the following is TRUE about this reaction ?
A
Exothermic if B < 0
B
Endothermic if A > 0
C
Exothermic if A > 0 and B < 0
D
Endothermic if A < 0 and B > 0

## Explanation

$\Delta$Go = $\Delta$Ho - T$\Delta$So

Given that A and B are non-zero constants, i.e., A = $\Delta$Ho, B = $\Delta$So

If $\Delta$Ho is positive means reaction is endothermic.
3

### JEE Main 2019 (Online) 12th January Morning Slot

For diatomic ideal gas in a closed system, which of the following plots does not correctly describe the relation between various thermodynamic quantities ?
A B C D ## Explanation

CP does not changes with change in pressure.
4

### JEE Main 2019 (Online) 12th January Evening Slot

The combination of plots which does not represent isothermal expansion of an ideal gas is – A
A and D
B
B and D
C
B and C
D
A and C

## Explanation

For isothermal process of ideal gas,

PV = constants = K

$\therefore$  P = ${k \over v}$

So, the graph between P and ${1 \over v}$ is straight line passing through the origin. So, graph (A) is correct.

As PV = K so the P and V curve is hyperbola. So, graph (B) is wrong.

In isothermal PV = constant.

So, graph (C) is right.

We know internal energy (u) = ${f \over 2}$ nRT. Internal energy is function of temperature (T) only. So, U does not change when volume(V) changes. So, graph (D) is wrong.