Chemical Kinetics - Result Question 14

####17. Which of the following lines correctly show the temperature dependence of equilibrium constant, $K$, for an exothermic reaction?

(2018 Main)

(a) $A$ and $B$

(b) $B$ and $C$

(c) $C$ and $D$

(d) $A$ and $D$

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Answer:

Correct Answer: 17. (c)

Solution:

  1. From thermodynamics,

$$ \ln k=\frac{-\Delta H^{\circ}}{R T}+\frac{\Delta S^{\circ}}{R} $$

Mathematically, the equation of straight line is

$$ y=c+m x $$

After comparing Eq. (ii) with (i) we get, slope $=\frac{-\Delta H^{\circ}}{R}$ and intercept $=\frac{\Delta S^{\circ}}{R}$

Now, we know for exothermic reaction $\Delta H$ is negative (-)ve. But here,

$$ \text { Slope }=\frac{-\Delta H^{\circ}}{R} \text { is positive } $$

So, lines $A$ and $B$ in the graph represent temperature dependence of equilibrium constant $K$ for an exothermic reaction as shown below



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