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The dissociation of PCl5 into PCl3 and Cl2 is endothermic. Which graph correctly represents the variation of ln(Kp) with 1/T for this reaction?

1. ln Kp vs 1/T is a straight line with negative slope
2. ln Kp vs 1/T is a horizontal straight line
3. ln Kp vs 1/T is a straight line with positive slope
4. ln Kp vs 1/T is a curved graph

Correct answer: ln Kp vs 1/T is a straight line with negative slope

Solution

By the van't Hoff relation, a plot of ln Kp against 1/T is linear with slope -deltaH/R; since the dissociation is endothermic (deltaH > 0), the slope is negative.

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