Textbook problem 5J.11

[Ruby Stuart 1I]

Question: Predict whether each of the following equilibria will shift toward products or reactants with a temperature increase:

b) X2(g) >< 2X (g), where X is a halogen

Hi, I'm confused on why part b would be considered endothermic?
Also, what happens to K in exo/endothermic reactions if temperature is decreased instead of increased?

[Krishnaya Coleman Salgado 1I]

Hi!

The process of X2 becoming 2X requires dissociation, in other words, the breaking a bond. Halogens are diatomic molecules. The breaking of any bond requires heat--while this may seem counterintuitive, I like to think of it in terms of efficiency for diatomic molecules, breaking a bond is going from an efficient and stable form to a less efficient, less stable form of the element. Requiring heat means that the reaction is endothermic.

For the second part of your question, with a temp. decrease, the opposite would happen. If an endothermic reaction (+delta H) when heated results in the formation of more products, when cooled, it results in the formation of more reactants.

Hope this helps!