Question

Consider the following reaction:
HCN <--> H+ + CN- delta H = 315kJ
If 1.00 mole of CN- in the form of NaCN was added to equilibrium mixture, describe the effect on [H+], [HCN], [CN-], pH, and heat.

Answer 1

This ia Le Chatelier's Principle problem. When working with those problems I find it best to include heat as part of the equation. for dH = + it means endothermic; for dH = - it means exothermic. Therefore, I add heat to the appropriate side.
HCN + heat ==> H^+ + CN^-

Le Chatelier's Principle says that a system in equilibrium will try to undo what we do to it.
So adding CN^- to a system in equilibrium means the system will shift to the left (in order to try and use up the added CN^-). That means HCN increases, H^+ decreases, etc.
Note: The CN will increase because we have added CN^-; however, it will not be the sum of what we added and what it was originally because some of the CN is lost when it shifts to the left to form HCN.