Predict the sign of delta H for each reaction?

Question

Predict the sign of delta H for each of the following reactions and explain your prediction

a) Li (g) + Na- (g) -> Li- (g) + Na (g)
b) Na+ (g) + K (g) -> Na (g) + K+ (g)

Answer 1

Delta H is enthalpy, or the heat content of a chemical species.

a) negative - A sodium anion is going to be higher in energy than a lithium anion, as sodium is more electropositive. Hence, energy will be released from the charge transfer reaction.

b) positive - A potassium cation is going to be higher in energy than a sodium cation, for the same reason. So energy would need to be put into the system in order for this charge transfer to take place.