How to calculate the energy required for the gas phase process represented by Na(g) + Br(g) -> Na+(g) + Br- (g)

Answer 1

You want to use Hess' Law together with the ionization potential for Na and the electron affinity for Br. The IP and Ea are available in tables in your text or on the web and Hess' Law will be in your text or you can search for it on the internet.