Question

Estimate the change in energy when solid rubidium reacts with gaseous diatomic chlorine

Answer 1

You can find the change in enthalpy by looking up the standard enthalpy of formation of RbCl. (If you can't find it, you can probably estimate it by extrapolating the enthalpies of formation of NaCl and KCl.) To get the change in energy you'll have to correct for the work done when the Cl2 disappears.

Another possibility is to look up the IP and EA of Rb and Cl, respecively, and the bond energy of the Cl-Cl bond, and calculate the lattice energy of RbCl through something like the Kapustinkii equation:

http://en.wikipedia.org/wiki/Kapustinskii_equation

You can put all those pieces together in a Born-Haber cycle to determine the energy of RbCl versus the elements.