If the formation of NaCl proceeds according to the following equation and change in enthalpy, what is the reaction and change in enthalpy for the decomposition of 2 moles of NaCl? Na + ½Cl2→NaCl, ΔH = - 411 kJ

Given that the equation is balanced you can apply the stoichiometry.

Multiply the entire reaction by 2 and reverse it to get the decomposition ot 2 mols of NaCl

2NaCl → 2Na + Cl2

The change in entalphy is positive (instead of negative) and of double magnitude: 822 kj.