Ethalpy reactions

Question

The following reaction is endothermic.

1. MgCl₂(s) Mg(s) + Cl₂(g)    ΔH(1) = 641 kJ

Calculate the enthalpy change for the reaction of the elements to form one mole of MgCl₂(s).

Mg(s) + Cl2(g)MgCl2(s)    ΔH(2) = ??? kJ

Answer 1

The reaction in question is the reverse of the reaction that is given with a ∆H = 641 kJ.

When we reverse a reaction, we simply change the sign of the value for ∆H.

Mg(s) + Cl2(g) => MgCl2(s) ... ∆H = -641 kJ