Question

How do I use Hess's law to calculate enthalpy change? (when given two reactions)

(1)Pb(s) + Cl2(g) --> PbCl2(s) Delta H(1)= -359.4 kJ
(2)Ni(s) + Cl2(g) --> NiCl2(s) Delta H(2) = -305.3 kJ

This is the third reaction:
(3) PbCl2(s) + Ni(s) --> Pb(s) + NiCl2(s) Delta H(3) = ? kJ

I'm confused is it -664.7 or -54.1 or am I wrong? :(

Answer 1

Do this.
Reverse equation 1 and add to equation 2. Do and check to see that this adds to equation 3 which is what you want. Since you reversed equation 1 you want to change sign on dH for that equation and add to dH for equation 2 to obtain the dH for the final equation (eqn 3).