Hess’s Law

[JulieAljamal1E]

Can someone explain how to do problem 4d.19 from the 7th edition step by step? It asks to calculate the reaction enthalpy for H2 + Br2 -> 2HBr given the following data: NH3 + HBr-> NH4Br delta H= -188.32 kJ., also given N2+3H2->2NH3 delta H=-92.22 kJ, and given N2+4H2+Br2->2NH4Br delta H= -541.66 kJ. Where do you start and how do you know what to rearrange? Thank you

[melodyzaki2E]

I would put each equation on top of each other, then cancel out the products that are used up as reactants in the next step (you may have to multiply to cancel them out exactly), then just add up all the enthalpy changes when you are left with your desired chemical reaction.

[almaochoa2D]

Also you might have to switch some of the equations and make the products the reactants. If you do so make sure the multiply the enthalpy by -1 of the specific reaction.