Hess’s Law

Moderators: Chem_Mod, Chem_Admin

JulieAljamal1E
Posts: 71
Joined: Fri Sep 28, 2018 12:24 am

Hess’s Law

Postby JulieAljamal1E » Sat Jan 26, 2019 12:08 am

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
Posts: 61
Joined: Fri Sep 28, 2018 12:18 am

Re: Hess’s Law

Postby melodyzaki2E » Sat Jan 26, 2019 10:44 am

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
Posts: 67
Joined: Fri Sep 28, 2018 12:23 am

Re: Hess’s Law

Postby almaochoa2D » Sat Jan 26, 2019 11:07 am

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.


Return to “Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)”

Who is online

Users browsing this forum: No registered users and 2 guests