# Hess's Law

15+ Year Member
Hello,

I had a question about Hess's Law. I am taking Gen Chem and this subject was a little bit conffusing. I went to a couple of tutors and their view on it was pretty much same as the teacher and no one really gave me a prcticle simplified way of looking at this problem.

It's basically trying to figure out Delta H of a reaction indirectly using components of the reaction in other reactions. Does anyone know a simple Hess Law for kids maybe? I can fudge this on a test but I really want to learn this stuff, when MCAT kreeps up on me in the future.

#### LoveDoc

##### Respect the Rhesus Monkey
7+ Year Member
15+ Year Member
Dr. G,

I took my G-chem including Hess' Law this weekend and got an A.

Do you have specific questions?

The only advice I can offer w/o knowing specifics is remember you can only use Hess when you are given Heat of Formation for all compounds. You can also use it if you are given Heat of Formation for all compounds except 1 but given Heat of Reaction for the rxn.

Give me specifics and I'll try to respond. PM me if you'd like.

#### An Yong

##### Senior Member
7+ Year Member
15+ Year Member
I thought hess's law basically says that the delta enthalpy or a rxn is the sum of each delta enthalpy for each step. So basically, to get a delta H for a certain rxn, all you have to do is add up all the delta H's for each step.

I'm pretty sure the professor has to give you all the enthalpy's for each step... if not then the only way i can think of getting the enthalpy for a particular step in the mechanism is to get a table of enthalpies and use the formula : Delta H = Bonds Broken - Bonds Formed

I hope that helps

OP
B

15+ Year Member
Hey thanx guys for responding. I am getting the teacher to help me on it this friday. We have a test next week so i am jumping on it before. Basically I know that all the Delta H have to be added at the end. My question was how to manipulate the equations so the compnents that you don't need cross cancel. I have the hardest time with this, it really shouldn't be that hard from what I see but then I always get stuck and run into trouble. Thanx guys I appreciate the responces. Good job on the A!

#### CaNEM

##### Senior Member
10+ Year Member
15+ Year Member
Hess's Law:

Don't smoke by the gas pump.

#### LoveDoc

##### Respect the Rhesus Monkey
7+ Year Member
15+ Year Member
Dr. G,

That makes sense. I hope my next comment helps you tremendously.

When I start w/ a Hess' Law question, I immediately label the compounds/elements of the 2 given reactions as R (reactants) or P (products) or 0 (zero-cancels out) depending on where they appear in the rxn i'm solving for.

This helps me so much b/c it tells me if one side of the rxn is R the other side has to be P w/ no additional knowledge.

It also helps me to begin visualizing what will cancel out and start formulating which coefficients I need to multiply by.

I really hope this helps. And I am confident you will receive an A also. Go meet the prof...maybe she/he will write a LOR one day.

Thanks!

OP
B

15+ Year Member
Thank You very much Doc I appreciate it.

#### dpark74

##### Senior Member
7+ Year Member
15+ Year Member
Be careful about states of matter. It is vital to know that there is a delta H associated from going from a solid to liquid to gas. Some equations involve these changes and are crucial to finding the correct answer.

Also, the equation as written Reactants -> Products. Delta H is energy needed to break bonds (reactants) - energy released from bonds formed (products). Also, be careful about stoichiometry as this is needed to find the total number of bonds broken and formed.