Hess's Law

Welcome to our free

Chemistry Homework Help

forum.

Please read the forums rules before posting.

Find out how you can Help My Chemistry Tutor

[rotorflyr84]

Just wondering if anyone has any "tricks of the trade" per say, for these types of equations? I'm having trouble solving for the net equation give two or three other equations.

For example:

Calculate the enthalpy change for the reaction
P[4]O[6] + 2O[2] --> P[4]O[10]

givinen the following enthalpies of reaction:

P[4](s) + 3O[2](g) --> P[4]O[6](s)       Delta H = -1640.1 kJ     (Eq. 1)

P[4](s) + 5O[2](g) --> P[4]O[10](s)      Delta H = -2940.1 kJ    (Eq. 2)

My assumption, is that I have to reverse equation 1 and leave equation 2 as is. When this is done, my P4 will cancel out, but I'm stuck with what to do with the 3O2 and the 5O2?

I appreciate your help!!!

[Chemistry Tutor]

well it works just like an algebra equation

if you have 5 O₂'s on the left of the yield sign and 3O₂'s on the right

then that is a net 2 O₂'s on the left which satisfies our requirements

[rotorflyr84]

Ok. That makes sense. I thought it was strictly addition with Hess's Law. I had no idea that you could use addition or subtraction when moving variables from one side of the equation to the other. Thanks a lot. Appreciate it!

[Chemistry Tutor]

no problem..how'd you hear about us?