# Hess's Law!!!!!!

#### intagurl

##### Full Member
10+ Year Member
7+ Year Member
Using Hess's law what is the Hrxn for 2C(s) + O2(g) -> 2CO(g):

C(s) + O2(g) -> CO2(g); H=-393.7 KJ/mol
CO(g) + 1/2 O2(g) -> CO2(g); H= -283.3 KJ/mol

answer is -220.8 KJ
WHY?!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Please Help!

#### Ex_EE

##### Full Member
10+ Year Member
2C + 2O2 -> 2 CO2 = 2*(-393.7)
reverse the next one
2CO2 -> 2CO + O2 = 2*(283.3)

Add the two equations and you get.
2C(s) + O2(g) -> 2CO(g) = 2(-393.7) + 2(283.3) = -220.8

#### intagurl

##### Full Member
10+ Year Member
7+ Year Member
Thank you, Ex_EE!
I'm just wondering,
isn't it Hrxn= Hpro-Hrea?
Why do you add those two?

#### ananda

##### Full Member
10+ Year Member
Thank you, Ex_EE!
I'm just wondering,
isn't it Hrxn= Hpro-Hrea?
Why do you add those two?

Well if you take a look, both the products and the reactants are negative. So basically you are doing [2(-prod) - 2(-reactact)] and if you simplify the minus with the negative reactant, it becomes a plus, hence :
it is (-product) + (reactact). Hope this helps!

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