I need help on two questions!!! PLEASE HELP!

Question

Calculate the ∆H for the following reaction:

2H2O2 🡪  2H2O   +  O2   

You are given these two equations:

2H2  +  O2 🡪   2H2O  ∆H = -572 kJ

H2  + O2 🡪 H2O2  ∆H = -188 kJ 

Calculate the ∆H for the following reaction:

NO  +  O 🡪 NO2    

You are given these three equations:

O2 🡪 2O   ∆H = +495 kJ

2O3 🡪 3O2  ∆H = -427 kJ

NO + O3 🡪 NO2 + O2   ∆H = -199 kJ 

Answer 1

Look for unique species that must be present in the final equation. However you linearly combine (multiply and add or subtract, you do the same things to the enthalpies of reaction.

You need 2H₂O on the right and that is what rxn 1 does.

You need 2H₂O₂ on the left but I have 1 on the right in rxn 2. Therefore I multiply by -2 (negative switches direction of reaction

Add rxn1 to -2(rxn2) and you will get the final reaction. ΔH = ΔH₁ -2ΔH₂

In the 2nd set you have an intermediate that came in, O₃, and you need to get rid of it:

-1/2 rxn1 to get one O on left side

rxn3 as is to get NO₂ on right side

-1/2 rxn2 to get rid of O₃ introduced by rxn3

Let's use O₂ as a check: 1/2 on right, 3/2 on left, 1 on right -- balanced (goes away)

I'll let you work out the rest.