How can I apply Hess's Law to solve a thermodynamics homework problem?

[courtrigrad]

Hello all

Given

(a) N2(g) + O2(g) --> 2NO(g) ∆H = + 180. 7 kJ

(b) 2NO(g) +O2(g) --> 2NO2(g) ∆H = -113.1 kJ

(c) 2N20(g) --> 2N2(g) --> O2(g) ∆H = -163.2 kJ

Desired Reaction: N2O (g) + NO2(g) --> 3NO (g)

Can someone please tell me where to start? How do I apply Hess's Law? I know I have to work backwards. However, how do I do this in a systematic way?

Any help is appreciated

Thanks

 

[apchemstudent]

Hello all

Given

(a) N2(g) + O2(g) --> 2NO(g) ∆H = + 180. 7 kJ

(b) 2N)(g) +O2(g) --> 2NO2(g) ∆H = -113.1 kJ

(c) 2NO2(g) --> 2N2(g) --> O2(g) ∆H = -163.2 kJ

Desired Reaction: N2O (g) + NO2(g) --> 3NO (g)

Can someone please tell me where to start? How do I apply Hess's Law? I know I have to work backwards. However, how do I do this in a systematic way?

Any help is appreciated

Thanks

please rewrite the equations. Just look at B), for instance, it's not even balanced... and C makes no sense since how can you get O2 from 2 N2...

 

[dextercioby]

A good place would be to state the equations correctly:
$$N_{2}+O_{2}\rightarrow 2NO$$ $$(\Delta H)_{1}=+180.7 kJ$$

$$2NO+O_{2}\rightarrow 2NO_{2}$$ $$(\Delta H)_{2}=-113.1 kJ$$

$$2NO_{2}\rightarrow N_{2}+2O_{2}$$ $$(\Delta H)_{3}=-67.6 kJ$$

Now,which is the quantity u want to compute and what's the reaction u wish to get??

Daniel.

 

[courtrigrad]

I want to compute ∆H for

N2O (g) + NO2(g) --> 3NO (g)

 

[apchemstudent]

I think C should be 2 N2O(g) -> 2N2(g) + O2(g)...

Balance the reactions so that you will get rid of the unwanted reactants or products... For instance, N2 + O2 -> 2NO

you you might want to multiply the reaction by 2 since N2 is not wanted and there's an N2 in reaction C of the product. I am assuming you know they will cancel out when added together...

 

[courtrigrad]

yes but is there an actual method to solve the problem? Or do you have to just guess and check?