Salters AS question

OKay...

i) N2 + 2O2 ---> N2O4 (all gases)

iii) H₁ = (4 x 54) + (5 x -20)
H₂ = (4 x -393) + (12 x -286) + (9 x 0)

iv) H = H₂ - H₁

make sense?

Thank you,

I get i) and iii) but not ii)

The actual question is:

Use the data in the table and the diagram to calculate values for H1 and H2 following the steps given

Ahh okay, well the top equation is the main equation you want to find the enthalpy change for

The bottom part of the diagrams are the elements in their standard states, the enthalpy of formation of a compound (e.g. H2)/CO2 etc) is defined by the energy required to form it from these elements in their standard states.

The diagram is showing the hess cycle, with H1 and H2 being the enthalpy of formations for the reagents and products of the main equation.

Ahhh I get it

So to calculate I basically multiply the amount of substance in moles by the standard enthalpy change to form the products and the same for the reactants?

Then for change in H, would it always be change in hf in products - change in H in reactants? Or does it change?

For formation, always products - reactants

Cool, thank you some much,

Just one more quick question...

Is this the same for combustion as well? and reaction ? (not sure if thats the other one but I know it begins with an r)

I think it's better if you do it using the cycle, than learning the rules. It's very difficult for me to explain over the internet how to do that, but I'll try

You want to go from the top left to the top right by going down through the bottom part. The H1 arrow is pointing in the wrong direction to do this, so you need to reverse the direction. this has the effect of changing the sigh i.e. H1 to - H1. Now add H2 since it is pointing in the right direction.

Overall H = - H1 + H2 = H2 - H1