Consider the following two thermochemical equations

Question

N2+2.5O2-> N2O5(s) Delta H=xkJ
N2+2.5O2-> N2O5(g) Delta H=ykJ

The enthalpy change in kJ for the sublimation of one mole of N205 solid to gas would be represented by the quantity
a)x+y
b)x-y
c)y-x
d)-x-y

I am thinking it's D but i am still confused :S

Answer 1

It is not d.
Reverse equation 1 (and the sign of DH) and add to equation 2 and see if you don't get the sublimation equation.
N2O5(s) ==> N2O5(g)

Answer 2

Look this is what i did
You multiply the first reaction by -1
so you have -x for the reactant (since its what you started with)
and you have y for the product (since it makes a gas)

So enthalpy change is Delta H=>H(products)- H(reactants)
which means its y-(-x)
negatives cancel out
giving you y+x or x+y

Is that right? That was my first answer but now i'm not sure -_-

And btw thank you a lot for helping :)

Answer 3

No, that isn't right.

N2+2.5O2-> N2O5(s) Delta H=xkJ
N2+2.5O2-> N2O5(g) Delta H=ykJ

Reverse equation 1.
N2O5(s) ==> N2 + 5/2 O2 DH = -x kJ.
Then add equation 2.
N2 + 5/2 O2 ==> N2O5(g) DH = y kJ.
-----------------------------------
N2 and 5/2 O2 cancel leaving
N2O5(s) ==> N2O5(g) DH = sum of the above DH values which is -x+y
right?

Answer 4

So it's C

Answer 5

You MAY multiply by -1 as you did but then you ADD the two kJ values since you ADDED the two equations. Your error is the products - reactants thing. That doesn't apply here; that's how the original value of x and y were determined.

Answer 6

Yes, c is correct.