Question

Consider the following two thermochemical equations

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

Assuming it's a)

Answer 1

No. Sublimation is changing from solid to gas. Reverse equation 1 and add to equation 2. You should get
N2O5(s) ==> N2O5(g), then reverse kJ for equation 1 and add to equation 2.

Answer 2

So the answer is a?

Answer 3

I don't think so.

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 = -xkJ
N2 + 5/2 O2 ==> N2O5(g) DH = +ykJ.
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N2O5(s) ==>N2O5(g) DH = -x+y