The Student Room Group

enthalpy change for combustion?

in my OCR A - a level chemistry book it says that enthalpy of formation is always- enthalpy change of products - enthalpy change of reactants
for enthalpy of combustion it states=
enthalpy change of reactants - enthalpy change of products

for the latter, using that has only given me the wrong answer, and even mark schemes use the first one, products - reactants, for enthalpy change of combustion as well as formation.

im rlly confused, which one is right? i know for combustion the answer must always be negative since it is exothermic but i still dont get why both are used in different places
Reply 1
I always learnt it as if using formation then. Reactants are taken apart so you use -enthalpy change or reactant +enthalpy change of products

But with combustion the reactants are broken apart (combusted ) so you do
+Enthalpy change of reactants -enthalpy change of products (as products are formed and not combusted)
Reply 2
Original post by Aya.J
I always learnt it as if using formation then. Reactants are taken apart so you use -enthalpy change or reactant +enthalpy change of products
But with combustion the reactants are broken apart (combusted ) so you do
+Enthalpy change of reactants -enthalpy change of products (as products are formed and not combusted)



I think I explained this badly so sorry if I've confused you 🥲
Reply 3
haha its fine im already pretty confused lmao
wait so like, you take the enthalpy change of reactants as + and then subtract the enthalpy change of products? which im assuming you also make +? or can we just use formation and take the negative of the difference between enthalpy of products and reactants?
Reply 4
Original post by Nat4695
haha its fine im already pretty confused lmao
wait so like, you take the enthalpy change of reactants as + and then subtract the enthalpy change of products? which im assuming you also make +? or can we just use formation and take the negative of the difference between enthalpy of products and reactants?

have you tried drawing a hess cycle? I find it easier to see it visually and write the values on - if you go with the arrow its the same sign, if against the arrow you flip. Not sure. if you are asking something different though haha :smile:
Reply 5
Original post by bn2608
have you tried drawing a hess cycle? I find it easier to see it visually and write the values on - if you go with the arrow its the same sign, if against the arrow you flip. Not sure. if you are asking something different though haha :smile:

im gonna try that now haha thanks
Reply 6
Original post by Nat4695
in my OCR A - a level chemistry book it says that enthalpy of formation is always- enthalpy change of products - enthalpy change of reactants
for enthalpy of combustion it states=
enthalpy change of reactants - enthalpy change of products
for the latter, using that has only given me the wrong answer, and even mark schemes use the first one, products - reactants, for enthalpy change of combustion as well as formation.
im rlly confused, which one is right? i know for combustion the answer must always be negative since it is exothermic but i still dont get why both are used in different places

Try drawing a Hess diagram if you're stuck. That can add more substance to the formulas that you learn...
Reply 7
Original post by Nat4695
im gonna try that now haha thanks

best way I remember it is enthalpy change of formation 'flies' so the arrows go up, and then for enthalpy change of combustion, the arrows go down, if that's helpful!
Reply 8
Original post by bn2608
best way I remember it is enthalpy change of formation 'flies' so the arrows go up, and then for enthalpy change of combustion, the arrows go down, if that's helpful!

Learning the definitions also helps a ton... e.g. the formation arrow goes up because you're forming a compound from its elements

Quick Reply

Latest