# chemistry aqa as level bond enthalpy question

https://filestore.aqa.org.uk/sample-papers-and-mark-schemes/2021/november/AQA-74042-QP-NOV21.PDF page 20 question 8.2 i got the answer as 170 but the correct answer is 252. i don't get what we do with the +41 kJ/mol given for h2o can someone explain
bond enthalpy is defined as the energy needed to break (one mole of) a bond in a gasous molecule. it is measured when the molecule is gaseous, since if it were a liquid or a solid the energy put in is spent turning it into a gas, not breaking the bond. knowing this, you can set up a new equation where the water formed is instead gaseous, with a new enthalpy change that takes this into account.

hope this helps, and i hope you can try the question again and get +252 kJ mol-1!

new equation enthalpy change

bond enthalpy calculation

Original post by 0gg
bond enthalpy is defined as the energy needed to break (one mole of) a bond in a gasous molecule. it is measured when the molecule is gaseous, since if it were a liquid or a solid the energy put in is spent turning it into a gas, not breaking the bond. knowing this, you can set up a new equation where the water formed is instead gaseous, with a new enthalpy change that takes this into account.
hope this helps, and i hope you can try the question again and get +252 kJ mol-1!

new equation enthalpy change

bond enthalpy calculation

hi thanks i get it. i do have one question tho, why are u multiplying 4 with 41 when u're calculating the new enthalpy change shouldn't you do 8 (cus 4x2) + -2046? bc there are 2 bonds (between O and H) in H2O
(edited 4 weeks ago)
Original post by ricecakes1
hi thanks i get it. i do have one question tho, why are u multiplying 4 with 41 when u're calculating the new enthalpy change shouldn't you do 8 (cus 4x2) + -2046? bc there are 2 bonds (between O and H) in H2O

the enthalpy change from water to steam is for the whole molecule, it's not a bond enthalpy (:
Original post by 0gg
the enthalpy change from water to steam is for the whole molecule, it's not a bond enthalpy (:

oh right idk what i was thinking lol, thank u
Original post by ricecakes1
oh right idk what i was thinking lol, thank u
nw ((::