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Aqa a level chemistry help needed asap

http://pmt.physicsandmathstutor.com/download/Chemistry/A-level/Past-Papers/AQA-Old/Unit-5/January%202013%20QP%20-%20Unit%205%20AQA%20Chemistry%20A-level.pdf
1(c)(iii) please can someone help i got +410 but the mark scheme says -410
I don't understand how this is possible as the enthalpy of formation = products-reactants. I've been doing this for a long time and have always gotten the answer right, Why would it be negative
if 768-358=410?
Original post by alevelssss
http://pmt.physicsandmathstutor.com/download/Chemistry/A-level/Past-Papers/AQA-Old/Unit-5/January%202013%20QP%20-%20Unit%205%20AQA%20Chemistry%20A-level.pdf
1(c)(iii) please can someone help i got +410 but the mark scheme says -410
I don't understand how this is possible as the enthalpy of formation = products-reactants. I've been doing this for a long time and have always gotten the answer right, Why would it be negative
if 768-358=410?


sum(bonds products) - sum(bonds reactants)

1/2Cl2 + 3/2F2 --> ClF3

Using values from my data book:

3 x (Cl-F) - 1/2(Cl-Cl) - 3/2(F-F) =

(3 x 255) - (0.5 x 242) - (1.5 x 159) = 765 - 121 - 239 = +405 kJ

... as pointed out below, this is nonsense, as bond formation is exothermic.

The equation should read:

+ sum(bonds reactants)+ -sum(bonds products) = -405 kJ

EDIT: I am quite clearly going senile. Please call the men in white coats to haul me away and shoot me. It would be the kindest thing to do ...

For the second time in a week!

:getmecoat:
(edited 5 years ago)
Reply 2
Avatar for alevelssss
alevelssss
OP
11
Original post by charco
sum(bonds products) - sum(bonds reactants)

1/2Cl2 + 3/2F2 --> ClF3

Using values from my data book:

3 x (Cl-F) - 1/2(Cl-Cl) - 3/2(F-F) =

(3 x 255) - (0.5 x 242) - (1.5 x 159) = 765 - 121 - 239 = +405 kJ


Thats not the right answer lmao, The right answer is -410kj.
Reply 3
Avatar for alevelssss
alevelssss
OP
11
Original post by alevelssss
Thats not the right answer lmao, The right answer is -410kj.


I made the same mistake aswell
making bonds always releases energy. breaking bonds always costs energy.

so it should be [sum (bonds reactants) - sum (bonds products)]

in other words, the sum of reactant bond energies is energy that goes in, and the sum of product bond energies is energy you get back out.
Reply 5
Avatar for alevelssss
alevelssss
OP
11
Original post by julietlima3
making bonds always releases energy. breaking bonds always costs energy.

so it should be [sum (bonds reactants) - sum (bonds products)]

in other words, the sum of reactant bond energies is energy that goes in, and the sum of product bond energies is energy you get back out.


Ahhh makes sense now thank you!! should I always use this principle when working with bond enthalpies instead of formation data?
Original post by alevelssss
Ahhh makes sense now thank you!! should I always use this principle when working with bond enthalpies instead of formation data?


yes, it would help to understand the principle behind bond enthalpies, so you don't have to rely on blindly applying a formula you memorised but may not fully understand

remember that bond enthalpy is defined as the energy required to break the bond. which is why bond enthalphy values are always positive. but the energy released by making the bond is the exact same, it's just a negative value because it's energy being released
(edited 5 years ago)

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