Chem2 Energetics question

Hello,

There's a question where I'm slightly confused how the mark scheme displays the answer
It asks to calculate the bond enthalpy of C=C - QP here http://papers.xtremepapers.com/AQA/Chemistry/2008%20Jun/AQA-CHM2-W-QP-JUN08.PDF , question 2d and MS is also here http://papers.xtremepapers.com/AQA/Chemistry/2008%20Jun/AQA-CHM2-W-MS-JUN08.PDF

-240 = (6 x 412) + 2 C=C + 348 + ( 2 x 436) – [ ( 3 x 348) + (10 x 412)
This is simplified to
-240 = 3692 - 5164 + 2 C=C
2 C=C = -1232
C=C= -616 kJ/mol

BUT, in the mark scheme it says that the answer is positive and not negative. Have I made a mistake in my calculation? If so where, or...is the mark scheme wrong?

Reply 1

Yasmin5

Original post by JackTheNerd

Hello,

-240 = (6 x 412) + 2 C=C + 348 + ( 2 x 436) – [ ( 3 x 348) + (10 x 412)
This is simplified to
-240 = 3692 - 5164 + 2 C=C
2 C=C = -1232
C=C= -616 kJ/mol

BUT, in the mark scheme it says that the answer is positive and not negative. Have I made a mistake in my calculation? If so where, or...is the mark scheme wrong?

Hiya Jack,

It's the calculations!

- 240 = 3692 - 5164 + 2C=C
- 240 = - 1472 + 2C=C
- 240 - (- 1472) = 2C=C
- 240 + 1472 = 2C=C
1232 = 2C=C
C=C = 616 kJ/mol

Broke it down a bit more to show you the whole process :smile:

Good luck for the exam on the 4th
(I presume EdExcel, same as me)

Reply 2

JackTheNerd

Original post by Yasmin5

Good luck for the exam on the 4th
(I presume EdExcel, same as me)

Oh ok thank you. So do I always work around the C=C instead of moving it to the other side of the equals sign?

I'm with AQA but yeah I.do.have a.chemistry exam on the 4th, good luck for you too!

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