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w12qp12
Q11. This question does seem a bit confusing at the start. We rely on the definition of bond energy as much as possible to make our choices easier.
Bond energy is the energy required to break 1 mole of covalent bonds (with the species in gaseous phase)
Notice that option A is purely a bond breaking reaction, for the other options, there are bond breaking and bond formation, so its sensible to focus on option A.
However, option A, is actually breaking of n covalent bonds.
XYn --> nX + nY
So we need to take energy change of option A and divide by n, to get energy required to break 1 mole of covalent bonds
Its like
CH4 --> C + 4H
we need to divide this energy change by 4 to get the energy for 1 C-H bond.
Q23. It took me a while to understand what the question is describing.
The molecule must be broken in two and
1) contain same carbon atoms
2) both parts must have isomers
So this eliminates D, as 9 carbons molecule cannot form 2 smaller parts with equal carbons.
Isomers from when hydrocarbon have 4 or more carbon, so minimum we must have 8 carbons in the original chain to start with.
The bottom text is the examiners report, if what I typed earlier don't make sense.
"29% of candidates chose the correct answer, C. The most commonly chosen incorrect answer was A, chosen by 41% of candidates. If n=4 (option A) then the molecule C4H10 must be splitting to give C2H6 and C2H4, neither of which have structural isomers. The answer is C because if n=8 the products of cracking are
C4H10 and C4H8, both of which have structural isomers
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