Q22.
(a) (i) 2C6H12O6 3CH3COCH3 + 3CO2 + 3H2O
Or multiples
1
(ii) to speed up the reaction
OR
(provide a) catalyst or catalyses the reaction or biological catalyst
OR
release / contain / provides an enzyme
Ignore “fermentation”
Ignore “to break down the glucose”
Not simply “enzyme” on its own
1
(b) (i) CH3CH(OH)CH3 + [O] –> CH3COCH3 + H2O
Any correct representation for the two organic structures. Brackets not essential.
Not “sticks” for the structures in this case
1
(ii) Secondary (alcohol) OR 2° (alcohol)
1
(c) M1 q = m c ΔT
OR q =150 × 4.18 × 8.0
Award full marks for correct answer
In M1, do not penalise incorrect cases in the formula
M2 = (±) 5016 (J) OR 5.016 (kJ) OR 5.02 (kJ)
(also scores M1)
M3 This mark is for dividing correctly the number of kJ by the number
of moles and arriving at a final answer in the range shown.
Using 0.00450 mol
therefore ΔH = − 1115 (kJ mol−1 )
OR − 1114.6 to − 1120 (kJ mol−1 )
Range (+)1114.6 to (+)1120 gains 2 marks
BUT − 1110 gains 3 marks and +1110 gains 2 marks
AND − 1100 gains 3 marks and +1100 gains 2 marks
Award full marks for correct answer
In M1, do not penalise incorrect cases in the formula
Penalise M3 ONLY if correct numerical answer but sign is incorrect; (+)1114.6 to (+)1120 gains 2 marks
Penalise M2 for arithmetic error and mark on
If ΔT = 281; score q = m c ΔT only
If c = 4.81 (leads to 5772) penalise M2 ONLY and mark on for M3 = − 1283
Ignore incorrect units in M2
If units are given in M3 they must be either kJ or kJ mol−1 in this case
3
(d) M1 The enthalpy change / heat change at constant pressure when
1 mol of a compound / substance / element
M2 is burned / combusts / reacts completely in oxygen
OR
burned / combusted / reacted in excess oxygen
M3 with (all) reactants and products / (all) substances in standard /
specified states
OR
(all) reactants and products / (all) substances in normal states under standard conditions / 100 kPa / 1 bar and specified T / 298 K
For M3
Ignore reference to 1 atmosphere
3
(e) M1
Σ B (reactants) − Σ B (products) = ΔH
OR
Sum of bonds broken − Sum of bonds formed = ΔH
OR
2B(C−C) + B(C=O) + 6B(C−H) + 4B(O=O) (LHS)
− 6B(C=O) − 6B(O−H) (RHS) = ΔH
M2 (also scores M1)
2(348)+805+6(412)+4(496) [LHS = 5957]
(696) (2472) (1984)
− 6(805) − 6(463) [RHS = (−) 7608] = ΔH
(4830) (2778)
OR using only bonds broken and formed (5152 − 6803)
M3
ΔH= − 1651 (kJ mol−1)
Candidates may use a cycle and gain full marks.
Correct answer gains full marks
Credit 1 mark for (+) 1651 (kJ mol−1)
For other incorrect or incomplete answers, proceed as follows
• check for an arithmetic error (AE), which is either a transposition error or an incorrect multiplication / addition error; this would score 2 marks (M1 and M2)
• If no AE, check for a correct method; this requires either a correct cycle with 4O2, 3CO2 and 3H2O OR a clear statement of M1 which could be in words and scores only M1
Allow a maximum of one mark if the only scoring point is LHS = 5957 (or 5152) OR RHS = 7608 (or 6803)
Award 1 mark for + 1651
3
(f) For the two marks M1 and M2, any two from
• heat loss or not all heat transferred to the apparatus or heat absorbed by the apparatus or (specific) heat capacity of the apparatus not considered
• incomplete combustion / not completely burned / reaction is not complete
• The idea that the water may end up in the gaseous state (rather than liquid)
• reactants and / or products may not be in standard states.
• MBE data refers to gaseous species but the enthalpy of combustion refers to liquids in their standard states / liquid propanone and liquid water in standard states
• MBE do not refer to specific compounds OR MBE values vary with different compounds / molecules OR are average / mean values taken from a range of compounds / molecules
Apply the list principle but ignore incomplete reasons that contain correct chemistry
Ignore “evaporation”
Ignore “faulty equipment”
Ignore “human error”
Not enough simply to state that “MBE are mean / average values”
2
[15]