Kaplan error? Please help

[phosphatase]

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QUESTION:
5. Compound ΔH of formation
(kcal/mole, 25 °C, 1 atm)
H2
O(l) –68.4
H2O(g) –57.8
C2H6(g) –20.2
C2H4(g) +12.5
C2H2(g) +54.2
CO(g) –26.4
CO2(g) –94.1
Based on the values contained in the table, what is the
heat of combustion of one mole of ethylene at 298K
and 1 atm pressure?
A. 316.1 kcal
B. 12.5 kcal
C. –291.1 kcal
D. –316.1 kcal
E. –337.3 kcal


My answer: C2H4+3O2-->2CO2+2H2O+X
X=(-12.5)-(2*94.1)-(2*68.4)=312.5

Kaplan answer: = 2(-94) + 2(-68.4) -(12.5) -3(0) = -337.3 kcal/mol

Kanplan is wrong, agree?

 

[The Anhedonia]

unfortunately, i cannot answer this specific question for you (i have brain-dumped all DAT related materials since i took it). but a general rule of thumb - ALL test prep books are prone to error. imo kaplan especially

 

[phosphatase]

thanks, but heat of combustion should be a positive number, am I right?

I just feel very uncomfortable seeing a 20 line kaplan explanation which I think is totally wrong. Definitely not a typo.

 

[wantVCUdental]

QUESTION:
5. Compound ΔH of formation
(kcal/mole, 25 °C, 1 atm)
H2
O(l) –68.4
H2O(g) –57.8
C2H6(g) –20.2
C2H4(g) +12.5
C2H2(g) +54.2
CO(g) –26.4
CO2(g) –94.1
Based on the values contained in the table, what is the
heat of combustion of one mole of ethylene at 298K
and 1 atm pressure?
A. 316.1 kcal
B. 12.5 kcal
C. –291.1 kcal
D. –316.1 kcal
E. –337.3 kcal


My answer: C2H4+3O2-->2CO2+2H2O+X
X=(-12.5)-(2*94.1)-(2*68.4)=312.5

Kaplan answer: = 2(-94) + 2(-68.4) -(12.5) -3(0) = -337.3 kcal/mol

Kanplan is wrong, agree?

Kaplan is CORRECT, ΔH=ΔHproduct-ΔHreactant (Hess Law)

 

[The Anhedonia]

well, no. think broadly about what the question is asking (forget numbers for a second).

combustion = heat is released = delH must be negative.


i found this website has a decent explanation: http://pslc.ws/fire/cellulos/combcals.htm

 

[The Anhedonia]

VCUdental, my "well,no" is not directed toward you, it's to the op 🙂

 

[wantVCUdental]

VCUdental, my "well,no" is not directed toward you, it's to the op 🙂

relax 🙂, i don't attack anyone (EXCEPT THE DAT!!!!! which is coming up for me next week *teeth grind + knife sharpening!!!)

 

[phosphatase]

thanks for all your help! but I think you all are wrong.

The question is asking heat of combustion, (not delH) which should be positive,

http://en.wikipedia.org/wiki/Heat_of_combustion


Please correct me