Did Kaplan Butcher this question?

[MiasmaticHelium]

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Based on the values in this table, what is the heat of combustion of one mole of ethlene at 298K and 1 atm of pressure?

Relevant Table Information:
Heat of formation variables:
H2O(l) = -68.4 kcal
C2H4(g) = 12.5 kcal
CO2 (g) = -94.1 kcal

Answer choices:
A. 316.1 kcal
B. 12.5 kcal
C. -291.1
D. -316.1
E. -337.3

My steps:
Balance EQ: C2H4 + 3 O2 --> 2 CO2 + 2 H20

Only CO2 and H20 equate to formation reactions, considering they are on product side.. & Hf [(products)-(reactants)] .:. [2(-94.1)+2(-68.4)]-[-12.5] negative sign because C2H4 is not represented as formation (product) in balanced equation.. .:. sign flip is appropriate

my work indicates a value of -312.5...kaplan says -337.3... the only place our work differs is that they made C2h4 positive..therefore resulting in (-325)-(12.5) compared to my (-325)-(-12.5)

am I correct here or did I over think the sign flip?

 

[Cawolf]

You are over thinking it:

Heat of formation is a standard value, and it can be positive. This simply indicates that ethylene is formed endothermically. It does not mean that it is a product.

 

[MiasmaticHelium]

Ah, okay. Thank you. The "formation" part, seemed to me at least, to imply that it must correspond to products. Not hard stuff but somehow some of those little things seem to get lost in the "noise" (noise as in overwhelming amount of information committed to memory)

 

[Cawolf]

It would be better to develop an understanding of what the terms mean.

Enthalpy of formation is just the amount of heat released or absorbed when some bond or bonds are formed.

So by having the products - reactants, you are saying bonds formed - bonds broken.