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Topic: What does standard enthalpy mean?  (Read 1441 times)

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Offline sharbeldam

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What does standard enthalpy mean?
« on: March 25, 2023, 06:18:57 AM »
Here is the original question:
The standard enthalpy of reaction for the oxidation of benzoic acid (C7H6O2) is -6454 kJ at 293.15K. Suppose 1.785g of benzoic acid would be oxidized at 293.15K in a bomb calorimeter with a total calorimeter constant C_v = 5.02 kJ/K. Which of the following answers is correct?

1) ΔU°r = -6461.4 kJ
2) T(final) = 316.95 K
3) q_(v,bomb and water) = 47.15 kJ
4) q_(v,bomb and water) = 94.33 kJ

I really just want to know what is standard enthalpy , if it's per 1 mole of benzoic acid, then answer D would be correct.

BUT, if we type the reaction
2 C7H6O2 + 15 O2 -> 14 CO2 + 6 H2O
the reaction enthalpy is per 2 moles.

do you guys think the 6545 is released per 1 mole or 2 moles of benzoic acid oxidizing?
(The right answer is C, but i really dont get it, standard should mean per 1 mole not per reaction).

Thanks a lot
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Offline mjc123

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Re: What does standard enthalpy mean?
« Reply #1 on: March 25, 2023, 05:14:36 PM »
The options are 1, 2, 3 and 4 and you think the answer is D?

Seriously, I think the question is ambiguous. I would naturally read it the way you do, and assume it's -6454 kJ/mol benzoic acid. But a quick Google gives the enthalpy of combustion of BA as -3227 kJ/mol. So when the question says "The standard enthalpy of reaction for the oxidation of benzoic acid" it must mean the balanced equation with integer coefficients, as you have written it, where 1 mole of "reaction" involves 2 moles of BA. but this is not obvious, it should have been specified more clearly. (Unless the full original version of the question included the equation?)

Offline sharbeldam

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Re: What does standard enthalpy mean?
« Reply #2 on: March 25, 2023, 06:50:35 PM »
Thank you!!
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Offline Enthalpy

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Re: What does standard enthalpy mean?
« Reply #3 on: March 30, 2023, 05:21:25 AM »
Double-check benzoic acid's heat of combustion from its heat of formation: -385kJ/mol.

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