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Topic: Benzene bond enthalpy question  (Read 213 times)

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Offline Johnny 2375

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Benzene bond enthalpy question
« on: September 11, 2019, 12:57:00 PM »
Can't seem to figure out this one could you help me please thanks.

Calculate the average C-C bond enthalpy in benzene (C6H6) given the following data:

C(s) → C(g)
enthalpy change = +715 kJmol-1

H2(g) → 2 H(g)
enthalpy change = +436 kJmol-1

C6H6(l) → C6H6(g)
enthalpy change = +31 kJmol-1

enthalpy of formation (C6H6(l)) = +49 kJmol-1

E (C-H) = +413 kJmol-1
« Last Edit: September 11, 2019, 01:14:40 PM by Johnny 2375 »

Offline Corribus

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Re: Benzene bond enthalpy question
« Reply #1 on: September 11, 2019, 01:20:02 PM »
It is the forum rule that you have to show your work to receive help.
What have you tried so far?
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Johnny 2375

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Re: Benzene bond enthalpy question
« Reply #2 on: September 11, 2019, 01:27:23 PM »
I got the right answer which is 507 KJ mol-1 but my working out was wrong and accidental

Offline Johnny 2375

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Re: Benzene bond enthalpy question
« Reply #3 on: September 11, 2019, 01:29:09 PM »
formulated this equation:  6C + 3H2 → C6H6   

Then I tried using hess's law

Offline Corribus

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Re: Benzene bond enthalpy question
« Reply #4 on: September 11, 2019, 01:52:39 PM »
"working out was wrong and accidental" is rather vague, particularly since you say you got the right answer. Also, your formulated equation doesn't include any states of matter, which are obviously important to setting up the problem.

Here's a hint: start with the definition of the enthalpy of formation in terms of some of the chemical species noted in the information given to you. Then you can work backwards.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Johnny 2375

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Re: Benzene bond enthalpy question
« Reply #5 on: September 11, 2019, 01:54:42 PM »
I don't understand why there are 3 equations its confusing

Offline Corribus

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Re: Benzene bond enthalpy question
« Reply #6 on: September 11, 2019, 02:09:27 PM »
Ok, look: One isolated molecule of benzene contains 6 C-H bonds and 6 C-C bonds. You could theoretically form these bonds from 6 lone carbon atoms and 6 lone hydrogen atoms, all in the gaseous state.  Whatever energy is stored in a benzene molecule from a starting state of 6 isolated carbon and 6 isolated hydrogen molecules goes into forming those bonds. You have to use gaseous states here because condensed phases have intermolecular energies to worry about. (The problem uses moles of molecules, but the idea here is the same.)

Represented by a chemical equation, this is 6C(g) + 6H(g) :rarrow: 1 C6H6

You have to assemble all those other pieces of information to find out the enthalpy change associated with this process. Your starting point, which is the tricky part to see, is the enthalpy of formation of liquid benzene. The enthalpy of formation has a very specific definition. I suggest you look up the definition of the enthalpy of formation, and write a chemical equation as it relates to benzene. You can be pretty sure it will involves some of the species in the equations you are given, and I suggest you keep close track of the phases of matter, because that's important. Then you can use a standard Hess law approach to determine the delta H of the above reaction.

There's one more logic step after that, but see if you can get to that point and the rest isn't too hard.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Johnny 2375

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Re: Benzene bond enthalpy question
« Reply #7 on: September 11, 2019, 02:15:04 PM »
Why isn't it 3H2 instead of 6H

Offline Johnny 2375

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Re: Benzene bond enthalpy question
« Reply #8 on: September 11, 2019, 02:21:08 PM »
((6(413)+31+6x)) - ((6(715)+6(436)) = 49

6x - 4397 = 49

6x = 4446
x= 741

I don't know what i'm doing wrong

Offline Corribus

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Re: Benzene bond enthalpy question
« Reply #9 on: September 11, 2019, 03:01:56 PM »
I don't know either, because that's just a bunch of numbers. In chemistry you have be careful about writing out exactly what you're doing, and few problems won't benefit from explicitly writing out a chemical reaction formula. This is especially the case if you're trying to get someone to help you.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Johnny 2375

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Re: Benzene bond enthalpy question
« Reply #10 on: September 11, 2019, 03:03:23 PM »
What is the working out?

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