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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: azzarn on April 23, 2009, 09:23:05 PM

Title: Thermochemistry problem
Post by: azzarn on April 23, 2009, 09:23:05 PM
Hey all, This is a problem I have tried repeatedly, and am confident of my solution, yet it differs from the book answer. I'm hoping that some of you can try the problem and post your solution, so I can finally prove my chem teacher is wrong about something. For that reason, I will not influence how to do the problem, just the question and needed numbers.

Calculate the amount of heat involved in condensing 100.00g of benzene gas (C6H6) at 80.00 degrees celsius to liquid benzene at 25.00 degrees celsius.
Specific Heat:1.72
Boiling Point:80 celsius
Heat of Vaporization:30.8

Good Luck, and please show work. (I know it is hard on a computer)
Title: Re: Thermochemistry problem
Post by: lancenti on April 24, 2009, 02:31:45 AM
We need the units, whether it's J/K or kJ/K, kCal/K makes a difference.
Title: Re: Thermochemistry problem
Post by: Borek on April 24, 2009, 03:08:22 AM
Quote from: azzarn on April 23, 2009, 09:23:05 PM
Hey all, This is a problem I have tried repeatedly, and am confident of my solution, yet it differs from the book answer. I'm hoping that some of you can try the problem and post your solution, so I can finally prove my chem teacher is wrong about something. For that reason, I will not influence how to do the problem, just the question and needed numbers.

Sorry, it won't work this way. You will either show your work so that we can check it, or you will not get any help. That's a forum policy.
Title: Re: Thermochemistry problem
Post by: UG on April 24, 2009, 04:51:16 AM
Come on mate! If you're confident of your answer, post it!  :)
Title: Re: Thermochemistry problem
Post by: azzarn on April 24, 2009, 07:00:13 AM
Alright sorry all, this is how I thought to do it.
First we need to find the energy of the phase change, where q=m*Change in HVAP.
So: (100g/78g/mol)=1.28mol*30.8kJ/mol=39.42kJ. But the problem is, the equation calls for mass, but the constant in in moles/kJ. Here, I converted to moles.
Then: Q=M*C*Delta T so Q=100g*1.72j/g*C *55 degrees Celsius. so Q=9460J, or 9.46 kJ.
Using Hess's Law we can add those two numbers, yielding 48.9 kJ of energy.

That is how I thought to to it, but how we did it in class dealt with proportions. Does anyone see any problems in my work?

Thanks for looking!
Title: Re: Thermochemistry problem
Post by: Borek on April 24, 2009, 07:17:34 AM
Quote from: azzarn on April 24, 2009, 07:00:13 AM
48.9 kJ of energy.

Looks OK to me.

Quote
That is how I thought to to it, but how we did it in class dealt with proportions.

And you got different result? Can you show how it was done in the class?