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#### Zoloft

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##### Confused about heat of a reaction
« on: January 16, 2009, 12:11:49 AM »
OK, so I'm suppose to calculate how many grams of propane are required to heat all of the water in a 50 gallon water heater from 23.4 C to 65.0 C if the water heater (the jacket which contains the water) itself has a heat capacity of 23.4 J/C and the heater also uses 2.3 kg of copper pipe.

SO, would I use -qsys = qsurr

And set the heat of water equal to the heat of the water heater?

My problem is the question of where the water heater's capacity comes into place, and whether or not I also need the specific heat capacity of copper.

Furthermore, I can't think of a way to properly use Hess's Law (except to calculate the heat released by burning 1 mole of propane).

Any direction/help? I've done numerous problems, but nothing like this so far, and it's getting  quite complex, I think...

Also, is 50 gallons are equivalent to 189,270 grams, right?

#### Borek

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##### Re: Confused about heat of a reaction
« Reply #1 on: January 16, 2009, 03:14:39 AM »
Start calculating amount of heat needed to heat up everything that has to be heated. IMHO that means water, piping and jacket, although from what you have posted it is not entirely clear, perhaps question wording is a little bit lousy.
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#### Zoloft

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##### Re: Confused about heat of a reaction
« Reply #2 on: January 16, 2009, 03:44:39 AM »
Hmmm, I think I figured it out.

Apparently, I should consider the heat of water, the jacket, and the copper as one system.

Add their heat (joules), and divide by the amount of heat of the reaction of 1 mole of propane (2000 something).

This would leave the units in mols of propane, so then just convert that to grams.

Is this right?

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