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### Topic: heat of combustion  (Read 12160 times)

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#### psychfan29

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##### heat of combustion
« on: October 23, 2007, 11:49:57 AM »
A 2.839g sample of C2H40 was burned in a bomb calorimeter whose total heat capacity is 16.77 kJ/degree Celsius. The temperature of the calorimeter increases from 22.62 degrees C to 26.87 degrees C.  What is the heat of combustion per mole of C2H4O?

a) -260 kJ/mol
b) -8.90 X 10^3 kJ/mol
c) -3.14 X 10^3 kJ/mol
D) -1.10 X 10^3 kJ/mol
E) -3.93 kJ/mol
F) -61.2 kJ/mol

I have no clue how to even start this problem.  I know that the *delta*T is 4.25 degrees C.  I don't know how to set it up, though.  Do I have to write a balanced chemical equation? I'm so confused.

#### LQ43

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##### Re: heat of combustion
« Reply #1 on: October 23, 2007, 01:07:36 PM »
2 different steps here

1. Need to find
Heat absorbed by the calorimeter = Heat capacity x delta T

2. Heat absorbed by the calorimeter is the same #joules as the heat given off by the combustion (they are actually opposite in sign, one is + the other is -) but for this use the value as +

a) Heat of combustion (for 2.839 g)  = answer from 1

b) how many moles in 2.839 g ?

Calculate heat of combustion PER mole using a) and b) (what's the math?)

#### psychfan29

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##### Re: heat of combustion
« Reply #2 on: October 23, 2007, 01:27:01 PM »
The only thing I don't understand is if I'm supposed to use the + then how can I get a negative answer?

#### LQ43

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##### Re: heat of combustion
« Reply #3 on: October 28, 2007, 11:48:49 AM »
sorry, typo there, the heat of the calorimeter is + , the heat of combus is -