Homework Help: Chemistry

Posted by Heather on Tuesday, October 9, 2012 at 7:46pm.

The combustion of 0.0222g of isooctane vapor, C8H18(g), at constant pressure raises the temperature of a calorimeter 0.400 degrees C. The heat capacity of the calorimeter and water combined is 2.48kJ/degrees C. Find the molar heat of combustion of gaseous isooctane.

The balanced cchemical equation is:
C8H18(g)+12(1/2)O2(g) -> 8CO2(g)+9H2O(l)

Chemistry - DrBob222, Tuesday, October 9, 2012 at 8:02pm
q = 2480 J/C x 0.400 C = ?
q/gram = q/0.0222
q/mol = q/g x molar mass = 9q/0.0222) x molar mass
Chemistry - Heather, Tuesday, October 9, 2012 at 8:22pm
I got the answer 5,110 kJ/mol. But the answer is supossed to be negative. Why should it be negative?
Chemistry - DrBob222, Tuesday, October 9, 2012 at 9:43pm
Because it's an exothermic reaction. dHrxn = - (q/0.0222) x molar mass

The water T was raised which means it absorbed heat; therefore, the heat had to come from the reaction and that makes it exotherrmic.

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