# Chemistry / Stoichiometry

When 1 mol of CH4 is burned, 490 kJ of energy are released as heat.
CH4 + O2 -> 2H2 + CO2
If 3.2g of CH4 are burned, what is the value of the heat of the reaction?

The answer worked in class was -180 kJ, but while I was redoing problems, I got the answer 98 kJ (both approximations according to sfs). When I was working the
stoichiometry, I wrote that each mole of CH4 burned produced 490 kJ, like the question said, so my equation was:
(3.2gCH4 * 1molCH4 * 490kJ)/
(16.05gCH4 * 1molCH4)
= 98 kJ (app.)

However, in the class working, we multiplied the kJ by -2 for some reason:
(3.2gCH4 * 1molCH4 * -890kJ)/
(16.05gCH4 * 1molCH4)
= 108 kJ (app.)

Is there a reason to change the number of kJ per mole of CH4 to -890, or was there a mistake?

1. 👍 0
2. 👎 0
3. 👁 137

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