<span>C6H12 = 6x12 + 6x1 = 78.
The equation indicates that 2x78 = 156g benzene will produce 6542kJ.
Using proportions you can then calculate that
x/6542kJ = 7.9g / 156g
x = 331.3kJ = 331300J.
heat = mass x ΔT x 4.18J/g°
ΔT = 331300J / (5691g x 4.18J/g°) = 13.9°
final temp = 21 + 14° = 35°C</span>
So if the sum of the enthalpies of the reactants is greater than the products, the reaction will be exothermic. If the products side has a larger enthalpy, the reaction is endothermic. You may wonder why endothermic reactions, which soak up energy or enthalpy from the environment, even happen
