Rather than writing down q_reaction to mean "Heat flow in a reaction", we can try and find some quantity which is easier to relate to the real world. Thermodynamics uses the concept of enthalpy: in a reaction done at constant pressure, the change in enthalpy is equal to q_reaction.

DH = (H_products- H_reactants) = q_reaction

Note that DH for an exothermic reaction is negative by this definition since q is negative for an exothermic reaction. DH is positive for an endothermic reaction.

Example: A 34.0 gram sample of water is cooled from 30.0 to 25.0 ^oC. If the specific heat of water is 4.18 J/g^oC, what is the change in enthalpy, DH?

Solution: Use the relations DH = q_reaction and q = m*c*DT for a process where a substance undergoes a temperature change

m = 34.0 g
c = 4.18 J/g^oC
DT = (30.0-25.0) = 5.0 ^oC

q = 34.0g* 4.18 J/g^oC * 5.0 ^oC = 7.1x10² J
DH = q = 7.1x10² J

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