Enthalpy: tracking energy exchange
At constant pressure, enthalpy change is related to heat exchanged by the system. An exothermic reaction generally has ΔH < 0.
The essential idea
At constant pressure, enthalpy change is related to heat exchanged by the system. An exothermic reaction generally has ΔH < 0.
Example
A combustion releases energy to the surroundings: the system loses energy, giving a negative ΔH under the usual convention.
Mini problem
What happens to ΔH if a reaction is reversed?
Show the solution
The sign reverses: a reaction at −100 kJ becomes +100 kJ when written in the opposite direction.
Common mistake
Do not replace reasoning with a memorized rule. Always check geometry, units, coefficients or model assumptions.
Guided exercise
A reaction as written has ΔH = −250 kJ. What is ΔH when the equation is reversed?
Show the detailed solution
Reversing the equation reverses the sign: ΔH = +250 kJ.
Avoid this
ΔH belongs to the reaction as written. Multiplying the whole equation by two also multiplies ΔH by two.