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
Answer: The sign reverses: a reaction at −100 kJ becomes +100 kJ when written in the opposite direction.
Explanation: Reverse the equation and ΔH changes sign; double the equation and ΔH doubles. What happens to ΔH if a reaction is reversed?
Method: Start from the relation used, substitute each quantity with its unit, then check the order of magnitude and the final unit.
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
Answer: Reversing the equation reverses the sign: ΔH = +250 kJ.
Explanation: Reverse the equation and ΔH changes sign; double the equation and ΔH doubles. Multiplying the whole equation by two also multiplies ΔH by two.
Method: Start from the relation used, substitute each quantity with its unit, then check the order of magnitude and the final unit.
Avoid this
ΔH belongs to the reaction as written. Multiplying the whole equation by two also multiplies ΔH by two.