Enthalpy and reaction heat
At constant pressure, a system's enthalpy change is related to heat exchanged. An exothermic reaction has a negative enthalpy change under the usual thermodynamic convention.
Core idea
At constant pressure, a system's enthalpy change is related to heat exchanged. An exothermic reaction has a negative enthalpy change under the usual thermodynamic convention.
A concrete example
A combustion can release substantial energy to the surroundings while having ΔH < 0: the sign describes the system being studied.
Why it matters
This concept connects symbolic scientific language with what actually happens at atomic, molecular or experimental scales. Before applying a formula, always check the quantities, units and assumptions of the model.
Check it yourself
Restate the idea without technical vocabulary, then repeat the reasoning with a simple value. If your explanation and calculation tell different stories, revisit the assumptions.
A combustion can release substantial energy to the surroundings while having ΔH < 0: the sign describes the system being studied.