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Mendeleev · Chemistry pathway · level 2

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.

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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.

Go further

EnthalpySpecific heat
Thermochemical equation

ΔH belongs to the reaction as written

Reverse the equation and ΔH changes sign; double the equation and ΔH doubles.

Key idea

Enthalpy: tracking energy exchange

Key idea

Enthalpy: tracking energy exchange

À pression constante : ΔH = q_p · loi de Hess : les ΔH s’additionnent

At constant pressure, enthalpy change is related to heat exchanged by the system. An exothermic reaction generally has ΔH < 0.

Guided example

Enthalpy: tracking energy exchange

At constant pressure, enthalpy change is related to heat exchanged by the system. An exothermic reaction generally has ΔH < 0.

Put it into practice

Enthalpy: tracking energy exchange

Choose a concrete case linked to Enthalpy: tracking energy exchange. Write down the known information, what you are trying to find, and the units. Then state the rule, model or trend you will use. If a calculation is involved, keep the units visible at every line; if it is conceptual, state what changes and what remains fixed.

Check your answer

A strong answer contains four things: the starting situation, the variable that changes, the direction or numerical result, and one check. The check can be a unit, an order of magnitude, a limiting case or a comparison with a familiar example.

Periodic table