Enthalpy change (ΔH)

Definition

Enthalpy change ($\Delta H$) is the amount of heat absorbed or released by a system at constant pressure during a chemical reaction. It indicates whether a reaction is endothermic (absorbs heat) or exothermic (releases heat).

5 Must Know Facts For Your Next Test

1. $\Delta H$ is positive for endothermic reactions and negative for exothermic reactions.
2. The standard enthalpy change of formation ($\Delta H_f^\circ$) refers to the enthalpy change when one mole of a compound is formed from its elements in their standard states.
3. Hess's Law states that the total enthalpy change of a reaction is the sum of the enthalpy changes for each step, regardless of the pathway taken.
4. $\Delta H$ can be calculated using bond enthalpies by subtracting the total bond energies of the reactants from that of the products: $\Delta H = \sum E_{bonds \ broken} - \sum E_{bonds \ formed}$.
5. Calorimetry experiments can determine $\Delta H$ by measuring temperature changes in a known quantity of water.

Review Questions

• What does it mean if $\Delta H$ for a reaction is negative?
• How does Hess's Law relate to calculating $\Delta H$ for complex reactions?
• Describe how you would use bond enthalpies to calculate $\Delta H$.

Related terms

Endothermic Reaction: A chemical reaction that absorbs heat from its surroundings, indicated by a positive $\Delta H$.

Exothermic Reaction: A chemical reaction that releases heat to its surroundings, indicated by a negative $\Delta H$.

Calorimetry: A technique used to measure the amount of heat involved in a chemical or physical process.