Delta H from Bond Energies

Part of the Energy, Rates and Equilibrium section of Coddy's Chemistry journey. Lesson 7 of 35.

$\Delta H$ = energy to break the reactants' bonds minus energy from making the products' bonds.

$\Delta H = \sum E_{broken} - \sum E_{made}$

H₂ + Cl₂ → 2HCl: broken 436 + 243 = 679; made 2 × 432 = 864.

One H2 and one Cl2 molecule become two HCl molecules

H₂ + Cl₂ → 2HCl: one H-H and one Cl-Cl broken, two H-Cl made

$\Delta H = 679 - 864 = -185\ \mathrm{kJ}$ : exothermic.

H2 and Cl2 climb 679 kJ to separate atoms, then fall 864 kJ as two H-Cl bonds form, ending 185 kJ lower

Breaking bonds goes up, making bonds comes down

Challenge

Hydrogen and chlorine react: H₂ + Cl₂ → 2HCl. The bond energies are H-H $436\ \mathrm{kJ/mol}$ , Cl-Cl $243\ \mathrm{kJ/mol}$ and H-Cl $432\ \mathrm{kJ/mol}$ . Find the enthalpy change $H$ .

Try it yourself

Solve for $H$

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Energy, Rates and Equilibrium

1Energy in Reactions

Exothermic and Endothermic Energy Diagrams Enthalpy Change Activation Energy Recap - Hand Warmer

4Rates of Reaction

Collision Theory Concentration and Rate Temperature and Rate Surface Area Catalysts Recap - Speed Trials

7Practice Problems

Recap - Methanol Plant Recap - Butane Lighter Recap - Limestone Kiln

2Bond Energies

Breaking and Making BondsDelta H from Bond EnergiesHess's Law Energy and Moles Recap - Fuel Value

5Equilibrium

Reversible Reactions Dynamic Equilibrium The Equilibrium Constant Writing K Solids and Liquids in K Recap - K Builder

3Changes of State

Heating Curves Melting and Boiling Energy of State Changes Recap - Ice to Steam

6Changing an Equilibrium

Le Chatelier's Principle Changing Concentration Changing Pressure Changing Temperature The Haber Process Recap - Factory Settings