Energy & Chemical Change
Bond energy, Hess's law, activation energy
Energy and Chemical Change
Exothermic reactions release energy (delta H < 0). Endothermic reactions absorb energy (delta H > 0). Activation energy (Ea) is the minimum energy needed for a reaction. Hess's law: total enthalpy change is independent of the route.
Example
Applying Hess's Law
Given: C + O2 -> CO2 (delta H1 = -393 kJ). CO + 0.5O2 -> CO2 (delta H2 = -283 kJ). Find delta H for C + 0.5O2 -> CO. By Hess's law: delta H = delta H1 - delta H2 = -393 - (-283) = -110 kJ.
Note
Exam Tip
Draw energy diagrams showing reactants, products, Ea, and delta H. For Hess's law, reverse equations (change sign) or multiply (multiply delta H). Bond energy: delta H = sum(bonds broken) - sum(bonds formed).
Key Vocabulary
ExothermicA reaction that releases energy to the surroundings
EndothermicA reaction that absorbs energy from the surroundings
Activation energyThe minimum energy required for a chemical reaction to start
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Exothermic
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