Energy Change

Define enthalpy change (ΔH).

The heat energy absorbed or released during a chemical reaction at constant pressure.

ΔH < 0 means...

Exothermic — heat is released to surroundings. Products have LOWER energy than reactants.

ΔH > 0 means...

Endothermic — heat is absorbed from surroundings. Products have HIGHER energy than reactants.

Give two exothermic examples.

Combustion: CH₄ + 2O₂ → CO₂ + 2H₂O + energy.

Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.

Give two endothermic examples.

Photosynthesis: 6CO₂ + 6H₂O + energy → C₆H₁₂O₆ + 6O₂.

Thermal decomposition: CaCO₃ → CaO + CO₂.

Formula for ΔH using bond energies.

ΔH = Energy in (bonds broken) − Energy released (bonds formed).

Negative = exothermic; positive = endothermic.

Define activation energy (Eₐ).

The minimum energy needed for a reaction to take place.

Define activated complex

The unstable transition state from reactants to products. (Exists at maximum energy point)

What does a catalyst do?

Provides an alternative pathway with lower Eₐ so the reaction is faster. ΔH is unchanged.