Enthalpy and Activation Energy

Chemical reactions move energy around. Some release thermal energy to the surroundings (exothermic). Others take in thermal energy from the surroundings (endothermic). Enthalpy change tells us how much energy moves, and activation energy explains why reactions need a “push” to start.

Enthalpy change, $\Delta H$

• $\Delta H$ is the thermal energy change of a reaction (measured in kJ/mol).
• Exothermic: heat leaves the system, surroundings warm up, $\Delta H$ is negative.
• Endothermic: heat enters the system, surroundings cool down, $\Delta H$ is positive.
• Bond breaking absorbs energy (endothermic). Bond making releases energy (exothermic).

Overall enthalpy change depends on both:

$$\Delta H = \sum (\text{energy to break bonds}) - \sum (\text{energy released making bonds})$$

Activation energy, $E_a$

• $E_a$ is the minimum energy particles must have when they collide to react.
• Analogy: reactants must climb a small hill before rolling down to products. The hill height is $E_a$.
• Higher temperature gives particles more kinetic energy, so more collisions have at least $E_a$.
• A catalyst provides an alternative pathway with a lower $E_a$. It speeds up the reaction but does not change $\Delta H$ or the amounts of products at the end.

Reaction pathway diagrams

• Vertical axis: energy. Horizontal axis: progress of reaction.
• Exothermic: products lower than reactants; $\Delta H$ arrow points down; $\Delta H < 0$.
• Endothermic: products higher than reactants; $\Delta H$ arrow points up; $\Delta H > 0$.
• $E_a$ is from reactants up to the peak. With a catalyst, the peak is lower.

Real‑world links

• Exothermic: combustion in fuels; hand warmers.
• Endothermic: instant cold packs; photosynthesis needs energy from light.

Common misconceptions

• “Breaking bonds releases energy” is incorrect; it absorbs energy.
• A catalyst does not change $\Delta H$ or the final amounts of products; it only lowers $E_a$.
• Temperature change of the surroundings shows energy flow direction, not reaction speed by itself.