Endothermic and exothermic reactions

1. Understanding Energy Changes in Reactions

• Chemical reactions involve energy changes when bonds are broken and new bonds form.
• Exothermic reactions release energy, while endothermic reactions absorb energy.
• ΔH (enthalpy change) determines whether a reaction is exothermic $(-ΔH)$ or endothermic $(+ΔH).$

2. Exothermic Reactions

• Definition: More energy is released when new bonds form than is absorbed to break the existing bonds.

• $ΔH$ is negative $(-ΔH).$

• Products have lower energy than reactants.

• Example: Combustion of coal: $C + O_2 \rightarrow CO_2 + \text{energy}$

• Graph Representation:

  • Reactants start at a higher energy level.
  • Activation energy $(Ea)$ is required to start the reaction.
  • Products end at a lower energy level.

3. Endothermic Reactions

• Definition: More energy is absorbed to break bonds than is released when new bonds form.

• $ΔH$ is positive $(+ΔH).$

• Products have higher energy than reactants.

• Example: Dissolving ammonium nitrate in water: $NH_4NO_3 + H_2O + \text{energy} \rightarrow NH_4^+ + NO_3^-$

• Graph Representation:

  • Reactants start at a lower energy level.
  • Activation energy $(Ea)$is required to initiate the reaction.
  • Products end at a higher energy level.

4. Law of Conservation of Energy

• Energy cannot be created or destroyed—it is transferred between the system (reaction) and surroundings.
• In exothermic reactions, energy is transferred to the surroundings.
• In endothermic reactions, energy is taken from the surroundings.

5. Key Takeaways

✅ Exothermic reactions release energy ($ΔH$ is negative, products have lower energy).

✅ Endothermic reactions absorb energy ($ΔH$ is positive, products have higher energy).

✅ Energy changes follow the Law of Conservation of Energy.

✅ Graphs help visualise energy changes in reactions.