Exothermic vs. Endothermic Reactions

Overview

Chemical reactions involve breaking and forming chemical bonds. These processes are associated with energy changes. Reactions are classified as either exothermic or endothermic based on whether they release or absorb energy, respectively.

Bond Breaking and Bond Making

• Bond Breaking: Requires energy input to overcome the attractive forces holding atoms together. This is an endothermic process.
• Bond Making: Releases energy as atoms form new bonds and achieve a more stable state. This is an exothermic process.

Enthalpy (H) and Enthalpy Change (ΔH)

• Enthalpy (H): The chemical energy of a substance, also known as its heat content.

• Enthalpy Change (ΔH): The difference in enthalpy between products and reactants. It indicates the overall energy change during a reaction.

  • ΔH = ΣH(reactant bonds broken) - ΣH(product bonds formed)

Exothermic Reactions

• Definition: Reactions that release energy into the surroundings, usually as heat. The energy released in forming new bonds is greater than the energy required to break existing bonds.
• Enthalpy Change: ΔH is negative (ΔH < 0).
• Examples: Combustion, neutralisation reactions.
• Energy Profile Diagram: The products have lower energy than the reactants. The energy is released to the surrounding

Endothermic Reactions

• Definition: Reactions that absorb energy from the surroundings. The energy required to break existing bonds is greater than the energy released in forming new bonds.
• Enthalpy Change: ΔH is positive (ΔH > 0).
• Examples: Photosynthesis, melting ice.
• Energy Profile Diagram: The products have higher energy than the reactants. The energy is absorbed from the surrounding

Comparison Table

Energy Profile Diagrams

Energy profile diagrams illustrate the energy changes during a reaction. They show the relative energy levels of reactants, products, and the activation energy.

• Exothermic: The products are at a lower energy level than the reactants. The difference represents the energy released.
• Endothermic: The products are at a higher energy level than the reactants. The difference represents the energy absorbed.

(Description: Diagrams would show a curve representing the reaction pathway. For exothermic, the curve goes from a higher energy level (reactants) to a lower energy level (products). For endothermic, the curve goes from a lower energy level (reactants) to a higher energy level (products). The peak of the curve represents the activation energy.)

Activation Energy

• Definition: The minimum energy required to start a chemical reaction. It’s the energy needed to break the initial bonds and form the activated complex (transition state).
• Represented on energy profile diagrams as the height of the energy barrier.

Molar Enthalpy Changes

• Enthalpy change expressed per mole of a specific reactant or product (kJ mol⁻¹).
• Allows for quantitative comparison of energy changes in different reactions.

Enthalpy Changes for Mixtures

• Enthalpy change expressed per gram of a mixture (kJ g⁻¹).
• Useful for comparing the energy content of different fuels.

Thermochemical Equations

• Balanced chemical equations that include the enthalpy change (ΔH).
• Specify the physical states of reactants and products (s, l, g, aq) because the state affects the enthalpy.
• Example:
  • Combustion of methane:
    CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) ΔH = -890 kJ mol⁻¹ (exothermic)

Key Differences Summarized

Exothermic reactions release heat (ΔH < 0), while endothermic reactions absorb heat (ΔH > 0).