5 - Chemistry Energetics

Exothermic and Endothermic Reactions

Definitions

• Exothermic Reaction:

  • Transfers thermal energy to the surroundings.

  • Results in an increase in temperature of the surroundings.

  • Reaction examples include:

    • Combustion reactions (e.g., burning fuel in the presence of oxygen).

    • Neutralization reactions (e.g., acid-base reactions).

    • Most oxidation reactions.

• Endothermic Reaction:

  • Absorbs thermal energy from the surroundings.

  • Leads to a decrease in temperature of the surroundings.

Bond Energy Considerations

• Breaking Bonds:

  • Is typically endothermic.

  • Requires more energy input to break bonds than the energy released from breaking them, overriding the reaction's overall energy output.

  • Example: Breaking a marker requires added energy, while the sound and thermal energy released is minimal compared to that energy input.

• Making Bonds:

  • Is exothermic as more energy is released than absorbed.

  • A reaction is classified as exothermic if the energy required to break bonds is less than the energy released when new bonds are formed.

Activation Energy and Enthalpy Change

• Activation Energy (Ea):

  • The minimum energy required for colliding particles to react.

• Enthalpy Change (ΔH):

  • Represents the difference in energy between reactants and products.

  • Formula: ΔH = (Energy of Reactants) - (Energy of Products).

  • If ΔH is negative (ΔH < 0), the products have less energy than the reactants, indicating an exothermic reaction.

  • If ΔH is positive (ΔH > 0), the products contain more energy than the reactants, indicating an endothermic reaction.

Summary of Energy Changes

• In summary, ΔH is:

  • Negative for exothermic reactions (indicating energy release).

  • Positive for endothermic reactions (indicating energy absorption).

Visual Representation of Energy Changes

• Graphical representation typically depicts:

  • For Exothermic Reactions:

    • Energy level decreases as reactants transform into products.

    • Energy change is negative.

  • For Endothermic Reactions:

    • Energy level increases as reactants are converted to products.

    • Energy change is positive.