Physical Science Matric Revision: Energy Changes and Rates of Reactions

Revision Notes for CAPS Physical Science Grade 12

Chemical Change: Energy Changes and Rates of Reactions


Understanding energy changes and rates of reactions is crucial in Physical Science as it explains how chemical reactions occur and how energy is involved in these reactions. Mastery of these concepts enables predicting reaction behavior, which is essential in fields like chemistry, environmental science, and engineering.

Essential Learning Objectives

  • Understand the Law of Conservation of Energy.
  • Differentiate between exothermic and endothermic reactions.
  • Understand factors that influence the rate of reactions.
  • Apply principles of chemical kinetics and equilibrium.

Key Points

Law of Conservation of Energy

  • Definition: Energy cannot be created or destroyed; it only transfers from one form to another.
  • Implication: Total energy within a closed system remains constant.

Energy Profiles and Enthalpy

  • Bond Energy: Amount of energy required to break a bond; higher bond energy indicates a stronger bond.
  • Heat of Reaction (ΔH): Change in energy when reactants form products.
  • Formula: ( \Delta H = H_{\text{products}} – H_{\text{reactants}} )
  • Exothermic Reactions: ( \Delta H ) is negative (releases heat).
  • Endothermic Reactions: ( \Delta H ) is positive (absorbs heat).
  • Activation Energy: Minimum energy needed to start a reaction.

Rates of Reactions

  • Rate Formula:
  • ( \text{rate} = \frac{\Delta[\text{Reactants}]}{\Delta t} ) or ( \frac{\Delta[\text{Products}]}{\Delta t} )
  • Factors affecting Rate:
  • Nature of Reactants: Faster reactions with inorganic substances and small molecules.
  • Surface Area: Increased surface area accelerates the reaction.
  • Concentration: Higher concentration increases reaction rate.
  • Pressure: For gases, higher pressure speeds up reactions.
  • Temperature: Higher temperature increases reaction rate.
  • Catalysts: Substances that increase reaction rate without being consumed.

Real-World Applications

  1. Exothermic Reactions:
  2. Example: Combustion of fuels like gasoline.
  3. Calculation:

    • If ( \Delta H = -280 \, \text{kJ/mol} ), it indicates heat release.
  4. Endothermic Reactions:

  5. Example: Photosynthesis in plants.
  6. Calculation:

    • For ( \Delta H = +300 \, \text{kJ/mol} ), it indicates heat absorption.
  7. Rate Determination:

  8. Experiment: Reaction of hydrochloric acid with magnesium ribbon.
  9. Observation: Faster reaction with powdered magnesium than with strips due to increased surface area.

Common Misconceptions and Errors

  • Misconception: All spontaneous reactions are fast.
  • Correction: Spontaneity depends on thermodynamic favorability, not the rate.
  • Error: Confusing activation energy with the overall energy change.
  • Strategy: Use energy profile diagrams to differentiate.

Practice and Review

Practice Questions

  1. Calculate the enthalpy change if ( H_{\text{products}} = 100 \, \text{kJ} ) and ( H_{\text{reactants}} = 150 \, \text{kJ} ).
  2. Describe the effect of increasing temperature on the rate of reaction between sodium thiosulfate and hydrochloric acid.
  3. Graph the energy profile for an exothermic reaction.


  1. [ \Delta H = 100 \, \text{kJ} – 150 \, \text{kJ} = -50 \, \text{kJ/mol} ]
  2. The rate increases because particles move faster and collide more frequently with sufficient energy.
  3. Start with high reactant energy, show a peak (activation energy), and end at a lower product energy level.

Connections and Extensions

  • Link with Equilibrium: Reaction rates help understand equilibrium positions, influenced by Le Chatelier’s Principle.
  • Real-world application: Chemical engineers use these concepts to design reactors for efficient product synthesis.

Summary and Quick Review

  • Energy Conservation: Total energy remains constant.
  • Enthalpy Change: Indicates heat absorbed or released.
  • Factors Influencing Reaction Rates: Nature of reactants, surface area, concentration, pressure, temperature, and catalysts.

Additional Resources

  • Videos: Khan Academy’s “Rates of Reactions” and “Energy Changes”.
  • Online Articles: Chemguide’s “Energy Profiles” and “Factors Affecting Reaction Rates”.
  • Platforms: PHET Interactive Simulations.


  • Extract information from Module 4 of the provided files【4:10†source】 .

By utilizing these key points, practice problems, and connections, you can grasp how energy changes and rates of reactions work together in chemical processes.

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