Enthalpy Change, Activation Energy, Catalysts & Temperature

This is part of Year 11 HSC Chemistry course under the topic of Energy Changes in Chemical Reactions. 

HSC Chemistry Syllabus

  • Conduct practical investigations to measure temperature changes in examples of endothermic and exothermic reactions

        Enthalpy Change, Activation Energy, Catalysts & Temperature

        This video discusses changes in energy of a reaction, the concept of enthalpy,  and introduces the thermodynamic natures of reactions; endothermic and exothermic. 

        What is Enthalpy?

        Enthalpy is the chemical potential of a substance and enthalpy change thus represents the change in energy that occurs during a reaction, applicable to both chemical and physical changes. Enthalpy changes are more significant in chemical reactions than physical reactions, indicated by their larger delta H (`\DeltaH`) value. 


        Types of Enthalpy Changes

        1. Endothermic Reactions: These types of reactions are characterised by a positive `\Delta H` where the reaction absorbs energy. 

        2. Exothermic Reactions: These types of reactions are identified by a negative `\Delta H` , indicating the release of energy
        Similar to chemical bonds, the breaking of intermolecular forces requires energy to be absorbed. This is because it is only be absorbing the energy that it is able to be utilised to break the bond. This can be thought of analogously as the breaking of a stick which requires energy.
        In the opposite scenario, forming intermolecular forces causes the release of energy.

        Energy Profile Diagrams

        • Energy profile diagrams illustrate the energy levels of exothermic and endothermic reactions.
        • The left side of the energy profile diagram shows the enthalpy of reactants, while the right side of the diagram represents the enthalpy of products.
        • The "activation energy" is depicted as a peak, indicating the minimum energy required for a reaction.
        • `\Delta H` is the difference in enthalpy between reactants and products, decreasing in exothermic and increasing in endothermic reactions.

        Temperature Changes in Reactions

        Temperature is a measure of a system's heat energy. Endothermic reactions are ones which absorb energy from their surroundings. This leads to a decrease in surrounding temperature as heat energy has since been removed. Similarly, exothermic reactions release energy, increasing the temperature of their surroundings. The principle of conservation of energy implies that energy absorbed or released is transformed into chemical potential energy. 


        Next Section:  ΔH of Combustion and Neutralisation, Calorimetry