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 with enthalpy change representing the change in energy that occurs as a result of chemical reaction. The concept of enthalpy change is applicable to both chemical and physical changes although it is more significant in chemical reactions than physical reactions. This is demonstrated by their larger delta H (`\DeltaH`) values in comparison to physical changes. 


        Types of Enthalpy Changes

        1. Endothermic Reactions: These types of reactions are characterised by a positive `\Delta H`. The positive delta H value indicates that the reaction absorbs energy.  

        2. Exothermic Reactions: These types of reactions are identified by a negative `\Delta H`. The negative delta H value indicates that the reaction releases energy. 

        Similar to chemical bonds, the breaking of intermolecular forces requires energy to be absorbed. 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 to occur successfully. 
        • `\Delta H` is the difference in enthalpy between reactants and products, decreasing in exothermic and increasing in endothermic reactions.


        Catalysts and Their Effects 

        Catalysts are substances that accelerate the rate of a chemical reaction without undergoing any permanent chemical change themselves. They are pivotal to numerous chemical processes, from industrial manufacturing to biological systems. 

        How catalysts work:

        • Lowering Activation Energy:
          • The primary role of a catalyst in a chemical reaction is to lower the activation energy. Activation energy is the minimum energy required for reactants to transform into products. 
          • By providing an alternate pathway with lower activation energy, catalysts enable more reactant molecules to have enough energy to undergo the reaction at a given temperature.
          • The effect of this is that the catalysts increase the rate at which a reaction occurs in both the forward and reverse directions. 


        Temperature Changes in Reactions

        Temperature is a measure of a system's kinetic energy (you can read more about the relationship between temperature, heat, and energy here). Endothermic reactions are ones which absorb energy from their surroundings. This leads to a decrease in the surrounding temperature as energy has been transferred away. Similarly, exothermic reactions release energy, increasing the temperature of their surroundings by transferring energy.

        The concepts of exothermic and endothermic reactions can be visualised in the diagram below where the transfer of energy from a system/reaction to the surroundings indicates an exothermic reaction while the transfer of energy from the surroundings to the system/reaction indicates an endothermic reaction. 


        Next Section:  ΔH of Combustion and Neutralisation, Calorimetry