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
  • Construct energy profile diagrams to represent and analyse the enthalpy changes and activation energy associated with a chemical reaction (ACSCH072)
  • Model and analyse the role of catalysts in reactions (ACSCH073) 

        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. 

         

        Temperature and Enthalpy Change

        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. 

        Temperature is a measure of a system's kinetic energy. In endothermic reactions, energy is absorbed by the surroundings to break intramolecular bonds and intermolecular forces. This process can be  thought of analogously as the breaking of a stick which requires energy. This absorption of energy in an endothermic reaction leads to a decrease in the surrounding temperature. Conversely, exothermic reactions form intramolecular bonds and intermolecular forces to release energy, increasing the temperature of their surroundings. 

        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. 

         

         

        While total enthalpy can't be measured, changes in enthalpy can be calculated using the formula 
        $$\Delta H = \text{energy absorbed in reactant bond breaking + energy released in product bond forming}$$

        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.

         

        Activation Energy and Catalysts

        Most reactions need an ignition step before they become spontaneous, like a campfire requiring small amounts of fuel before fully igniting. This initial energy barrier is called activation energy `(E_a)`. Catalysts can lower the activation energy of a reaction, increasing reaction rates. 

        The blue line in the energy profile diagram below demonstrates the decrease in activation energy after adding a catalyst into a reaction mixture. 

         

        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. 

         

        Thermochemical Equations

        Thermochemical equations include enthalpy change (`\Delta H`) on the right side in kilojoules (kJ). 


        $$2H_2O(l) \rightarrow 2H_2(g) + O_2(g) \hspace{3cm} \Delta H = 572 \text{ } kJ$$

        • sign: positive `\Delta H` indicates an endothermic reaction while a negative `\Delta H` indicates an exothermic reaction.  
        • Magnitude: The amount of energy absorbed or released by a reaction is directly proportional to the amount of substance involved in reaction. 

         

        Enthalpy Changes in Physical Changes

        State changes involve enthalpy change due to the formation or breaking of intermolecular forces that occur. The enthalpy changes which occur with changing states of water are demonstrated by the following equations. 

        $$H_2O(s) \rightarrow H_2O(l) \hspace{3cm} \Delta H = +7 \text{ } kJmol^{–1}$$ 

        $$H_2O(l) \rightarrow H_2O(g) \hspace{3cm} \Delta H = +44 \text{ } kJmol^{–1}$$

         

         

        Next Section: Enthalpy Change (ΔH) in Ionic Compound Dissolution

         

        BACK TO MODULE 4: DRIVERS OF REACTION