Hess's Law

This is part of Year 11 HSC Chemistry course under the topic of Enthalpy and Hess's Law

HSC Chemistry Syllabus

  • Investigate Hess’s Law in quantifying the enthalpy change for a stepped reaction using standard enthalpy change data and bond energy data, for example:

– Carbon reacting with oxygen to form carbon dioxide via carbon monoxide


    • Apply Hess’s Law to simple energy cycles and solve problems to quantify enthalpy changes within reactions, including but not limited to:

    – Enthalpy changes involved in photosynthesis

    – Enthalpy changes involved in respiration

        Hess's Law

        This video will introduce Hess's Law and explore how the principles of Hess's Law apply to help determine the enthalpy of particular reactions.  


        What is Hess's Law?

        Hess's Law is a chemical principle which states that the overall enthalpy change of a reaction is independent of the pathway that is taken to achieve the reaction



        Consider the following diagram:





        1. Two-Step Reaction to Form `CO_2`
          The blue arrows represent the formation of `CO_2` in a two-step process – first producing `CO` by oxidising carbon, then oxidising `CO` to form `CO_2`. 

        2. Direct Combustion
          The black arrow shows the direct combustion of carbon to create `CO_2`


        Note: The enthalpy change values of both pathways are identical as they produce the same product from the same starting reactants. 


          Applying Hess's Law: Practical Examples 

          1. Heat of Combustion
            Combustion is exothermic, indicating that more energy is released than absorbed. We can consider the combustion of an alcohol to illustrate this. When an alcohol has been combusted, energy was first absorbed to break C-C and C-H bonds, then energy was released to form C=O double bonds and O-H bonds. The formation particularly of the C=O double bond leads to a relatively large amount of energy in comparison to the energy used to break the C-C and C-H bonds. 

          2. Effect of Carbon-Hydrogen Bonds:
            In compounds, adding more carbon-hydrogen bonds results in a more negative delta H, thus releasing more energy per molecule of the fuel. 

          3. Respiration and Photosynthesis (answered in video) 


          Previous Section: Calculating the Enthalpy of Formation

          Next Section: Entropy and Gibbs Free Energy ( ΔG)