Hess's Law

 

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

HSC Chemistry Syllabus

  • Explain the enthalpy changes in a reaction in terms of breaking and reforming bonds, and relate this to:

– The Law of Conservation of Energy 

    • 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 Explained

          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.  

           

          Enthalpy Change Formula 

          The formula for enthalpy change can be given by

           

          $$\Delta H = \Sigma \Delta H_f^{\circ} \text{(products)} – \Sigma \Delta H_f^{\circ} \text{(reactants)} $$

          where:

          $$\text{Bonds of reactants that are broken = } –\Sigma \Delta H_f^{\circ} \text{(reactants)}$$

          $$\text{Bonds of products that are formed = } \Sigma \Delta H_f^{\circ} \text{(products)}$$

           

          `\Delta H` is largely equal to the difference in energy absorbed when breaking bonds (endothermic) and energy released when forming bonds (exothermic).

          There are relatively small contributions to `Delta H` from energy changes involved in other processes in a chemical reaction such as state changes. This is discussed in more detail here. For the sake of simplicity, we will assume these energy changes are negligible when explaining Hess's law.

           

          Hess's Law

          The diagram above demonstrates the concept of Hess's Law 

          “The total enthalpy change of a chemical reaction is the same, regardless of the pathway taken provided the initial and final conditions are the same”

          It can also be summarised by the following diagram

           

           

          The importance of Hess's Law stems from its use in calculating enthalpy change values where direct measurement is difficult. In the previous example demonstrated by the diagram, the enthalpy of formation for carbon monoxide is difficult to measure because the combustion of carbon often produces both carbon monoxide and carbon dioxide. However by using Hess's Law, the enthalpy of formation value can be calculated by obtaining the enthalpy of formation values for the complete combustion of carbon and combustion of carbon monoxide. 

          Hess's Law also serves as an important concept as it demonstrates how chemical reactions abide by the Law of Conservation of Energy.

          Hess's Law Formula

          Hess's law demonstrates how enthalpy change is a state function/path independent. 

          Using Hess's Law the formula for `\Delta H` can be derived to be

           

          $$\Delta H _\text{overall} = \Delta H_1 + \Delta H_2 + \Delta H_3 + ...$$

          `\Delta H` is unchanged regardless of reaction pathway. 

           

          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)

             


             

            BACK TO MODULE 4: DRIVERS OF REACTION