Enthalpy Change (ΔH) in Ionic Compound Dissolution

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

HSC Chemistry Syllabus

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

–  Dissociation of ionic substances in aqueous solution

      Enthalpy Change (ΔH) in Ionic Compound Dissolution

      This video discusses changes in energy during ionic compound dissociation. There are three separate steps involved in the dissociation of Ionic compounds and the magnitude of energy changes in each step will determine whether the reaction is ultimately endothermic or exothermic. 

       

      Dissolution in Ionic Compounds

      Dissolution is a fundamental chemical process where a solute becomes incorporated into a solvent to result in the formation of a solution. Ionic compounds are substances which are made up of positively and negatively charged ions held together by electrostatic forces known as ionic bonds. 

      The dissolution of these compounds occurs when they interact with a solvent that can overcome these ions bonds. There are three stages which are involved in the dissolution of ionic compounds and understanding them can help us determine the overall thermochemical nature of the reaction. 

       

      Steps: 

      1. Separation of Water Molecules: 
        The intermolecular forces which hold together water molecules; hydrogen bonds, dipole-dipole interactions, and dispersion forces, are overcome to separate water molecules from one another. This stage is typically endothermic.

      2. Dissociation of Ionic Compounds:
        Ionic compounds separate into individual ions, requiring the overcoming of lattice energy. Factors like ionic bond strength, ionic radius, and charge influence this energy. This step is also typically endothermic.  Steps 1 and 2 occur simultaneously. 


      3. Hydration of Ions:
        The free cations and anions interact with water molecules, forming ion-dipole forces. This step is typically exothermic. 

       

       

       

      Overall Thermochemical Nature:

      The overall enthalpy change of dissolution depends on the sum of the individual enthalpy changes of the three steps above and varies among different substances. If the total energy which is absorbed in steps 1 and 2 are greater than that which is released in step 3, the reaction would be considered to be endothermic. If the energy released in step 3 is greater than the energy absorbed in steps 1 and 2, the reaction would be considered to be exothermic. 


      Combustion

      $$CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l)$$

      In combustion reactions between a fuel and an oxidiser (like oxygen), reaction can be either complete (producing `CO_2` and water) or incomplete (producing `CO` and soot). Combustion involves both bond breaking (endothermic process) and bond forming (exothermic process). Because more energy is released in the bond formation process than is absorbed in the bond breaking process, combustion reactions are exothermic. 

       

      Acid-Base Neutralisation

      Acid-base reactions involve hydrogen ion transfer, typically forming water and a salt. For example: 

      $$HCl(aq) + NaOH(aq) \rightarrow H_2O(l) + NaCl(aq)$$

      Like combustion reactions, more energy is released from the bond forming process than the bond breaking process. Thus neutralisation reactions are overall exothermic. 

       

      Previous Section Enthalpy Change, Activation Energy, Catalysts & Temperature

      Next Section: ΔH of Combustion and Neutralisation, Calorimetry

       

       

      BACK TO MODULE 4: DRIVERS OF REACTION