Enthalpy of Neutralisation Theory

This is part of the HSC Chemistry course under the topic Properties of Acids and Bases

HSC Chemistry Syllabus

  • Conduct a practical investigation to measure the enthalpy of neutralisation (ACSCH093)

Enthalpy Change of Acid-base Reactions

Enthalpy change of neutralisation (acid-base) reactions is the molar change in energy. 

This video explains why neutralisation reactions are exothermic. In particular, why the enthalpy change depends on the strength of acid and base involved in the reaction.

 

Why is Neutralisation Exothermic?

Neutralisation reactions are exothermic because the energy released from the formation of water is greater than the energy absorbed in the reaction.

Recall that

  • Breaking chemical bonds is endothermic (absorbs energy), and forming chemical bonds is exothermic (releases energy). 

    Enthalpy Change of Neutralisation Between Strong Acids & Bases

    Neutralisation involving strong acids and strong bases have the same enthalpy change (∆H).

    This is because

    • Strong acids and strong bases are completely dissociated so no chemical bonds will be broken 
    • Energy is released from the formation of O–H bond in water molecules. 

    For example, the neutralisation between hydrochloric acid (HCl) and sodium hydroxide (NaOH):

     

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

     

    Both HCl and NaOH solutions contain completely dissociated HCl and NaOH. When they are mixed, the hydrogen ions will react with hydroxide ions without breaking any bonds. When a water molecule is formed, energy is released, causing the neutralisation to be exothermic.

     

       
      This is represented by the net ionic equation: 
       
      $$H^+(aq) + OH^–(aq) \rightarrow H_2O(l) \hspace{1cm} \Delta H = –57.1 \hspace{0.1cm} kJ \hspace{0.1cm} mol^{–1}$$
       
      Since neutralisation is exothermic, the temperature of the resulting solution always increases. The ∆H value suggests 57.1 kJ of energy is released when 1 mole of water molecules is formed. The amount of energy released is proportional to the number of water molecules formed.

        Enthalpy Change of Neutralisation Involving Weak Acids

        The enthalpy change of neutralisation involving weak acids are smaller in magnitude compared to that between strong acids and strong bases.

        This is because weak acids/bases are partially dissociated, which means not all hydrogen ions are free to react with hydroxide ions. Some energy is required to break the chemical bonds in weak acid molecules to produce hydrogen ions.

        For example, at equilibrium, only a small amount of acetic acid molecules are dissociated. Most hydrogen ions are still bound to acetic acid molecules. The forward reaction (dissociation) is endothermic as denoted by its positive ∆H. 

         

          $$CH_3COOH(aq) \leftrightharpoons H^+(aq) + CH_3COO^-(aq) \hspace{1cm} \Delta H = + 1.0\hspace{0.1cm} kJ \hspace{0.1cm} mol^{–1}$$

           

          When a solution of NaOH is added, the hydroxide ions cause more acetic acid molecules to be deprotonated:

          $$CH_3COOH(aq) + OH^-(aq) \rightarrow H_2O(l) + CH_3COO^-(aq) \hspace{1cm} \Delta H = -56.1 \hspace{0.1cm} kJ \hspace{0.1cm} mol^{–1}$$

           

          The overall enthalpy of this neutralisation reaction is less negative compared to the one between HCl and NaOH because 1.0 kJ is absorbed for every mole of acetic acid deprotonated.

            

          BACK TO MODULE 6: ACID/BASE REACTIONS