Enthalpy of Neutralisation

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

Change in Enthalpy of Neutralisation

  • Neutralisation reactions are exothermic. This means the products are more enthalpically stable and have less energy than the reactants.
    • Counterintuitively, breaking bonds absorbs energy, forming bonds releases energy. 
  • Neutralisation involving strong acids and strong bases have the same molar heat of reaction (∆H) because they completely dissociate in water to form the same amount of H+ and OH

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

Net ionic equation for neutralisation: 
 
$$H^+(aq) + OH^–(aq) \rightarrow H_2O(l) \hspace{1cm} \Delta H = –57 \hspace{0.1cm} kJ \hspace{0.1cm} mol^{–1}$$
 
  • The temperature of a neutralisation mixture thus always increases.
  • The enthalpy of neutralisation for the ionisation of weak acids and weak bases differs because of the existence of conjugate acids and bases which are able to donate and accept protons respectively. 

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

 

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

Measuring Heat of Neutralisation 

  •  The molar heat of neutralisation is the enthalpy change when 1 mole of water is formed from the reaction of an acid and a base

  • Assumptions: 
    • No energy is lost to surrounding
    • The specific heat capacity of the solution which is formed from neutralisation is the same as water
    • the ΔH of the dislocations of the acid and base do not contribute to the energy changes which are experienced by the solution

 

  • Heat of neutralisation is calculated using the formula

$$q = mC \Delta T$$

Where: 

  • q = quantity of heat (amount of energy absorbed by the solution in J)
  • m = mass of the final solution (unit depends on c - usually kg)
  • c is the specific capacity of solution
  • ΔT is the change in temperature given in Kelvin (interconvertible with Celsius)

 

Specific Heat Capacity

 

  • The specific heat capacity of a solution is the amount of energy which is required to raise the temperature of a substance by 1K per unit mass

  • Water has a specific heat capacity of 4.18 x 103 J kg-1 K-1

     

    Molar Enthalpy of Neutralisation

    $$q = - \Delta H \times n$$

     Where:

    • ΔH is negative (exothermic)
    • n is the number of moles of water formed 
    • q is positive (energy is absorbed by solution)