Question

Heat of neutralization of NaOH and HCl is -57.46kJ/equivalent. The heat of ionization of water in kJ/mol is:
(A) -57.46
(B) +57.76
(C) 114.92
(D) +114.92

Answer 1

Hint: Neutralization reaction and ionization reaction are opposite reactions. If in one type energy is released then in the other type the energy is absorbed. Also neutralization reaction is related to the acid base concept.


Complete step by step solution:
The enthalpy change that takes place when one equivalent of an acid and one equivalent of a base undergo neutralization to form water and salt. To neutralize the acid an equal amount of base has to be added. During the titration, the point of neutralization is also called the point of equivalence. In this reaction there will be no extra protons or hydroxide ions after the reaction. The pH after the neutralization will be equal to 7. Here energy is released when a water molecule is formed. The reaction is carried out in standard conditions.
The heat released (Q) is given by,
$Q = m{c_p}\vartriangle T$
Here$m$ = mass of the solution
${c_p}$ = specific heat capacity of the solution,
$\vartriangle T$= temperature change observed during the reaction.
Using this equation the standard enthalpy change $\vartriangle H$ is obtained by the following equation.
$\vartriangle H = - \dfrac{Q}{n}$
Where n = number of moles.
When a strong acid reacts with a strong base, salt and water is formed. The reaction is as follows:
$HA + BOH \to AB + HOH$
As the acid and base are fully dissociated, the cation and anion are involved in neutralization reaction. The enthalpy change for this reaction is -57.62kJ/mol. Here, energy is released as a bond between proton and hydroxide ion is formed.
Enthalpy of ionization is the energy required to convert a neutral species to charged ions. This is an endothermic reaction as energy is required to break a bond. Water after getting ionized will be converted to protons and hydroxide ions. This process is exactly opposite to that of neutralization. Thus the magnitude of enthalpy will be constant. The energy released in case of neutralization will be the energy absorbed in the ionization reaction.


Thus the enthalpy of ionization is +57.47kJ/mol.


Note: The number of equivalents is equal to the product of number of moles and valence factor. In this example the valence factor is one. Thus the number of equivalents is equal to the number of moles.