Question

The heat of neutralisation of \(\mathrm{HCl}\) and \(\mathrm{NaOH}\) is \(13.7 \mathrm{kcal}\) and that of \(\mathrm{NaOH}\) and \(\mathrm{CH}_{3} \mathrm{COOH}\) is \(13.2\) kcal. Heat of ionisation of \(\mathrm{CH}_{3} \mathrm{COOH}\) is (a) \(13.3 \mathrm{kcal}\) (b) \(13.7 \mathrm{kcal}\) (c) \(0.5 \mathrm{kcal}\) (d) none of these

Short answer

The heat of ionisation of \(\mathrm{CH}_{3} \mathrm{COOH}\) is 0.5 kcal.

Step-by-step solution

Understand the Given Information

The heat of neutralisation of \(\mathrm{HCl}\) and \(\mathrm{NaOH}\) is -13.7 kcal. Heat of neutralisation of \(\mathrm{NaOH}\) and \(\mathrm{CH}_{3} \mathrm{COOH}\) is -13.2 kcal.

Use The Formula For Heat Of Ionization

The heat of ionization of \(\mathrm{CH}_{3} \mathrm{COOH}\) can be determined by subtracting the heat of neutralization of the strong acid and strong base from the heat of neutralization of the weak acid. So, \(\Delta H_{ionization} = \Delta H_{neutralization_{weak}} - \Delta H_{neutralization_{strong}}\)

Substitute The Values Into The Formula

Substituting the values into the formula, gives \(\Delta H_{ionization} = -13.2 - (-13.7) = 0.5 \) kcal.

Key concepts

Heat of Neutralisation

Heat of neutralisation is a term used in thermochemistry to describe the heat released when an acid and a base react to form water and a salt. This reaction is fundamental to acid-base chemistry, and the heat evolved, or absorbed, provides insight into the strength of the reacting acid and base.

For a strong acid reacting with a strong base, the heat of neutralisation tends to be a constant value, typically around 13.7 kcal/mol for reactions occurring at standard conditions. However, when a weak acid or a weak base is involved, the heat of neutralisation is usually less than this standard value since additional energy is consumed in dissociating the weak acid or weak base.

In the context of the given exercise, the difference in heat of neutralisation between the reaction involving strong acid-strong base (HCl and NaOH) and the reaction with a weak acid (CH3COOH) and strong base (NaOH) is attributed to the heat of ionisation of the weak acid. Knowing this allows us to calculate the additional energy involved in the ionisation process of the weak acid.

Thermochemistry

Thermochemistry focuses on the energy changes that occur during chemical reactions, particularly the transfer of heat. It is rooted in the first law of thermodynamics, which states that energy cannot be created or destroyed, only transformed from one form to another. In chemical processes, the quantities of heat exchanged with the environment are essential for understanding reaction energetics.

For example, when an acid reacts with a base, the thermochemical aspect involves the measurement of the heat of neutralisation. The change in heat, or enthalpy change (ΔH), of a reaction can tell us whether the process is exothermic (releasing heat) or endothermic (absorbing heat).

Discussions on thermochemistry are pivotal for students to understand why certain reactions occur spontaneously, and others require additional energy. The exercise provided exemplifies how thermochemistry is used in calculating heat changes in acid-base reactions, thereby allowing us to deduce properties like the heat of ionisation.

Acid-Base Reactions

Acid-base reactions are chemical processes where acids donate protons to bases. In the context of our exercise, HCl, a strong acid, donates a proton to NaOH, a strong base, resulting in the formation of water and sodium chloride. Similarly, acetic acid (CH3COOH), a weak acid, donates a proton to NaOH, forming water and sodium acetate.

The heat of neutralisation for strong acid-strong base reactions is more significant than that for weak acid strong base reactions. This is because strong acids and bases fully dissociate in water, releasing more energy upon their complete reaction. Conversely, weak acids only partially dissociate; thus, not all the available energy is released during neutralisation.

Understanding these reactions allows students to grasp key concepts like the differences between strong and weak acids/bases and the implications for reaction enthalpy. This knowledge is not only critical for solving chemistry problems but also helps in predicting the outcomes of acid-base interactions, which are ubiquitous in both laboratory and real-world scenarios.