Question

Heat of neutralisation of a strong acid and weak base is less than \(57.3 \mathrm{~kJ} \mathrm{~mol}^{-1}\). This is because (a) all the acid is not ionised (b) all the base is not ionised (c) the ionisation is complete but further reaction does not take place (d) some of the heat is transferred to the surroundings

Short answer

The correct answer is (b) not all the base is ionised.

Step-by-step solution

Understand the concept of neutralisation

A neutralisation reaction occurs when an acid reacts with a base and produces a salt and water. The heat released during this reaction is called the heat of neutralization. When a strong acid reacts with a strong base, the heat of neutralisation is always -57.1 kJ/mol because the reaction is complete and results in the formation of water in a very exothermic reaction.

Analyse the given alternatives

Let's break down each of the given alternatives. Option a suggests that not all the acid is ionised. But the problem statement mentions that it's a strong acid. By definition, strong acids are completely ionised in solution. Hence, option a is incorrect. Now let's explore option b - not all the base is ionised. A weak base does not ionise completely in solution. So, this option seems possible. Option c - complete ionisation but no further reaction. This goes against the fundamental principle of neutralization reaction where heat is generated. So, this is not correct. Option d suggests that some heat is transferred to the surroundings. This is true for all reactions but doesn't account for the heat of neutralisation being less.

Final conclusion

From the analysis, option b seems to be the most valid choice. Not all of the weak base is ionised in the solution, which means that not all base molecules can react with acid molecules. This would cause less heat to be released than the standard heat of neutralization for a strong acid and strong base reaction.

Key concepts

Neutralization Reaction

In a neutralization reaction, an acid and a base combine to form water and a salt. This type of chemical reaction is fundamental in chemistry. A classic example of this process is mixing hydrochloric acid (HCl) with sodium hydroxide (NaOH) to produce water (H₂O) and sodium chloride (NaCl). When these reactants mix, they neutralize each other, meaning they cancel each other’s properties. The result is a solution that is closer to a neutral pH, which is around 7. During this reaction, heat is also typically released, which is known as the heat of neutralization.

• Strong Acid + Strong Base = Heat released is consistent and well-defined.
• Strong Acid + Weak Base = Heat released is less because the base does not completely ionize.

Understanding this process helps to explain why the heat of neutralization differs based on the strength of the acid and the base involved.

Strong Acids

Strong acids are those that fully dissociate in water, releasing all their hydrogen ions into the solution. This means that in an aqueous solution, nearly 100% of a strong acid's molecules donate protons. Common examples of strong acids include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃). When a strong acid reacts with a base, it strongly increases the hydrogen ion concentration. This complete dissociation is crucial for a high heat release during neutralization, as the more hydrogen ions available to react, the more exothermic the reaction will be.

• Full ionization ensures predictable reaction strength.
• Results in a well-defined amount of heat released when reacting with strong bases.

This concept helps in understanding why strong acids tend to lead to more consistent heat of neutralization compared to weak acids or bases.

Weak Bases

Weak bases are substances that partially dissociate in solution. This means they do not fully donate their hydroxide ions (OH⁻) when dissolved in water, unlike strong bases that dissociate completely. Examples of weak bases include ammonia (NH₃) and methylamine. Because weak bases don’t fully ionize, they have fewer available ions to interact with the hydrogen ions from an acid. This partial ionization is key to understanding why the heat of neutralization is less than expected. Not all base molecules react with acid molecules, resulting in less heat being released.

• Partial ionization leads to less heat being released.
• Causes their reactions to be less exothermic compared to strong bases.

This concept is crucial for understanding scenarios like the strong acid and weak base reaction, which results in a lower heat of neutralization.

Ionization

Ionization refers to the process by which an atom or a molecule acquires a charge by gaining or losing electrons to form ions. In the context of acids and bases, ionization is especially important as it determines how readily these substances can react in solution. For acids, ionization means releasing hydrogen ions ( H⁺ ) into the solution. For bases, it's about releasing hydroxide ions ( OH⁻ ). The degree to which these substances ionize categorizes them as strong or weak. Strong acids and bases fully ionize, meaning the reaction is efficient and the heat of reaction is substantial, as seen in complete neutralization reactions.

• Complete ionization results in predictable and high heat of reaction.
• Partial ionization results in a subdued chemical reaction and less heat production.

Understanding ionization helps explain the variances in reaction enthalpy between different strengths of acids and bases.