Problem 4 - Chemistry in 30 days

Question

Heat of neutralization of an acid with a base is \(13.7 \mathrm{kcal}\) when (a) both acid and base are weak (b) acid is weak and base is strong (c) acid is strong and base is weak (d) both acid and base are strong

Step-by-Step Solution

Answer

(d) both acid and base are strong.

Step 1: Understand Heat of Neutralization

Heat of neutralization is the amount of heat energy released when one equivalent of an acid reacts with one equivalent of a base to form water and a salt.

Step 2: Identify Type of Acid and Base

Read the options carefully: (a) both weak, (b) weak acid, strong base, (c) strong acid, weak base, (d) both strong. Heat of neutralization is maximum when both the acid and base are strong because the reaction is entirely in the aqueous phase with complete ionization.

Step 3: Apply Concept of Strong Electrolytes

For strong acids and bases, neutralization involves the complete ionization of both. Thus, the standard heat of neutralization for strong acid and strong base is typically around 13.7 kcal per equivalent, which suggests the reaction goes to completion with the maximum amount of energy release.

Step 4: Evaluate Each Option

Since both the acid and base need to be strong for complete and maximum energy release, we conclude that the correct situation where heat of neutralization is 13.7 kcal is when both acid and base are strong.

Key Concepts

Strong Acids and Bases

Strong acids and bases are substances that dissociate completely in water. This means that when you have a strong acid like hydrochloric acid (HCl) or a strong base like sodium hydroxide (NaOH), they fully separate into their ions when dissolved.

This complete dissociation is why these substances are referred to as strong electrolytes.

• For strong acids: They release a high concentration of hydrogen ions (H⁺) in solution.
• For strong bases: They release a high concentration of hydroxide ions (OH⁻) in solution.

The complete ionization leads to the release of a significant amount of heat during neutralization reactions, contributing to the high heat of neutralization value of 13.7 kcal which happens when both the acid and base involved are strong.

Ionization

Ionization is the process where atoms or molecules gain or lose electrons, becoming ions. In the context of acids and bases, ionization refers specifically to the dissociation of these substances into ions in water.

When you have strong acids and bases, their complete ionization means that they split completely into ions. For acids, this process can be represented as: \[ ext{HA} ightarrow ext{H}^+ + ext{A}^- \] And for bases: \[ ext{BOH} ightarrow ext{B}^+ + ext{OH}^- \] The complete ionization for strong acids and bases ensures that there are ample ions available to react, which is why we observe a significant release of energy, or heat, during neutralization.

Neutralization Reactions

Neutralization reactions occur when an acid reacts with a base to produce water and a salt. This type of reaction is an exothermic process, meaning it releases heat.

The typical equation for a neutralization reaction is: \[ ext{H}^+ + ext{OH}^- ightarrow ext{H}_2 ext{O} \] For strong acids and bases, the neutralization reaction happens quickly and with the complete ionization of reactants. This efficiency in transforming reactants to products, and the fact that it occurs completely in aqueous solution, leads to the maximum heat release observed as 13.7 kcal. During the neutralization, the

• H⁺ ions from the acid combine with OH⁻ ions from the base, forming water.
• Salt is formed from the remaining ions left in the solution.

The precise measurement of heat released in such reactions helps chemists understand the energy dynamics at play with different strengths of acids and bases.