Question

Explain whether the equivalence point of each of the following titrations should be below, above, or at pH 7: (a) \(\mathrm{NaHCO}_{3}(\text { aq) titrated with } \mathrm{NaOH}(\mathrm{aq}) ; \text { (b) } \mathrm{HCl}(\mathrm{aq})\) titrated with \(\mathrm{NH}_{3}(\mathrm{aq}) ;\) (c) KOH(aq) titrated with HI(aq).

Short answer

(a) Above 7, (b) Below 7, (c) At 7

Step-by-step solution

Identify the Acid-Base Pairs

Look at each pair and evaluate if they are strong or weak acids or bases. For (a) NaHCO3 (a weak base) is being titrated with NaOH (a strong base), for (b) HCl (a strong acid) is titrated with NH3 (a weak base), and for (c) KOH (a strong base) is titrated with HI (a strong acid).

Determine the pH at Equivalence Point for Each Pair

The pH at the equivalence point for a titration between a strong acid and a strong base is 7. If the base is weak and the acid is strong, the pH will be less than 7, and if the acid is weak and the base is strong, the pH will be more than 7. Applying this understanding to each pair, (a) should have a pH above 7 because it is a strong base with a weak base, (b) should have a pH below 7 because it is a strong acid with a weak base, and (c) should have a pH of 7 because it is a strong acid with a strong base.

Key concepts

Acid-Base Titration

Acid-base titration is a technique used to determine the concentration of an unknown acid or base by reacting it with a known concentration of a titrant. The titrant is gradually added to the unknown solution until the reaction reaches an equivalence point, where the number of moles of acid equals the number of moles of base added.
The main goal is to find this equivalence point, which can be detected using pH indicators for weak acids and bases, or a pH meter for more precision.

• During a titration, the pH changes from one extreme to another, and the point at which it rapidly changes indicates the equivalence point.
• Indicators are substances that change color at a certain pH level, showing that the equivalence point has been reached.
• In many cases, titration curves can be plotted by measuring the pH of the solution as the titrant is added, providing a visual representation of the process.

Understanding how titrations work is crucial for calculating the unknown concentration and determining the pH changes during the procedure.
This procedure is very common in analytical chemistry and medical laboratories for various applications.

Weak and Strong Acids/Bases

In chemistry, acids and bases can be categorized as either weak or strong, which refers to their ability to dissociate in water.
Strong acids and bases dissociate completely, releasing more hydrogen ions (H⁺) for acids or hydroxide ions (OH^-) for bases.
Weak acids and bases, on the other hand, only partially dissociate.

• For a strong acid, all the molecules break apart completely in solution. An example of this is hydrochloric acid (HCl).
• A strong base like sodium hydroxide (NaOH) also dissociates completely, providing all its ions readily available in water.
• Conversely, a weak acid, such as acetic acid, only partially dissociates, which means not all of its molecules release H⁺ ions.
  Similarly, a weak base like ammonia (NH₃) partially accepts H⁺ in water, making it less effective.

Knowing whether a substance is a weak or strong acid or base helps predict the pH of a solution at the equivalence point.
It also informs the selection of appropriate indicators for titration.

pH at Equivalence Point

The pH at the equivalence point of a titration depends on the nature of the acid and the base involved in the reaction. The equivalence point is the stage in the titration when the total number of moles of the acid equals the total number of moles of the base.
However, the pH at this point is not always neutral (pH 7); it varies according to the strength of the acids and bases being titrated.

• In a titration involving a strong acid and a strong base, the equivalence point is typically at pH 7, due to the complete neutralization of H⁺ and OH^- ions.
• For a strong acid with a weak base, the equivalence point pH will be less than 7. This is because, at the equivalence point, the conjugate acid of the weak base will hydrolyze, making the solution slightly acidic.
• Conversely, when a weak acid reacts with a strong base, the pH at the equivalence point is more than 7, as the conjugate base of the weak acid will make the solution slightly basic.

Understanding what dictates the pH at the equivalence point aids in determining the endpoint and selecting the correct indicator for the titration.
This is vital in accurately completing titration procedures and understanding the chemical interactions at play.