Question

The pH at the equivalence point of the titration of a strong acid with a strong base is \(7.0 .\) However, the \(\mathrm{pH}\) at the equivalence point of the titration of a weak acid with a strong base is above \(7.0 .\) Explain.

Short answer

At the equivalence point of a strong acid-strong base titration, the pH is 7 due to the complete neutralization and formation of water. For a weak acid-strong base titration, the pH is above 7 because the weak acid's conjugate base, which remains in the solution, increases the hydroxide ion concentration, making the solution basic.

Step-by-step solution

Understanding the Nature of Strong and Weak Acids

Recognize that strong acids dissociate completely in water to produce a high concentration of hydrogen ions \(H^+\)), while weak acids only partially dissociate and thus produce a lower concentration of \(H^+\)). Meanwhile, strong bases dissociate completely to produce hydroxide ions \(OH^-\)).

Recognizing the Equivalence Point of a Titration

The equivalence point of a titration occurs when the amount of acid equals the amount of base, meaning that all the \(H^+\)) ions have reacted with \(OH^-\)) ions to form water \(H_2O\)).

Explaining pH of Strong Acid with Strong Base at Equivalence Point

In the case of a strong acid and strong base titration, the \(H^+\)) ions from the acid and the \(OH^-\)) ions from the base react completely to form water. Since pure water has a pH of 7, the solution is neutral at the equivalence point.

Explaining pH of Weak Acid with Strong Base at Equivalence Point

When titrating a weak acid with a strong base, the weak acid partially dissociates, leaving behind its conjugate base in the solution. At the equivalence point, the strong base has neutralized the acid forming water but the conjugate base remains in the solution, which typically reacts with water to create a basic solution \(OH^-\)), thus increasing the pH above 7.

Key concepts

Understanding Acid-Base Titration

Acid-base titration is a method used to determine the concentration of an acid or a base in solution by adding a controlled amount of titrant to a known volume of the analyte until the reaction reaches its equivalence point.

During titration, a pH indicator or pH meter is often employed to detect when the equivalence point is reached. This point signifies that the moles of acid (\( H^+ \) ions) in the solution are stoichiometrically equivalent to the moles of base (\( OH^- \) ions), resulting in a neutralized solution. In practical terms, what you're witnessing in an acid-base titration is a carefully monitored neutralization reaction.

Distinguishing Between Strong and Weak Acids

It's essential to understand the difference between strong and weak acids to grasp the concept of titration. Strong acids, such as hydrochloric acid (HCl), dissociate completely in an aqueous solution, releasing a high concentration of hydrogen ions (\( H^+ \) ions), leading to a lower pH.

Conversely, weak acids, like acetic acid (CH3COOH), do not completely dissociate. Instead, they establish an equilibrium between the undissociated acid and the ions produced, resulting in a higher pH for a given concentration. This fundamental distinction explains why the equivalence point pH is different when titrating with a strong acid compared to a weak acid.

The Importance of pH Scale in Titration

The pH scale is a logarithmic scale used to specify how acidic or basic a water-based solution is. The scale ranges from 0 to 14, with 7 as the neutral midpoint—pure water's pH. Acids have a pH less than 7, while bases have pH greater than 7.

In the context of titration, the pH of the solution gives us crucial information about the progress of the reaction. At the start, the strong acid's low pH reflects its high concentration of hydrogen ions. As the strong base is added, the pH gradually increases until it reaches the equivalence point. For a strong acid and strong base titration, this point precisely corresponds to a neutral pH of 7 since all the ions have reacted to form water.

Role of the Conjugate Base

When you react a weak acid with a strong base, the acid's hydrogen ions (\( H^+ \) ions) are neutralized, while the remainder of the acid molecule—the conjugate base—remains in solution. This conjugate base tends to be a weak base and can react with water to produce hydroxide ions (\( OH^- \) ions), which raises the pH of the solution.

Thus, at the equivalence point of a titration involving a weak acid and a strong base, the solution contains the conjugate base of the weak acid, which gives the solution a pH higher than 7. This is a key factor that distinguishes the equivalence point pH in titrations involving weak acids versus strong acids and explains the occurrence of a basic pH at the equivalence point in such cases.