Question

Sketch titration curves for the following acid-base titrations: (a) \(\mathrm{HCl}\) versus \(\mathrm{NaOH},\) (b) HCl versus \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) (c) \(\mathrm{CH}_{3} \mathrm{COOH}\) versus \(\mathrm{NaOH}\). In each case, the base is added to the acid in an Erlenmeyer flask. Your graphs should show the \(\mathrm{pH}\) on the \(y\) axis and the volume of base added on the \(x\) axis.

Short answer

Graphs: (a) sharp rise at pH 7; (b) smooth, pH < 7 at equivalence; (c) buffered rise, pH > 7 at equivalence.

Step-by-step solution

Understand the Acid-Base Pair for Each Titration

Identify the type of acid and base for each titration pair: (a) for HCl and NaOH, both are strong, (b) HCl is strong and CH₃NH₂ is weak, (c) CH₃COOH is weak and NaOH is strong.

Sketching the Strong Acid-Strong Base Titration (HCl vs NaOH)

For (a): Start with a low initial pH because HCl is a strong acid. As NaOH is added, the pH increases steadily. At the equivalence point, the pH is 7. Beyond this point, the pH rises rapidly as more alkali is added.

Sketching the Strong Acid-Weak Base Titration (HCl vs CH₃NH₂)

For (b): Begin with a low initial pH. As CH₃NH₂ is added, the pH increases more gradually compared to (a). At the equivalence point, the pH is lower than 7, in the acidic range. After the equivalence point, the pH increases slowly.

Sketching the Weak Acid-Strong Base Titration (CH₃COOH vs NaOH)

For (c): Start with a moderate initial pH since CH₃COOH is a weak acid. As NaOH is added, pH increases, buffering the solution until it reaches the equivalence point. The equivalence point pH is above 7 due to the formation of acetate ions. Beyond the equivalence point, pH increases sharply.

Plotting the Titration Curves

Draw the volume of base added on the x-axis and pH on the y-axis for each titration. For (a), expect a sharp rise at equivalence point at pH 7. For (b), a smooth rise with equivalence point pH below 7. For (c), a buffered rise with equivalence point pH above 7.

Key concepts

Acid-Base Titration

Acid-base titration is a laboratory procedure that determines the concentration of an acid or base using a neutralization reaction. During titration, a solution of known concentration, called the titrant, is gradually added to a solution of unknown concentration until a reaction reaches completion. When the acid and base have completely reacted, the equivalence point is achieved.
The process involves:

• Starting with a solution set with a known volume of acid or base.
• Adding the titrant gradually, while stirring, until the reaction reaches the equivalence point.
• Using indicators or pH meters to detect the endpoint, which ideally is close to the equivalence point.
• Plotting the titration curve, with the volume of titrant on the x-axis and pH on the y-axis, provides a visual representation of the titration process.

The titration curve can give insights into the strength of the acids and bases involved. A steep rise at the equivalence point indicates a strong acid and base pairing, while a more gradual slope might point to the involvement of a weak acid or base.

Equivalence Point

The equivalence point in an acid-base titration is where the quantity of acid equals that of the base added. At this point, the acid is completely neutralized by the base. The position of the equivalence point can vary:

• For a strong acid-strong base titration, like HCl with NaOH, the equivalence point is typically at pH 7.
• If a strong acid and a weak base are titrated, the equivalence point occurs below pH 7, resulting in an acidic solution. For instance, HCl with CH₃NH₂.
• When a weak acid is titrated with a strong base, the equivalence point is above pH 7, resulting in a basic solution, as seen with CH₃COOH and NaOH.

Understanding where the equivalence point lands on the pH scale is crucial in determining the characteristics and outcomes of the titration. It is an essential aspect of plotting and interpreting titration curves.

Strong Acid

Strong acids completely dissociate in water, releasing all their hydrogen ions, equating to a high proton concentration in solution. This complete ionization results in an initial low pH value. Common strong acids include hydrochloric acid (HCl).

• Titration involving a strong acid, such as HCl, begins with a low pH due to the excess of hydrogen ions.
• As a base like NaOH is added, the pH rises quickly.
• The equivalence point for strong acid with a strong base titration will be near pH 7.
• After the equivalence point, any additional base will result in a significant increase in pH.

Due to their complete dissociation, strong acids are predictable in their behavior during titration, with clear and distinct titration curves.

Weak Acid

Weak acids only partially dissociate in water, which means not all of the acid molecules are ionized. This results in a relatively higher initial pH compared to strong acids. Acetic acid (CH₃COOH) is a common example of a weak acid.

• During titration, the initial pH is higher than that of a strong acid.
• The addition of a strong base like NaOH gradually increases the pH.
• The presence of a buffer region, where changes in pH are more gradual, can often be observed before reaching the equivalence point.
• The equivalence point is higher on the pH scale, often above pH 7, due to the formation of weak base conjugates such as acetate ions.

This subtle behavior during titration contrasts with strong acids, making precise calculations crucial in such reactions. Understanding weak acids is vital for interpreting their specific titration curves accurately.