Question

Sketch two pH curves, one for the titration of a weak acid with a strong base and one for a strong acid with a strong base. How are they similar? How are they different? Account for the similarities and the differences.

Short answer

The pH curves for the weak acid-strong base titration and strong acid-strong base titration both start at low pH values and increase during titration. However, the weak acid-strong base titration curve is more gradual and has an equivalence point with a pH greater than 7, while the strong acid-strong base titration has a steeper rise near the equivalence point and an equivalence point with a pH of 7. The differences in the curves are due to the weak or strong nature of the acids, their buffering capacities, and the products formed at the equivalence points.

Step-by-step solution

Initial pH

For both titrations, the initial pH of the solution is determined by the starting acid. 1.1 Weak Acid with Strong Base: For a weak acid, the initial pH is higher than that of a strong acid. This is because a weak acid does not completely ionize in the solution, meaning that there are fewer free H+ ions, resulting in a higher pH. 1.2 Strong Acid with Strong Base: For a strong acid, the initial pH is lower than that of a weak acid. This is because a strong acid completely ionizes in the solution, leading to more free H+ ions and lower pH.

pH during titration

The pH during the titration depends on the amount of added base and the buffering capacity of the respective acids. 2.1 Weak Acid with Strong Base: As we add a strong base to the weak acid, the pH increases slowly near the equivalence point, resulting in a gradual and curved shape of the pH curve. The curve has a more gradual rise and is more buffered due to the presence of the weak acid and its conjugate base present in the solution. 2.2 Strong Acid with Strong Base: When titrating a strong acid with a strong base, the pH changes rapidly around the equivalence point, creating a much steeper curve. Before reaching the equivalence point, the pH increases slowly, similar to the titration with a weak acid and strong base. However, the rapid increase in pH near the equivalence point indicates that the buffering capacity of the strong acid and strong base is limited and not as effective as that in the weak acid and strong base titration.

Equivalence point

The equivalence point is the point at which the moles of the added base are equal to the moles of the initial acid in the solution. 3.1 Weak Acid with Strong Base: For a weak acid with strong base titration, the equivalence point will have a pH greater than 7 due to the hydrolysis of the conjugate base formed in the reaction. 3.2 Strong Acid with Strong Base: The equivalence point for a strong acid with strong base titration falls at pH 7, as the reaction produces a neutral salt and water.

Final pH

The final pH depends on the excess base or acid in the solution after the titration. 4.1 Weak Acid with Strong Base: For the titration of a weak acid with a strong base, the final pH will be higher than that of a strong acid with a strong base titration, as there will be an excess of the strong base in the solution. 4.2 Strong Acid with Strong Base: For the titration of a strong acid with a strong base, the final pH will be lower than that of a weak acid with a strong base titration due to the excess strong base present in the solution.

Summary of similarities and differences

Similarities: - Both titrations start at low pH value, indicating an acidic environment. - Both titrations see the pH increasing gradually before reaching the equivalence point. - Both titrations result in an increase of pH during the titration process. Differences: - The initial and final pH values are different due to the weak or strong nature of the acid. - The curve of a weak acid with a strong base titration is more gradual, while the strong acid with strong base titration has a steeper rise near the equivalence point. - The equivalence point for a weak acid with a strong base titration has a pH greater than 7, whereas the equivalence point for a strong acid with a strong base titration has a pH of 7.