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

Both titrations show sigmoidal pH curves, with the weak acid-strong base titration having a higher initial pH value, gentler slope, and higher final pH value after equivalence, compared to the strong acid-strong base titration. These differences are attributed to the partial ionization of weak acids, rapid reaction of strong acids with strong bases, and the formation of a basic conjugate base in weak acid titrations. The pH at the equivalence point is around 7 for strong acid-strong base titrations and slightly higher for weak acid-strong base titrations due to the presence of the basic conjugate base.

Step-by-step solution

Titration of a weak acid with a strong base

For the titration of a weak acid with a strong base, the pH curve exhibits a sigmoidal shape. This is due to the weak acid only partially dissociating in solution, thus not all of its acidic protons are released. However, the strong base will completely dissociate, therefore reacting with the acidic protons present in the solution. To sketch this curve, follow these steps: 1. Draw the x-y axis, with the x-axis representing the volume of the strong base added and the y-axis representing the pH of the solution. 2. Begin by plotting an initial point with a low pH value due to the weak acid (e.g., pH around 3-4). This is the starting point before any base is added. 3. As the strong base is added, the curve will rise gradually, representing the base reacting with the acidic protons and increasing the pH of the solution. 4. Once near equivalence, the pH curve will rise sharply upward, reflecting the rapid consumption of the remaining acidic protons. Equivalence is reached when the moles of acidic protons equal the moles of hydroxide ions added. 5. After reaching equivalence, the pH curve will level off and maintain a high pH value (e.g., pH around 9-10) due to the presence of the strong base.

Titration of a strong acid with a strong base

In the case of titrating a strong acid with a strong base, the pH curve will also exhibit a sigmoidal shape, but with some differences. Both strong acid and strong base complete dissociate in the solution, contributing more protons (from the acid) and hydroxide ions (from the base). To sketch this curve, follow these steps: 1. Draw the x-y axis with the same labels as before, representing the volume of the strong base added and pH of the solution. 2. Begin by plotting an initial point with a low pH value, similar to the weak acid case, but lower due to the strong acid (e.g., pH around 1-2). 3. As the strong base is added, the curve will now rise more rapidly than the weak acid case, representing the higher concentration of acidic protons being neutralized by the hydroxide ions from the strong base. 4. The curve will steeply rise near equivalence when the remaining acidic protons are neutralized. 5. After reaching equivalence, the pH curve will level off and maintain a pH value of around 7. This is due to the formation of water from the strong acid and base.

Comparing the curves

Both pH curves have a sigmoidal shape as they rise gradually before dramatically increasing at equivalence, then leveling off after the equivalence point. However, the main differences are: 1. The initial pH of the weak acid is higher than that of the strong acid. 2. The reaction between weak acid and strong base causes the pH to increase more gradually compared to strong acid-strong base titration. This is because the weak acid only partially ionizes in water, while the strong acid completely ionizes and thus reacts more rapidly with the strong base. 3. The curve for the weak acid titration reaches a higher final pH value (e.g., around 9-10) because the remaining weak acidic protons form a basic conjugate base. 4. The pH at the equivalence point of the strong acid-strong base titration is around 7, while for weak acid-strong base titration, it is slightly higher. This is due to the presence of the basic conjugate base formed in the weak acid titration. In conclusion, the similarities between the pH curves are their sigmoidal shapes, while the differences lie in the initial pH values, the steepness of the curve, and the final pH values after the equivalence point. These differences are mainly due to the ionization behavior of weak vs. strong acids and the formation of conjugate base after titration.