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

In summary, two pH curves are sketched for a weak acid-strong base titration and a strong acid-strong base titration. They share similarities such as low starting pH, rapid pH increase near the equivalence point, and leveling off after the equivalence point. Differences include the initial pH, rate of pH increase, shape of the curve, and pH at the equivalence point. These differences arise from the properties of weak and strong acids concerning their dissociation and their interaction with strong bases during the titration process.

Step-by-step solution

Understand the behavior of weak/strong acids and bases during titration

In titration, a solution of known concentration (the titrant) is added to a solution of unknown concentration (the analyte) to determine the concentration of the analyte. The point at which the analyte has been completely neutralized by the titrant is known as the equivalence point. In our exercise, we have two scenarios: weak acid titrated with a strong base, and strong acid titrated with a strong base. Weak acids do not ionize completely, meaning only a small percentage of the acid molecules release hydrogen ions (H+) during the reaction. As a result, the pH at the equivalence point of the titration involving a weak acid will be more than 7 due to the presence of the conjugate base produced. On the other hand, strong acids ionize completely, releasing all their hydrogen ions during the reaction. This results in a pH at the equivalence point that is exactly 7.

Sketch the pH curves for the two titrations

First, let's sketch the curve for a weak acid with a strong base: 1. At the beginning, the pH is low due to the weak acid, but it is not as low as for a strong acid. 2. As the strong base is added, the pH increases slowly at first and then more rapidly as the equivalence point approaches. 3. At the equivalence point, the pH will be greater than 7 because the weak acid is neutralized by the strong base and a leftover conjugate base is produced, which is alkaline. 4. After the equivalence point, the curve becomes more horizontal as the excess strong base causes the pH to level off at a higher value. Now, let's sketch the curve for a strong acid with a strong base: 1. At the beginning, the pH is very low due to the strong acid. 2. As the strong base is added, the pH increases rapidly towards the equivalence point. 3. At the equivalence point, the pH is exactly 7 as the strong acid is neutralized by the strong base, leaving a neutral solution. 4. After the equivalence point, the curve becomes more horizontal as the excess strong base causes the pH to level off at a higher value.

Compare the two pH curves

Let's now analyze the similarities and differences between the two curves. Similarities: 1. Both curves start at a low pH and end at a high pH. 2. Both curves show a rapid pH increase near the equivalence point. 3. Both curves become more horizontal after the equivalence point. Differences: 1. The pH curve for a weak acid with a strong base starts at a higher pH than the curve for a strong acid with a strong base because weak acids are less acidic than strong acids. 2. The weak acid-strong base curve has a more gradual initial increase in pH as compared to the strong acid-strong base curve. 3. The pH at the equivalence point for the weak acid-strong base titration is greater than 7, while it is exactly 7 for the strong acid-strong base titration. 4. The weak acid-strong base curve has a gentle sigmoidal shape whereas the strong acid-strong base curve has a steep sigmoidal shape.

Explain the similarities and differences

The similarities between the two curves are due to the common process of titration and the addition of a strong base to neutralize the acid. This causes an increase in pH and leveling off after the equivalence point. The differences between the curves can be explained by the different properties of strong and weak acids. Weak acids do not dissociate completely in a solution, so the initial pH is higher and the increase in pH is more gradual. At the equivalence point, the weak acid has been completely neutralized by the strong base, but the leftover conjugate base, being basic, causes the pH to be greater than 7. Meanwhile, a strong acid dissociates completely, resulting in a lower initial pH and a faster increase in pH. At the equivalence point, the strong acid and strong base neutralize each other, leaving a neutral solution with a exactly 7 pH.

Key concepts

Weak Acid Titration

When performing a weak acid titration, you start with a weak acid, which does not fully ionize in solution. As a result, only a fraction of its molecules release hydrogen ions \((H^+)\). This partial ionization means that the solution is only mildly acidic to start.
As you gradually add a strong base, the pH begins to rise. Initially, the increase is slow because the weak acid resists changes in pH due to its incomplete ionization.
As you approach the equivalence point, the pH climbs more quickly. At this stage, all the weak acid has been neutralized. The resulting solution contains its conjugate base, which has alkaline properties.
The pH at the equivalence point will be greater than 7, a characteristic of weak acid titrations with a strong base. Once past the equivalence point, the curve levels as excess strong base dominates the solution.

Strong Acid Titration

In a strong acid titration, the starting solution contains a strong acid, which completely ionizes, releasing all of its hydrogen ions. This leads to a very low initial pH.
As you add a strong base, the pH increases rapidly because the strong acid reacts easily with the base. There is little resistance to pH changes, unlike with a weak acid.
Upon reaching the equivalence point, the strong acid has been completely neutralized. This results in a solution with a pH of exactly 7, indicating a neutral water solution.
After the equivalence point, the pH levels off, similar to weak acid titrations, as the excess base governs the pH.

Equivalence Point

The equivalence point in titration is where the amount of titrant added perfectly neutralizes the analyte in the solution. For strong acids titrated with strong bases, this occurs when \(H^+\) ions from the acid perfectly counter the \(OH^-\) ions from the base.
In this scenario, the equivalence point results in a neutral solution, with a pH of exactly 7.
However, for a weak acid titrated with a strong base, the equivalence point is different. The weak acid is fully neutralized, but the conjugate base remains. Therefore, the pH rises above 7 at equivalence.
This occurs due to the basic nature of the conjugate base, making the solution on the alkaline side. Despite differing pH levels, in both cases, at equivalence point, the reaction between acid and base is complete.

Acid-Base Titration

Acid-Base titration involves the careful addition of a known acid or base to accurately determine the concentration of an unknown solution.
It requires measuring the pH change as the titrant is added. Plotting pH as a function of titrant volume results in a pH curve, crucial for identifying the equivalence point.
For educational purposes, it simplifies understanding how acids and bases interact. It demonstrates the neutralization process, resulting in salt and water.

• Titration of strong acids and bases reflects fast pH changes near the equivalence point due to the complete dissociation of ions.
• Weak acid titration shows gradual pH changes because of partial dissociation.

Strong Base

Strong bases like sodium hydroxide \((NaOH)\) dissociate completely in water, releasing hydroxide ions \((OH^-)\). These ions readily react with hydrogen ions \((H^+)\), significantly impacting the pH of a solution.
As a titrant, strong bases are highly effective because they swiftly neutralize both weak and strong acids. After the equivalence point in a titration, any additional strong base will cause a rapid increase in pH.
Due to their complete ionization, strong bases ensure a clear depiction of neutralization on a pH curve by creating a steep rise near the equivalence point for strong acid titrations.
In weak acid scenarios, they similarly define the equivalence point beyond which the solution becomes more alkaline, exhibiting a less steep curve as compared to strong acids.