Problem 40 - Chemistry: An Atoms-Focused Approach

Question

Phenolphthalein can be used as a color indicator to detect the equivalence points of titrations of samples containing either weak acids or strong acids, even though the pH values of the equivalence points vary depending on the identity of the acid. Explain how this is possible.

Step-by-Step Solution

Answer

Answer: Phenolphthalein can be used as a color indicator to detect the equivalence points of titrations of samples containing either weak acids or strong acids because it undergoes a sharp color change between pH values of 8.2 and 10. This wide range of pH values for the color change allows phenolphthalein to detect the equivalence point in titrations with varying pH values, regardless of whether the acid is weak or strong.

Step 1: Define equivalence point

The equivalence point in a titration is the point at which the amount of added titrant is chemically equivalent to the amount of analyte in the sample. In the context of acid-base titrations, this means that the moles of acid in the sample are equal to the moles of base added (or vice versa). At this point, the pH of the solution may vary depending on the strength of the acid/base involved, but the reaction between the acid and the base is complete.

Step 2: Understand the concept of pH indicators

A pH indicator is a substance that changes its color depending on the pH of the solution. This property can be used to visually detect the equivalence point in acid-base titrations by observing the color change of the indicator. To be effective, the pH indicator's color change must occur around the pH of the equivalence point for the acid and base being titrated.

Step 3: Explain the color change of phenolphthalein

Phenolphthalein is a pH indicator that is colorless in acidic solutions and turns pink in basic solutions. It undergoes a color change between pH values of about 8.2 (colorless) and 10 (pink). This means that phenolphthalein is useful for detecting the equivalence point in titrations involving weak acids and strong bases, as the pH of the solution will be in the range where phenolphthalein undergoes a color change when the reaction is complete.

Step 4: Explain how phenolphthalein can be used with different acids

Even though the pH values of the equivalence points for weak acids and strong acids vary, phenolphthalein can still be used effectively as a color indicator in both cases. This is because the color change of phenolphthalein occurs over a range of pH values, allowing it to detect the equivalence point in titrations with varying pH values. The sharp color change of phenolphthalein in the pH range of 8.2-10 allows a clear visual indication of when the equivalence point is reached, regardless of whether the acid is weak or strong.

Key Concepts

Equivalence Point

In the fascinating world of chemistry, titrations help us determine the composition of unknown substances.
One important concept in this process is the equivalence point. This is the magical moment when the amount of titrant added fully reacts with the substance being analyzed.
Specifically, in an acid-base titration, it is the point where the moles of acid equal the moles of base. At the equivalence point, the chemical reaction is complete. The composition of the solution is neutralized, meaning there’s neither excess acid nor excess base.

• For strong acid and strong base titrations, the equivalence point will occur at pH 7.
• For weak acid and strong base titrations, the equivalence point will occur at a pH greater than 7.
• For weak base and strong acid titrations, it will occur at a pH less than 7.

Understanding the equivalence point helps in customizing titration setups depending on the acids and bases involved.

pH Indicators

pH indicators play a crucial role in titrations by signaling the equivalence point.
They are substances that change their color based on the pH level of the solution.
They provide a visible cue that the equivalence point has been reached. Each pH indicator works within a specific range, which needs to match the expected pH at the equivalence point for them to be effective. Some widely used pH indicators include:

• Phenolphthalein: Changes from colorless to pink, best for pH 8.2 to 10.
• Methyl Orange: Changes from red to yellow, ideal for pH 3.1 to 4.4.
• Bromothymol Blue: Changes from yellow to blue, works best around pH 6.0 to 7.6.

Choosing the right indicator is key to accurate results. Understanding pH indicators makes titrations more accurate and visually clear.

Acid-Base Titrations

Acid-base titrations are a fundamental technique in chemistry used to find concentrations and compositions.
It involves the careful addition of an acid or base to an unknown solution to reach the equivalence point.
The process consists of:

• Choosing the reagents: Typically, a base is titrated against an acid or vice-versa.
• Selecting a suitable pH indicator: Ensures color change aligns with the equivalence point pH.
• Observing color changes: Indicates that the reaction has reached its equivalence point.

For example, phenolphthalein is a great indicator for titrations involving weak acids and strong bases due to its color change at pH 8.2-10. Even though different acids have different equivalence points, phenolphthalein’s range allows it to work effectively. Executing precise acid-base titrations can unveil important properties of substances, making it an essential skill in chemical analysis.