Book cover for Living by Chemistry

Living by Chemistry

Angelica M. Stacy

ISBN #9781464142314

2nd Edition

940 Questions

Group icon
129,188 Students Helped

Homework Questions

Right arrow
Summary
Learning Objectives
Key Concepts
Example Problems
Explanations
Common Mistakes

Summary

This section explains how acid-base indicators such as anthocyanin change color in response to variations in pH by shifting between two forms (HIn and In–). The balance between these forms, dictated by the concentration of H+ or OH– in the solution, results in observable color changes (red, purple, blue). The concept is further linked to reversible processes, where the reaction can shift direction depending on the addition of acid or base, offering both a molecular and practical view of chemical equilibrium.

Learning Objectives

1

Explain how acid-base indicators such as anthocyanin change color based on the pH of a solution.

2

Analyze the molecular basis of indicator color changes, including the roles of HIn and In– forms.

3

Write and balance chemical equations that represent the conversion between acid and base forms of indicators.

4

Differentiate between reversible processes and reactions that proceed to completion.

Key Concepts

CONCEPT

DEFINITION

Indicator

A large, complex molecule that changes color depending on the pH of the solution. In this lesson, the indicator is abbreviated as HIn in one form and In– in the other.

Anthocyanin

A pigment found in plants such as red cabbage, blueberries, and hydrangeas that acts as an acid-base indicator. It exists as HIn (red in acidic conditions) and as In– (blue in basic conditions).

pH

A measure of the acidity or basicity of a solution; lower pH indicates acid, higher pH indicates base.

Reversible Process

A process that can proceed in both the forward and reverse directions, such as the equilibrium between HIn molecules and In– anions when pH changes.

Chemical Equilibrium

The state in which both the forward and reverse reactions occur at the same rate, leading to constant concentrations of reactants and products.

Example Problems

Example 1

Explain how molecules account for the purple cabbage leaves.

Example 2

Explain how the two forms of an indicator are related to one another.

Example 3

Methyl red is an indicator that is red in acidic solution and yellow in basic solution. Draw three molecular views to show how the color changes from red to yellow. What color do you observe in between?

Example 4

Write a balanced chemical equation to describe what happens as the color changes from red to yellow for methyl red indicator using the general symbols for an indicator molecule and its anion.

Example 5

Write a balanced chemical equation to describe what happens as the color changes from yellow to red for methyl red indicator.
Scroll left
Scroll right

Step-by-Step Explanations

QUESTION

What happens when H+ ions are added to a basic solution containing mostly In– anions?

STEP-BY-STEP ANSWER:

Step 1: Recognize that in a basic solution, the indicator exists predominantly as In– (blue).
Step 2: When H+ ions are added, they react with In– anions according to the equation: H+(aq) + In–(aq) → HIn(aq).
Step 3: As In– anions are converted to HIn molecules, which are red, the solution changes from blue to purple and eventually to red as the reaction progresses.
Final Answer: Adding H+ ions shifts the equilibrium, converting blue In– anions to red HIn molecules, resulting in a color change from blue to purple to red.

Acid-Base Indicator Reaction in a Basic Solution

QUESTION

How does the color of an anthocyanin indicator change when OH– ions are added to an acidic solution?

STEP-BY-STEP ANSWER:

Step 1: In an acidic solution, the indicator exists mostly as red HIn molecules.
Step 2: Adding OH– ions removes H+ from the solution; this withdrawal causes some HIn molecules to dissociate into H+ and In–.
Step 3: As more OH– is added, the balance shifts progressively towards more In– formation, changing the color from red to purple and then to blue.
Final Answer: The addition of OH– ions depletes H+ and converts red HIn to blue In–, resulting in a color change from red to purple to blue.

Acid-Base Indicator Reaction in an Acidic Solution

Scroll left
Scroll right

Common Mistakes

  • Misinterpreting the purple color as a unique molecular species instead of a visual mix of red HIn and blue In– colors.
  • Assuming that the indicator reaction is a one-way process rather than reversible.
  • Overlooking the importance of equilibria in the color change, leading to errors in writing balanced chemical equations.
  • Confusing the role of pH; for instance, not recognizing that acid and base additions reverse the indicator equilibrium.