Book cover for Chemistry The Science in Context

Chemistry The Science in Context

Thomas R. Gilbert

ISBN #9780393615142

5th Edition

2,675 Questions

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191,124 Students Helped

Homework Questions

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Summary
Learning Objectives
Key Concepts
Example Problems
Explanations
Common Mistakes

Summary

This chapter covers the fundamentals of acid–base equilibria with an emphasis on understanding the behavior of strong and weak acids and bases. Key concepts such as Ka, Kb, pH, and pOH scales are discussed alongside the autoionization of water and the hydrolysis of salts. The text stresses the importance of RICE tables and equilibrium calculations in predicting pH changes, and it delves into how molecular structure influences acid strength, particularly in polyprotic acids. Mastery of these topics is essential for applications in both industrial and biological systems.

Learning Objectives

1

Explain the principles of acid–base equilibria in biological and industrial systems.

2

Differentiate between strong and weak acids and bases using Ka and Kb values.

3

Utilize RICE tables for equilibrium calculations to determine solution pH.

4

Analyze the impact of molecular structure on acid strength, including polyprotic acids.

5

Evaluate the hydrolysis of salts and predict if a solution will be acidic, basic, or neutral.

Key Concepts

CONCEPT

DEFINITION

Acid

A substance that donates a proton (H+) in solution, increasing the hydronium ion concentration.

Base

A substance that accepts a proton or donates an electron pair, often generating hydroxide ions (OH−) in solution.

Ka (Acid Dissociation Constant)

A quantitative measure of the strength of a weak acid in solution; it indicates the degree of ionization.

Kb (Base Dissociation Constant)

A quantitative measure of the strength of a weak base in solution, showing its tendency to accept a proton.

pH and pOH

Logarithmic scales used to express the acidity or basicity of a solution. pH measures hydrogen ion concentration, and pOH measures hydroxide ion concentration.

Autoionization of Water

The process by which water molecules dissociate into hydronium (H3O+) and hydroxide (OH−) ions, fundamental to acid–base chemistry.

Polyprotic Acids

Acids that can donate more than one proton per molecule, often ionizing in sequential steps.

Hydrolysis

A reaction involving the interaction of water with a substance, such as salts, that may result in an acidic or basic solution.

RICE Table

A structured approach (Reaction, Initial, Change, Equilibrium) for solving equilibrium problems in acid–base chemistry.

Example Problems

Example 1

Which of the lines in Figure P15.1 best represents the dependence of the degree of ionization of acetic acid on its concentration in aqueous solution?

Example 2

The graph in Figure P15.2 shows the percent ionization of two acids as a function of concentration in water. Which line describes the behavior of $\mathrm{HNO}_{3},$ and which line describes the behavior of acetic acid $\left(\mathrm{CH}_{3} \mathrm{COOH}\right) ?$ (Check your book to see figure)

Example 3

The bar graph in Figure P15.3 shows the degree of ionization of $1 \times 10^{-3} M$ solutions of three hypohalous acids: $\mathrm{HClO}$ HBrO, and HIO. Which bar corresponds to HIO? (Check your book to see figure)

Example 4

The bar graph in Figure P15.4 shows the degree of ionization of $1 M$ solutions of $\mathrm{HClO}, \mathrm{HClO}_{2},$ and $\mathrm{HClO}_{3}$ Which bar corresponds to HClO?? (Check your book to see figure)

Example 5

Figure $P 15.5$ shows a condensed molecular structure of pseudoephedrine, a widely used decongestant and stimulant. a. Is pseudoephedrine an acidic, basic, or neutral compound? b. Which functional group in its structure gives it the property you selected in part (a)? (Check your book to see figure)
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Step-by-Step Explanations

QUESTION

How do you determine the pH of a 0.1 M weak acid solution with a known Ka value?

STEP-BY-STEP ANSWER:

Step 1: Write the acid dissociation equation (HA ↔ H+ + A−) for the weak acid.
Step 2: Set up the RICE table with initial concentration 0.1 M for HA, and 0 M for H+ and A−.
Step 3: Define the change in concentration as -x for HA and +x for both H+ and A− as it dissociates.
Step 4: Write the expression for Ka: Ka = (x)(x)/(0.1 - x).
Step 5: Solve for x (assuming x is small, 0.1 - x approximates 0.1) to obtain the concentration of H+.
Step 6: Calculate the pH using the formula pH = -log[H+].
Final Answer: The calculated pH of the weak acid solution based on the value of x determined from the Ka expression.

Calculating pH of a Weak Acid Using a RICE Table

QUESTION

How can hydrolysis reactions predict the pH of a solution formed by dissolving a salt?

STEP-BY-STEP ANSWER:

Step 1: Identify the ions produced when the salt dissolves.
Step 2: Determine whether the cation or anion can undergo hydrolysis with water.
Step 3: Write the hydrolysis reactions for the corresponding ion(s).
Step 4: Identify if the reaction produces H+ (acidic) or OH− (basic), or if both occur (neutral).
Step 5: Use equilibrium expressions and possibly RICE tables to estimate changes in concentration.
Final Answer: The overall pH of the salt solution is predicted by the net effect of the hydrolysis reactions and the equilibrium constants involved.

Analyzing Salt Hydrolysis

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Common Mistakes

  • Confusing the roles of Ka and Kb, or mixing up acid and base dissociation constants.
  • Incorrect approximation in RICE table calculations, such as neglecting significant x values.
  • Overlooking the autoionization of water when calculating pH in very dilute solutions.
  • Failing to consider the effects of multiple ionization steps in polyprotic acids.
  • Misinterpreting salt hydrolysis reactions which can lead to incorrect predictions of the solution’s pH.