WebLecture: Common Ions
Kotz and Triechel, Chemistry and Chemical Reactivity Chapter 17: Sections 1-3.
Our main goal in this chapter is to understand and apply the principles of the common ion effect. This concept grows out of conservation of energy, and Le Chatelier's principal that when a system is in equilibrium, it will respond to a disturbance by attempting to return to equilibrium or establishing new equilibrium.
- 17.1 If we add an acid containing A- to the acid base reaction AH + BOH → BOH2+ + A-, Le Chatelier's principle will drive the reaction backwards to re-establish the equilibrium ratio. Adding a compound which dissociates to provide an ion common with another reaction will affect the equilibrium of the other reaction. This effect can be used to control the first reaction.
- 17.2 Solutions used to control acid-base reactions by reacting with added OH- ions and consuming H3O+ ions are buffers. Buffers are usually prepared from conjugate base pairs. Consider the reactions
- Acid Dissociation: AH + OH- → A- + H2O
If OH- is a stronger base than the conjugate base A- of AH, adding OH- will drive the reaction to the right and favor the production of A-; the additional OH- will be consumed.
- Base synthesis: H3O+ + A- → H2O + AH.
If the hydronium ion is a stronger acid than AH, adding H3O+ will drive this reaction to the right and favor the production of AH. The additional H3O+ will be consumed.
In each case, the equilibrium remains the same because the added ion is consumed by the existing weak acid or base in solution. The concentration of the hydronium ion in a buffer solution is given by [H3O+] = [acid]/[conjugate base] * Ka.
We can rewrite this in a more convenient form, the Henderson-Hasselbalch equation:
pH = pK log ( [conjugate base]/[acid] )
The strength of a buffer depends on the strength of the acid (indicated by Ka) and the ratio of concentrations of the acid with its conjugate base.
- 17.3 Acid-base titrations have different characteristics depending on the relative strengths of the components. The equivalence point is occurs when chemically equivalent quantities of acid and base have been mixed, and [H3O+] = [OH-]
- Strong acid — strong base: the equivalence point will be pH=7 and the concentrations will change rapidly at the equivalence point.
- Weak acid with strong base: the equivalence point will be above pH=7 due to the precence of the conjugate base of the weak acid. If we start with an acidic solution, the early part of the titration will be buffered, and require a large amount of the base to increase the pH.
- Strong acid with a weak base: a similar situation results when titrating a weak base with pH > 7 with a strong acid. The initial equilibrium is buffered by the presence of the conjugate acid of the weak base. The equivalence point occurs where pH < 7.
Because equivalence points can occur at pH values other than near 7, a variety of acid-base indicators are used to display when the pH value changes.
If you find these study methods useful, visit the Chapter 17 resources at Cengage "Brain" companion site for our text. Click on the chapter dropdown and select Chapter 17 to use the flashcards and glossary.
Use the MindTap reader version of the textbook to view videos and test your understanding with interactive checkpoints. Check OWLv2 for your homework problems.
Videos for Chapter 17: Principles of Chemical Reactivity: Other Aspects of Aqueous Equilibria
Review the Videos at Thinkwell Video Lessons.
- Under "EQUILIBRIUM IN AQUEOUS SOLUTIONS"
- Reactions of Acids and Bases
- An Introduction to Buffers
- Acidic Buffers
- Basic Buffers
- The Henderson-Hasselbalch Equation
- Acid-Base Titration
- Strong Acid-Strong Base Titration
- Weak Acide-Strong Base Titration
- Polyprotic Acid-Strong Base Titration
- Weak Base-Strong Acid Titration
- Acid-Base Indicators
Homework problems: See your Moodle assignment!
AP #14 GUIDED INQUIRY — Creating an effective buffer solution — Phase I
Building on what you learned with Experiment #13, design a buffer solution with a target pH and buffer capacity by selecting appropriate conjugate-acid base pairs.
- APGIE Investigation 16 Buffer Design: The Preparation and Testing of an Effective Buffer: How Do Components influence a Buffer’s pH and Capacity?
- IGHCE Lab 13.2 Quantify the common ion effect
- There is no alternate form for this lab
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