CHE 1302 – Basic Principles of Modern Chemistry II Week 8 Resources

By Patrick Olademehin (03-08-2021)

Hello everyone! The CHE 1302 resources for this week will cover key concepts in chapter 13 of the approved textbook, Chemistry: An Atoms First Approach by Zumdahl and Zumdahl. If you need a quick review on problem-solving strategies for questions in Chapter 12, check out last week’s resources using this link: https://www.baylor.edu/support_programs/doc.php/371246.pdf

Remember, we conduct a Group Tutoring session every Monday 5:30 – 6:30 PM. Reserve a spot early at https://baylor.edu/tutoring and endeavor to come with a specific question in mind. Keywords: Conjugate acid-base pair, Dissociation constant, Acid strength, Polyprotic acid, Acidic salt, pH. Topics Treated in Week 8 The key concepts covered in Chapter 13 include:

1. The Nature of Acids and Bases 2. Acid Strength 3. The pH Scale 4. Calculating the pH of Strong Acid Solutions 5. Calculating the pH of Weak Acid Solutions 6. Bases 7. Polyprotic Acids 8. Acid-Base Properties of Salts 9. The Effect of Structure on Acid-Base Properties 10. Acid-Base Properties of Oxides 11. The Lewis Acid-Base Model

Important Concepts from CHE 1301

1. Types of Chemical Reactions and Solution Stoichiometry – It is assumed that students are already familiar with Acid-Base reactions, Strong and weak acids, as well as net ionic equations from Chapter 6 of the approved textbook.

Highlights from Chapter 13 Topics

1. The Nature of Acids and Bases – Arrhenius postulated that Acids form H+ in water and Bases form OH- in water, while the Bronsted-Lowry model defines an Acid as a proton donor and a Base as a proton acceptor. An acid and a base that differ from each other by a single proton (H) are known as a conjugate acid-base pair.

2. Acid Strength – A strong acid dissociates completely in water while a weak acid only dissociates partially. A strong acid yields a weak conjugate base and its equilibrium

position lies far to the right. A weak acid gives a strong conjugate base and its equilibrium lies far to the left. A general representation of an acid dissociation reaction is shown below: HA (aq) + H2O (l) H3O+ (aq) + A- (aq) Acid Conjugate Base

K! =[𝐻3𝑂

+][𝐴−][𝐻𝐴]

Where Ka is the acid-dissociation constant.

3. The pH Scale – The pH scale is used to represent the acidity or alkalinity of a solution. Mathematically,

pH = - log [H+] At 25oC, pH + pOH = 14; Kw = [H+] [OH-] = 1.0 x 10-14; [H+] = [OH-] = 10-7

4. Calculating the pH of Strong Acid Solutions – Since strong acids ionize completely, we assume all the H+ (or H3O+) in a strong acid solution comes from the acid. Strong acids include: HCl, HBr, HI, HNO3, HClO3, HClO4 and H2SO4

5. Bases – Strong bases ionize completely in solution. They include all the hydroxides of

Group I and II, e.g., LiOH, NaOH, Mg(OH)2, Ca(OH)2. The equilibrium of strong bases lies to the right while that of weak bases lies to the left. A general base reaction is shown below:

B (aq) + H2O (l) BH+ (aq) + OH- (aq) Base Conjugate acid

6. Calculations Involving Weak Acids and Bases – A good revision of ICE concept in Chapter 12 is necessary here. It is very important to list the major species in solution before using the ICE method for the calculation. Here is a short video with a sample calculation: https://www.khanacademy.org/science/chemistry/acids-and-bases-topic/copy-of-acid-base-equilibria/v/weak-acid-equilibrium

𝐾𝑏 = [𝐵𝐻'][𝑂𝐻(]

[𝐵]

7. Polyprotic Acids – These are acids with more than one ionizable H per molecule. E.g., H2SO4 (diprotic), H3PO4 (triprotic). Polyprotic acids dissociate in steps and each step has a different Ka. For a given polyprotic acid: Ka1 >> Ka2 >> Ka3 The acid with higher Ka will dominate and the weaker acid can be ignored for calculation of H+. Check out a sample calculation here:

https://www.youtube.com/watch?v=0uzC7QzwX_o

8. Acid-Base Properties of Salts – Salt is another name for ionic compound. Salts that consist of the cations of strong bases and the anions of strong acids have no effect on [H+] when dissolved in water. A basic salt is one whose anion is the conjugate base of a weak acid, while an acidic salt is one whose cation is the conjugate acid of a weak base. Here’s a short video explaining how you know acidic, basic or neutral salt: https://www.khanacademy.org/science/chemistry/acids-and-bases-topic/copy-of-acid-base-equilibria/v/acid-base-properties-of-salts

9. The Effect of Structure on Acid-Base Properties – For a strong acid, we want H-A bond to

be polar in order for the acid to effectively donate H. Acid strength increases as H-A bond dissociation energy decreases.

10. The Lewis Acid-Base Model – A Lewis acid (e.g., BF3, Al3+) is an electron-pair acceptor and a Lewis base (e.g., NH3, F-) is an electron-pair donor. Lewis acids generally have vacant orbital to accept electrons while Lewis bases have excess electrons to donate.

Common Mistakes Students Make 1. Applying the solution to one type of problem to another one. Solving questions on this

chapter is strictly dependent on the major species in solution. 2. Assuming the value of Kw is always 10-14. Kw value is temperature dependent (Kw = 10-14

at 25oC). 3. Not using the 5% rule to see if their approximation is valid when solving a problem with

the ICE method.

4. Inability to know which acid/base is weak or strong. 5. Not being able to recognize a Lewis acid as an acid. 6. They sometime forget Kb can be obtained when you are given Ka. Note: Ka x Kb = Kw