Acid-Base Equilibria in General Chemistry
Explore acid-base equilibria concepts in general chemistry, covering topics such as autoionization of water, equilibrium constants of acids and bases, strong and weak acids, and more. Understand the behavior of different substances in solution and the relationship between Ka and Kb values.
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Module 13: Acid-Base equilibria General Chemistry Chem2331 Chem 2331: General Chemistry 1
Module 13 Module 13 Session 4 1. Video 1: Autoionization of water 1. Kw and concentrations of [OH-] and [H3O+] for a neutral, basic and acidic solution 2. Concentrations in equilibrium of [OH-] and [H3O+] 2. Video 2: Ka and Kb describe the strength of acid and bases 1. Finding concentrations in equilibrium 2. Effect of dilution 3. Concentration of OH- in acid solutions, concentration of H3O+ in basic solutions 3. Video 3: Ka and Kb are related through Kw 1. Decide the outcome of an amphoteric substance based on Ka and Kb Chem 2331: General Chemistry 2
Review from Monday In acid and base reactions, it is very important to write the right equations. What species are present when we dissolve the following substances in solution. Identify if the solution will be Acidic (has H3O+) Basic (has OH-) Neutral (has none) a) Na2SO3 b) NaOH c) NH3 d) HNO3 e) HNO2 f) KNO3 g) NH4ClO4 Chem 2331: General Chemistry 3
Equilibrium constant of acids and bases Weak and Strong acids Strong acids dissociate completely into ions in water. HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq) Ka >> 1 Weak acids dissociate very slightly into ions in water. HCN(aq) + H2O(l) H3O+(aq) + CN-(aq) Ka << 1 The Acid-Dissociation Constant [H3O+][CN-] [HCN] Ka = Chem 2331: General Chemistry 4
Equilibrium constant of acids and bases Kb is the equilibrium constant in which the base reacts with water to produce [OH-] and the conjugate acid [OH-][HCN-] [CN-] Kb = CN-(aq) + H2O(l) OH-(aq) + HCN(aq) Is H2PO4- in water an acid or a base, why? HPO42- + H3O+ Ka = 6.2x10-8 H2PO4- + H2O H3PO4 + OH- Kb = 1.3x10-12 Chem 2331: General Chemistry 5
Equilibrium constant of acids and bases Find online the Ka of the following compounds. Calculate Kb Kb = 10-14/Ka Acid Ka Conjugate base Nitric acid HNO3 Nitrous acid HNO2 Acetic acid CH3COOH Hydrochloric acid HCl Find online the Kb of the following compounds. Calculate Ka Ka = 10-14/Kb Base Kb Conjugate acid Ammonia NH3 Sulfate SO4(-2) Chem 2331: General Chemistry 6
Strong and weak acids and bases Which of these three acids has a larger Ka? CH3COOH H+ + CH3COO- ClCH2COOH H+ + ClCH2COO- Cl2CHCOOH H+ + Cl2CHCOO- Why does ethanol have a Ka=10-16 and phenol Ka=10-10 (hint: think about the stability of the products) Chem 2331: General Chemistry 7
Autoionization of water Auto-ionization of water Water is both an acid and a base H2O(l) + H2O(l) H3O+(aq) + OH-(aq) The Ion-Product Constant for Water [H3O+][OH-] = 1.0 x 10-14 at 25oC Kc = Kw = A change in [H3O+] causes an inverse change in [OH-]. Explain why in a neutral solution [H3O+] = [OH-] = 10-7 M ? Chem 2331: General Chemistry 8
Autoionization of water Calculate [H3O+] and [OH-] in a 0.1M of strong acid. HX + H2O X(-) + H3O(+) 0.1 / 0.1M 0.1M In all these problems we consider [H2O] constant and does not appear in any expression H2O(l) + H2O(l) H3O+(aq) + OH-(aq) 10-7 10-7 Chem 2331: General Chemistry 9
Autoionization of water Calculate [H3O+] and [OH-] in a 0.01M of strong base. H2O(l) + H2O(l) H3O+(aq) + OH-(aq) 10-7 10-7 Chem 2331: General Chemistry 10
Autoionization of water Acid Base [H+] 100 10-1 10-3 10-5 10-7 10-9 10-11 10-13 10-14 H+ H+ H+ H+ OH- H+ OH- OH- OH- OH- [OH-]10-14 10-13 10-11 10-9 10-7 10-5 10-3 10-1 100 Chem 2331: General Chemistry 11
Autoionization of water For each of the solutions below. 1) State if the solution is an acid solution, basic or neutral 2) Identify whether [H3O+] will be > , < or = 10-7M 3) Sort them in terms of increasing [H3O+] concentration: a. 1.0 M NaOH b. 1.0 M HCl c. 1.0 M K[CH3COO] d. 1.0 M NH4Cl e. 1.0 M BaCl2 Chem 2331: General Chemistry 12
Equilibrium constant of acids and bases If you have 0.5M CH3COOH. What is the concentration of H3O+? And OH-? Weak Acid CH3COOH + H2O H3O+ + CH3COO- Ka = 1.8x10-5 Chem 2331: General Chemistry 13
Equilibrium constant of acids and bases If you have 1.5M (CH3)2NH. What is the concentration of H3O+? And OH-? Weak Base (CH3)2NH2+ + OH- Kb = 5.9x10-4 (CH3)2NH + H2O Chem 2331: General Chemistry 14
