HX(s/l/g) → H+(aq) + X–(aq) Arrhenius Theory Acids ionize in water to produce hydrogen ions plus an anion. HX(s/l/g) → H+(aq) + X–(aq) Bases dissociate in water to produce hydroxide ions plus a cation. Svante Arrhenius (1859 – 1927) MOH(s) → M+(aq) + OH–(aq) Limitation: It failed to predict the acidic or basic properties of some compounds.

e.g. HCl(aq) + H2O(l) → H3O+(aq) + Cl–(aq) Modified Arrhenius Theory Acids are substances that react with water to produce hydronium ions. e.g. HCl(aq) + H2O(l) → H3O+(aq) + Cl–(aq) Bases are substances that react with water to produce hyrdoxide ions. Svante Arrhenius (1859 – 1927) e.g. NH3(aq) + H2O(l) → OH–(aq) + NH4+(aq) Limitations: There is no provision for reactions that do not occur in aqueous solution. Some substances have both acidic and basic properties: sodium bicarbonate (NaHCO3(s)), for example. These substances are called amphoteric.

The Proton Transfer Concept The Brønsted–Lowry concept does away with defining a substance as being an acid or a base. An entity is referred to as acting as an acid or acting as a base in the context of a specific reaction. Johannes Brønsted (1879 – 1947) A Brønsted–Lowry acid is a proton donor in a specific reaction. A Brønsted–Lowry base is a proton acceptor in a specific reaction. NOTE: The Bronsted-Lowry concept is not a theory since it does not explain why the reaction occurs nor does it predict which reaction will occur when a chemical entity is placed in a new situation. Thomas Lowry (1874 – 1936)

HCl(aq) + H2O(l) → H3O+(aq) + Cl–(aq) A Brønsted–Lowry reaction involves a single proton transfer from one entity (the acid) to another (the base). Hydrogen chloride loses a proton (H+(aq)) to water to form a hydronium ion and its conjugate base. As a result, it is classified as an acid. 1) hydrogen chloride in water H+ HCl(aq) + H2O(l) → H3O+(aq) + Cl–(aq) acid base Ammonia gains a proton (H+(aq)) from water to form its conjugate acid. As a result, it is classified as a base. 2) ammonia in water H+ NH3(aq) + H2O(l)  OH-(aq) + NH4+(aq) base acid Water has the ability to either accept or donate an proton, making it amphiprotic.

water as an acid water as a base HCO3–(aq) HPO42–(aq) HSO4–(aq) Other examples include: HCO3–(aq) HPO42–(aq) HSO4–(aq) H2PO4–(aq)

H3O+(aq) + NH3(aq) → H2O(aq) + NH4+(aq) 3) hydrochloric acid and ammonia solution H+ H3O+(aq) + NH3(aq) → H2O(aq) + NH4+(aq) acid base 4) ammonia gas and hydrogen chloride gas H+ HCl(g) + NH3(g) → NH4Cl(s) acid base Water does not have to be present!

HCO3-(aq) + H2O(l)  OH-(aq) + H2CO3(aq) The hydrogen carbonate ion is amphiprotic because it can either . . . a) accept a proton H+ HCO3-(aq) + H2O(l)  OH-(aq) + H2CO3(aq) base acid b) donate a proton H+ HCO3-(aq) + H2O(l)  H3O+(aq) + CO32-(aq) acid base The presence of the hydrogen carbonate ion makes sodium bicarbonate (baking soda) an amphoteric substance.

HCO3-(aq) + H2O(l)  OH-(aq) + H2CO3(aq) Bicarbonate ions can . . . 1) raise the pH of a strong acid solution H+ HCO3-(aq) + H2O(l)  OH-(aq) + H2CO3(aq) base acid 2) lower the pH of a strong base solution H+ HCO3-(aq) + H2O(l)  H3O+(aq) + CO32-(aq) acid base

Conjugate Acids and Bases A Brønsted–Lowry reaction results in an acid–base dynamic equilibrium, where the forward and reverse reactions occur at the same time and at the same rate. H+ base acid CH3COOH(aq) + H2O(l)  CH3COO-(aq) + H3O+(aq) acid base H+ A pair of substances that differ only by a proton is called a conjugate acid–base pair. conjugate pair 1.3% CH3COOH(aq) + H2O(l)  CH3COO-(aq) + H3O+(aq) conjugate pair

The weaker an acid, the stronger it’s conjugate base. 1.3% CH3COOH(aq) + H2O(l)  CH3COO-(aq) H3O+(aq) Acetic acid has a strong attraction for its own proton (i.e. doesn’t donate it very readily), so it is a weak acid. It’s conjugate base, the acetate ion (CH3COO–(aq)), is a stronger base than water. It has a greater attraction for protons. The weaker an acid, the stronger it’s conjugate base.

HCl(aq) + H2O(l) → H3O+(aq) + Cl–(aq) A hydrogen chloride molecule has a much weaker attraction for its own proton than water. This makes HCl(aq) a strong acid. The stronger an acid, the less it attracts its own proton and the stronger a base, the more it attracts another proton. The stronger an acid, the weaker it’s conjugate base.

OCl-(aq) + H2O(l)  HOCl(aq) + OH-(aq) Sample Diploma Question Prairie Chemical Inc. in Edmonton is a bulk manufacturer of bleach (NaOCl(aq)). The bleach reacts with water to form a solution with a pH of 10.87. OCl-(aq) + H2O(l)  HOCl(aq) + OH-(aq) The substance in the equation above that may act as an amphiprotic species is: OCl-(aq) H2O(l) HOCl(aq) OH-(aq)

NH4+(aq) + OH-(aq)  NH3(aq) + H2O(l) Sample Diploma Questions Two cleaning solutions were accidentally mixed. A strong smell of ammonia alerted a technician to the accident. After checking the labels of the cleaners and discovering that one container held NH4Cl(aq) and the other KOH(aq), the technician determined that the smell came from the following reaction: NH4+(aq) + OH-(aq)  NH3(aq) + H2O(l)   1. In this equilibrium, the Bronsted-Lowry acids are: A. NH3(aq) and H2O(l) B. NH4 +(aq) and H2O(l) C. NH3(aq) and OH-(aq) D. NH4 +(aq) and OH-(aq) 2. The conjugate base of N2H5+(aq) A. HOH(l) B. OH-(aq) C. N2H4(aq) D. N2H62+(aq)

Sample Diploma Questions 3. Match each acid or base in the following reaction, as numbered, with the corresponding term given below:   HSO4 –(aq) + HCOO-(aq)  HCOOH(aq) + SO4 2-(aq) 1 2 3 4 acid (Record in the first column) conjugate base (Record in the second column) base (Record in the third column) conjugate acid (Record in the fourth column) 4. A conjugate acid-base pair is: CH3COOH(aq) and CO3 2-(aq) HCO3-(aq) and CO3 2-(aq) H2S(aq) and H2PO4-(aq) OH-(aq) and CO3 2-(aq)

Homework: Read pgs. 722 – 726 pgs. 724, 726 Practice #’s 1 – 8

Predicting Acid-Base Reactions (A MAJOR CONCEPT!) One can now predict whether an acid-base reaction will take place by using both the collision reaction theory and the Bronsted-Lowry concept. According to both theories, the substance that has the greatest attraction for protons (i.e. the strongest base) will react with the substance that gives up its proton or protons most easily (i.e. the strongest acid), assuming that only one proton is transferred per collision.

Predicting Acid–Base Equilibria In a system that contains several different possible acid-base reactions, the only significant reaction is a proton transfer from the strongest acid present to the strongest base present. H+ strongest acid strongest base For an aqueous solution system, we represent all entities as they exist in solution. Strong Electrolytes: write in dissociated form ionic salts e.g. NaCl(aq) → Na+(aq) + Cl–(aq) strong acids e.g. HNO3(aq) + H2O(l) → H3O+(aq) + NO3–(aq) strong bases e.g. NaOH(aq) → Na+(aq) + OH–(aq)

Weak Electrolytes: write as is weak acids e.g. CH3COOH (aq) weak bases e.g. NH3(aq)

Steps For Predicting the Predominant Acid–Base Reaction 1) List all entities as they appear in solution, including H2O(l). SA SB 2) Label all possible aqueous acids and bases. 3) Label the strongest acid (SA) and strongest base (SB) using the table on pages 8 and 9 in the data booklet. 4) Write an equation showing the transfer of one proton from the strongest acid to the strongest base, with the products being the conjugate base and acid of the reactants. 5) Predict the position of the equilibrium, based on the fact that the side that is opposite the strongest acid is favoured. >50%  <50%  products favoured reactants favoured

CH3COOH(aq) + OH-(aq)  CH3COO-(aq) + H2O(l) S A A Na+(aq) OH–(aq) CH3COOH(aq) H2O(l) S B B metal ions are treated as spectators >50% CH3COOH(aq) + OH-(aq)  CH3COO-(aq) + H2O(l)

A S A NH3(aq) H2O(l) H3O+(aq) S B B The reaction is quantitative.

HCO3-(aq) + HPO42-(aq)  H2PO4-(aq) + CO32-(aq) Another Example Write the balanced acid–base equilibrium equation when aqueous potassium hydrogen carbonate (KHCO3(aq)) is mixed with aqueous sodium hydrogen phosphate (Na2HPO4(aq)). S A A A K+(aq) Na+(aq) HCO3–(aq) HPO42–(aq) H2O(l) B S B B <50% HCO3-(aq) + HPO42-(aq)  H2PO4-(aq) + CO32-(aq) Strongest acid, so reactants are favoured.

Perchloric acid is one of the six strong acids, so it forms H3O+(aq). Yet Another Example! Write the balanced acid–base equilibrium equation when perchloric acid (HClO4(aq)) is mixed with aqueous calcium hydroxide (Ca(OH)2(aq)). Perchloric acid is one of the six strong acids, so it forms H3O+(aq). S A A ClO4–(aq) H3O+(aq) Ca+(aq) OH–(aq) H2O(l) B S B B H3O+(aq) + OH-(aq)  H2O(l) + H2O(l)

Sample Diploma Problem 1. In which of the following reactions does equilibrium favor the products? HSO4-(aq) + F-(aq)  HF(aq) + SO4 2-(aq) HF(aq) + H2O(l) H3O+(aq) +F-(aq) HF(aq) + SO4 2-(aq)  HSO4-(aq) + F-(aq) CH3COOH(aq) + F-(aq)  HF(aq) + CH3COO-(aq)

Sample Diploma Problem 2. A student was asked to rank the relative strength of the following four acids: Formic Acid (HCOOH(aq)) Hydrazoic acid (HN3(aq)) Hypobromous acid (HOBr(aq)) Nitrous acid (HNO2(aq))   The student was given the following information. HNO2(aq) + HCOO-(aq)  NO2-(aq) + HCOOH(aq) (Products favored) HN3(aq) + OBr-(aq)  N3-(aq) + HOBr(aq) (Products favored) HN3(aq) + HCOO-(aq)  N3-(aq) + HCOOH(aq) (Reactants favored) Based on the reaction evidence, the four acids, ranked from strongest to weakest, are , , and . (record your four digit answer in the numerical response section on the answer sheet)

Sample Diploma Problem 3. Use the following information to answer the next question Chemical species HA 3-(aq) H3A-(aq) H2A 2-(aq) H4A(aq)   As a solution of NaOH(aq) is continuously added to the acid H4A(aq), a sequence of quantitative reactions occur. The order in which the species listed above would react is , , , and . (record your four digit answer in the numerical response section on the answer sheet)

Sample Diploma Problem 4. Vinegar (CH3COOH(aq)) and baking soda (NaHCO3(s)) are added to recipes to produce baked products with light, fluffy textures. The net ionic equation for the reaction that occurs is:   A. H3O+(aq) + HCO3-(aq)  CO2(g) + 2 H2O(l) B. HCO3-(aq) + CH3COO-(aq)  CH3COOH(aq) + CO3 2-(aq) C. CH3COOH(aq) + NaHCO3(aq)  NaCH3COO(aq) + CO2(g) + H2O(l) D. CH3COOH(aq) + HCO3 –(aq)  CH3COO-(aq) + CO2(g) + H2O(l)

Sample Diploma Problem 5. A reaction favoring the reactants in which HCO3-(aq) acts as an acid is: A. HCO3-(aq) + HBO3 2-(aq)  H2BO3-(aq) + CO3 2-(aq) B. HCO3-(aq) + HPO4 2-(aq)  H2PO4-(aq) + CO3 2-(aq) C. HCO3-(aq) + CH3COOH(aq)  H2CO3(aq) + CH3COO-(aq) D. HCO3-(aq) + HSO4 -(aq)  H2CO3(aq) + SO4 2-(aq)  

Homework: Read pgs. 727 – 731, including case study pg. 731 Practice #’s 9 – 17 pgs. 735 – 736 Section 16.2 Questions #’s 1 – 6 Do predicting acid-base reaction worksheet Do lab Exercises 16.B/16.C/16.D

Predicting Acid-Base Reactions Worksheet What is the generalization used to predict the position of an acid-base equilibrium? Predict the acid-base reaction for each of the following questions. Communicate your answer using the five-step method. 2. A nitrous acid spill is neutralized with sodium hydrogen carbonate (baking soda). 3. In a chemical analysis, a sample of methanoic acid is titrated in a quantitative reaction with standardized sodium hydroxide.

Predicting Acid-Base Reactions Worksheet Predict the acid-base reaction for each of the following questions. Communicate your answer using the five-step method. 4. Could hypochlorus acid be neutralized with baking soda? 5. Can small quantities of poisonous hydrocyanic acid be produced by quantitatively reacting sodium cyanide with hydrochloric acid? 6. In order to test the power of the Bronsted-Lowry concept, a student mixes solutions of sodium acetate with ammonium chloride. 7. Some excess hydrofluoric acid remains after a technician used some to etch glass. a. What readily available base could she use completely neutralize the acid? b. Predict the neutralization reaction assuming a quantitative reaction.