Unit 4: Equilibrium, Acids & Bases Part 2: Acids and Bases

1. Unit 4: Equilibrium, Acids & BasesPart 2: Acids and Bases Bronsted Lowry Acids and Bases Autoionization of Water pH Strong Acids and Bases Weak Acids and Bases Ionization Constants Buffers Titrations Lewis Acids and Bases

2. Review • Arrhenius Acid:Substance that increases the concentration of H+ ions when dissolved in water HCl (g) H+ (aq) + Cl- (aq) • Arrhenius Base:Substance that increases the concentration of OH- ions when dissolved in water. NaOH (s) Na+ (aq) + OH- (aq) H2O H2O

3. Review • The Arrhenius definition of acids and bases is limited to aqueous solutions • Two other common definitions for acids and bases. • Bronsted-Lowry acids and bases • Lewis acids and bases

4. Bronsted-Lowry Acids & Bases • Bronsted-Lowry Acid • any substance that can transfer a proton (H+ ion) to another substance • a proton donor HCl (g) + H2O(l) H3O+ (aq) + Cl- (aq) • A Bronsted-Lowry acid must have a hydrogen that can be lost as H+ B-L acid

5. Bronsted-Lowry Acids & Bases • Bronsted-Lowry Base • any substance that can accept a proton (H+ ion) from another substance • A proton acceptor HCl (g) + H2O(l) H3O+ (aq) + Cl- (aq) • A B-L base must have an atom with a lone pair of electrons that can form a new bond to a hydrogen ion. B-L acid B-L base

6. Bronsted-Lowry Acids & Bases • The H+ ion interacts strongly with the nonbonding pairs of electrons on water molecules, forming thehydronium ion • The hydronium ion is responsible for the characteristic properties of aqueous solutions of acids.

7. Bronsted-Lowry Acids & Bases • H3O+ is a more realistic depiction of the hydrogen ion in solution but for convenience we often use H+ (aq) to depict the hydrated hydrogen ion. HCl (g) + H20 (l)  H3O+(aq) + Cl- (aq) HCl (aq)  H+ (aq) + Cl- (aq)

8. Bronsted-Lowry Acids & Bases • Some substances like water are amphoteric • Capable of acting as either an acid or a base H2O (l) + HCl (g)  H3O+(aq) + Cl- (aq) NH3 (g) + H2O (l)  NH4+ (aq) + OH- (aq) base acid

9. Bronsted-Lowry Acids & Bases • Examples of other amphoteric substances include: • NaHCO3 • NaH2PO4 • NaHSO4 • Alcohols such as ethanol:

10. Bronsted-Lowry Acids & Bases • In any acid-base equilibrium, both forward and reverse reactions involve proton transfers. HNO2(aq) + H2O(l) NO2 –(aq) + H3O+(aq) • Forward Reaction: • B-L acid = • B-L base = • Reverse Reaction: • acid = • base =

11. Bronsted-Lowry Acids & Bases • The reaction between a BL acid and base produces a new acid (the conjugate acid) and a new base (the conjugate base). HNO2(aq) + H2O(l) NO2 –(aq) + H3O+(aq) • HNO2 and NO2- are called a conjugate acid-base pair. • H2O and H3O+ are also a conjugate acid-base pair. BL acid BL base conjugate base conjugate acid

12. Bronsted-Lowry Acids & Bases • Conjugate acid: • The acid formed when a base gains a proton • Conjugate acid of H2O • H3O + • Conjugate acid of SO4 2- • HSO4- • NOTE: The conjugate acid is always shown on the product side.

13. Bronsted-Lowry Acids & Bases • Conjugate base: • The base formed by removing a proton from an acid • Conjugate base of H2O • OH - • Conjugate base of H2SO4 • HSO4- • NOTE: The conjugate base is always shown on the product side.

14. Bronsted-Lowry Acids & Bases • Conjugate acid-base pair: • An acid and a base that differ only in the presence or absence of a single proton • HNO2 and NO2- • H3O+ and H2O • HCO3-and CO32- • NH4+ and NH3

15. Bronsted-Lowry Acids & Bases Example: Identify the acid, base, conjugate acid and conjugate base for the following reaction. HSO4– (aq) + CO32- (aq) SO42- (aq) + HCO3- (aq)

16. Bronsted-Lowry Acids & Bases Example: Identify the BL acid, BL base, conjugate acid and conjugate base in the following reactions.

17. Bronsted-Lowry Acids & Bases • In any acid-base equilibrium, both forward and reverse reactions involve proton transfers. HNO2(aq) + H2O(l) NO2 –(aq) + H3O+(aq) Conjugate base Conjugate acid acid base • How can we predict the position of the chemical equilibrium?

18. Bronsted Lowry Acids & Bases • The relative strengths of the acid and the conjugate acidcan be used to predict the position of the equilibrium. • Equilibrium favors the formation of the weaker acid. • The stronger acid more effectively loses a proton than its conjugate acid

19. Bronsted Lowry Acids & Bases • Every substance can be categorized as either: • strong acid • ionizes completely • weak acid • ionizes partially • solutions contain mixture of acid molecules, hydronium ion, and conjugate base • negligible acidity • no tendency to form H+ in solution

20. Bronsted-Lowry Acids & Bases • The seven most common strong acids: • HCl hydrochloric acid • HBr hydrobromic acid • HI hydroiodic acid • HNO3 nitric acid • HClO3 chloric acid • HClO4 perchloric acid • H2SO4 sulfuric acid You must know these acids by name and formula.

21. Bronsted-Lowry Acids & Bases • Examples of common weak acids: • acetic acid, • citric acid, • phosphoric acid • Examples of substances with negligible acidity: • CH4 • H2 • OH – • NH3

22. Bronsted-Lowry Acids & Bases • An inverse relationship exists between the strength of an acid and its conjugate base or between a base and its conjugate acid. • Strong acids form very weak conjugate bases (usually have negligible basicity). • Weak acids form stronger (but still fairly weak) conjugate bases • Substances with negligible acidity form very strong bases.

23. Bronsted-Lowry Acids and Bases The stronger the acid, the weaker the conjugate base The stronger the base, the weaker the conjugate acid

24. Bronsted-Lowry Acids & Bases Example:Does the following acid/base equilibrium favor the reactants or products?