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

HCl (g) H+ (aq) + Cl- (aq) 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

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

Bronsted-Lowry Acids & Bases 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

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

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

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)

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

Bronsted-Lowry Acids & Bases Examples of other amphoteric substances include: NaHCO3 NaH2PO4 NaHSO4

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 =

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

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.

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.

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

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)

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

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?

Bronsted Lowry Acids & Bases The relative strengths of the acid and the conjugate acid can 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 the conjugate acid

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

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.

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

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.

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

Bronsted-Lowry Acids & Bases Example: Use Fig. 16.4 to predict whether the equilibrium lies predominantly to the left or to the right: HSO4- (aq) + CO32- (aq) SO42- (aq) + HCO3 - (aq) Conj. base Conj. acid acid base