Acid-Base Theories The “Boyz”

Acid and Base Theories2 Arrhenius Theory of Acids Acid: molecular substances that breaks-ups in aqueous solution into H+ and anions H + (“hydrogen ions” or “protons”) H + gives acids its protons Example: HNO 3(aq)  H + (aq) + NO 3 – (aq)

Acid and Base Theories3 Common Acids

Acid and Base Theories4 Arrhenius Theory of Bases Base: releases OH – ions in aqueous solution OH – (hydroxide) OH – gives bases their properties Example: NaOH (aq)  Na + (aq) + OH – (aq)

Acid and Base Theories5 Common Bases

Acid and Base Theories6 Limitations of Arrhenius Theory H + does not exist in solution More likely to find H + attached to H 2 O (hydrated) H 3 O + Some bases, like ammonia, do not fit this definition, the solution is basic, but the compound does not dissociate, forming hydroxide ion NH 3 (g) + H 2 O (l)  NH 4 + (aq) + OH - (aq) Limited to the solvent water, but acid-base reactions can occur in other solvents

Bronsted and Lowry Johannes Bronsted Copenhagen (Denmark) Acid and Base Theories7 Thomas Lowry London (England)

Acid and Base Theories8 Brønsted–Lowry Theory of Acids and Bases Acid: proton (H+) donor HCl + NH 3  NH Cl – HCl donates a H + to NH 3 –H+ does not exist by itself

Acid and Base Theories9 Bases: accept a proton H 2 O + NH 3  NH OH – NH 3 accepts a H + from H 2 O A Brønsted -Lowry acid must have a H in its formula (like an Arrhenius acid) Any negative ion can be a Brønsted -Lowry base

Acid and Base Theories10 For an acid-base reaction: There must be a transfer of a proton A substance can behave as an acid, if another substance behaves as a base i.e. there is a proton donor (acid) and a proton acceptor (base) HCl (aq) + H 2 O (l)  H 3 O + (aq) + Cl - (aq) acid base conjugate conjugate acid base

Acid and Base Theories11 Conjugate acid-base pair The two molecules or ions related by transfer of a proton from one to the other Example. HCl (aq) and H 2 O (l) HCl donates the proton and H 2 O receives the proton

Acid and Base Theories12 Conjugate base of an acid The particle remaining when the proton is removed from the acid HCl (aq) – acid Remove proton  Cl - (aq) (conjugate base) Conjugate acid of a base The particle produced when a base receives a proton H 2 O (l) – base Add proton  H 3 O + (aq) (conjugate acid)

Conjugate Acid-Base Pair Practce H 2 SO 4 + H 2 O  HSO H 3 O + H 2 SO 4 & HSO 4 - H 3 O + & H 2 O HCl + NH 3  Cl - + NH 4 + HCl & Cl - NH 4 + & NH 3 NH 3 + H 2 O  NH OH - H 2 O + OH - NH 4 + & NH 3 Acid and Base Theories13

Acid and Base Theories14 Table 1: Comparing the Arrhenius and Brønsted-Lowry Theory TheoryArrheniusBrønsted-Lowry Acidany substance that dissociates to form H+ in aqueous solution any substance that provides a proton to another substance (or any substance from which a proton may be removed) Baseany substance that dissociates to form OH- in aqueous solution any substance that receives a proton from an acid (or any substance that removes a proton from an acid) Example HCl (aq)  H + (aq) +Cl - (aq) HCl (aq) +H 2 O (l)  H 3 O + (aq) +Cl - (aq)

Acid and Base Theories15 Strong and Weak Acids Strong acid: acid completely reacts with water –Completely ionizes in water Weak acid: acid reacts only slightly with water –Partially ionizes in water Common strong acids: HCl, HNO 3, H 2 SO 4 Most other acids are weak acids

Acid and Base Theories16 Strong and Weak Bases Strong base: base completely ionizes in water Weak base: only slightly ionizes in water Common strong bases: NaOH and KOH –True for most Group 1A and 2A hydroxides Common weak base: NH 3

Acid and Base Theories17 Amphiprotic A substance that can either donate or accept a proton H 2 O most common amphiprotic substance Reacts with base: H 2 O + NH 3  NH OH – Reacts with acid: H 2 O + HCl  Cl – + H 3 O +

Acid and Base Theories18 Acidic Anhydrides React with water to form acids Typically are nonmetal oxides CO 2 + H 2 O  H 2 CO 3 Basic Anhydrides React with water to form bases Typically are metal oxides CaO + H 2 O  Ca(OH) 2

Acid and Base Theories19 Neutralization Reaction of an acid and a base Acid + base  water + salt

Acid and Base Theories20 Assigned Questions: p. 379 # 2, 4 p. 386 # 15, 16 p. 389 # 18, 19, 20 p. 392 # 4 p. 392 # 8, 9, 11