Acids and Bases Chapters 14 and 15

Properties of acids Aqueous solutions have a sour taste. Acids change the color of acid-base indicators. Some acids react with active metals to release hydrogen gas, H2. Acids react with bases to produce salts and water (i.e. a neutralization reaction) Some acids conduct electric current.

Properties of bases Aqueous solutions taste bitter. Bases change the color of acid-base indicators. Dilute aqueous solutions of bases feel slippery (ex. soap). Bases react with acids to produce salts and water. (i.e. neutralization reactions) Bases conduct electric current.

Theories of acids and bases Theory Acid properties Base properties Arrhenius H+ ion produced OH- ion produced Brønsted-Lowry Proton donor Proton acceptor Lewis e- pair acceptor e- pair donor

Strong vs. weak Strong acids and bases are those that ionize completely in aqueous solution. They are also typically strong electrolytes. Weak acids and bases are those that do not ionize completely in aqueous solution. They are also typically weak electrolytes.

Common strong and weak acids Strong acids Weak acids H2SO4 + H2OH3O+ + HSO4- HSO4- + H2O  H3O+ + SO42- HClO4 + H2O H3O+ + ClO4- H3PO4 + H2OH3O++ H2PO4- HClO3 + H2O H3O+ + ClO3- HF + H2O  H3O+ + F- HCl + H2O  H3O+ + Cl- CH3COOH +H2OH3O+ + CH3COO- HNO3 + H2O  H3O+ + NO3- H2CO3 +H2OH3O+ + HCO3- HBr + H2O  H3O+ + Br- HCO3- + H2OH3O+ + CO32- HI + H2O  H3O+ + I- H2S + H2O  H3O+ + HS- HCN + H2O  H3O+ + CN- Page 474 of your textbook

Common strong and weak bases Strong bases Weak bases Ca(OH)2 Ca2+ + 2OH- NH3 + H2O  NH4+ + OH- Sr(OH)2  Sr2+ + 2OH- C6H5NH2+ H2O  C6H5NH3+ + OH- Ba(OH)2 Ba2+ + 2OH- NaOH  Na+ + OH- KOH  K+ + OH- RbOH  Rb+ + OH- CsOH  Cs+ + OH- p. 475 of text

Conjugate acid-base reactions HF (aq) + H2O (l)  F- (aq) + H3O+ (aq) c.a.1 c.b.2 c.b.1 c.a.2

Amphoteric substances are those that can react as either an acid or a base.

Oxyacids Molecules containing –OH groups (hydroxyl groups) can be acidic or amphoteric. The more oxygen atoms, more polar  more acidic

Oxyacids of chlorine

Ka for weak acids Acid-dissociation equilibrium constant (Ka) - A measure of the relative strength of an acid. For the generic acid dissociation reaction with water, HA (aq) + H2O (l)  H3O+ (aq) + A- (aq) Ka = [H3O+][A- ] [HA] As the Ka value of an acid increases, so does the strength of the acid. By definition: strong acid: Ka > 1 weak acid: Ka < 1

strong acid + water  weaker acid + weaker base (water acts as a strong base) weak acid + water ATTEMPTS TO CONVERT TO stronger acid + stronger base (water acts as a weak base) BUT the reaction cannot naturally proceed in this direction. This is why strong acids dissociate nearly completely whereas weak acids dissociate only slightly.

The larger the value of Ka, the stronger is the acid The larger the value of Ka, the stronger is the acid. Ka is a better measure of the strength of an acid than pH because adding more water to the acid solution will not change the value of the equilibrium constant Ka, but it will change the H+ ion concentration on which pH depends.

Kb for weak bases Base dissociation constant or equilibrium constant, Kb For the reaction in which the Arrhenius base, BOH, dissociates to form the ions OH- and B+: BOH  OH- + B+ For a Brønsted-Lowry base: B + H2O  BH+ + OH- Kb = [BH+][OH-] [B]

Kb provides a measure of the strength of a base if Kb is large, the base is largely dissociated so the base is strong if Kb is small, very little of the base is dissociated so the base is weak.

Self-ionization of water H2O(l) + H2O(l)  H3O+ (aq) + OH- (aq) Kw = [H3O+] [OH-] = 1 x 10-14 Measurements at 25˚C show that the [H3O+] and [OH-] are each 1 x 10-7 M

[H3O+] and [OH-] are inversely proportional When something is acidic [H3O+] > [OH-]; [H3O+] >1 x 10-7 M and [OH-] < 1 x 10-7 M When something is basic [H3O+] < [OH-]; [H3O+] <1 x 10-7 M and [OH-] > 1 x 10-7 M [H3O+] and [OH-] are inversely proportional

pH means proportion of H+ ion scale from 0 – 14 pH = -log[H3O+] pOH = -log[OH-] As Kw = 1 x 10-14 we can conclude that pH + pOH = 14

Basic pH 7-14 [H3O+] < [OH-]; [H3O+] < 1 x 10-7 M pOH [OH-] 1 14 1 x 10-14 M 1 x 10-1 M 13 1 x 10-13 M 2 1 x 10-2 M 12 1 x 10-12 M 3 1 x 10-3 M 11 1 x 10-11 M 4 1 x 10-4 M 10 1 x 10-10 M 5 1 x 10-5 M 9 1 x 10-9 M 6 1 x 10-6 M 8 1 x 10-8 M 7 1 x 10-7 M Acidic pH 0-7 [H3O+] > [OH-]; [H3O+] > 1 x 10-7 M [OH-] < 1 x 10-7 M pH = 7 NEUTRAL [H3O+] = [OH-] 1 x 10-7 M Basic pH 7-14 [H3O+] < [OH-]; [H3O+] < 1 x 10-7 M [OH-] > 1 x 10-7 M

pH of common items

Acid – base indicators compounds whose colors are sensitive to pH. Indicators change colors because they are either weak acids or weak bases. In basic solution HIn  H+ + In- In acidic solution

Indicators come in many different colors Indicators come in many different colors. The pH range over which an indicator changes color is called its transition interval.

Blue litmus paper turns red under acidic conditions and red litmus paper turns blue under basic (i.e. alkaline) conditions

A pH meter determines the pH of a solution by measuring the voltage between the two electrodes that are placed in the solution.

Neutralization reactions DR between strong acids and bases always form water and a salt (an ionic compound formed from the cation of the base and the anion of the acid). The net ionic equation is: H3O+ (aq) + OH- (aq)  2H2O (l)

Titration is the controlled addition and measurement of the amount of a solution of known concentration required to react completely with a measured amount of a solution of unknown concentration.

The point at which the two solutions used in a titration are present in chemically equivalent amounts is the equivalence point. The point in a titration at which an indicator changes color is called the end point.

Burettes for acid-base titrations The solution that contains the precisely known concentration of a solute is known as a standard solution. A primary standard is a highly purified solid compound used to check the concentration of the known solution in a titration. Burettes for acid-base titrations

Titration curve strong acid + strong base

Titration curve weak acid + strong base

Titration curves chemguide.co.uk

Buffers solutions composed of a weak acid or base and its salt (ex. CH3COOH and NaCH3COO) and can withstand large changes in pH. Buffer capacity is the amount of acid or base a buffer solution can absorb without a significant change in pH. This has very important biological implications.

Buffer examples Ex. Blood maintains a pH of 7.4. The conjugate acid-base pair that acts as a buffer system is H2CO3 and HCO3-. When there is excess acid present H3O+ + HCO3-  H2O + H2CO3 When there is excess base present OH- + H2CO3  H2O + HCO3-