Chapter 19: Acids & Bases

Properties of Acids & Bases Acid: substance dissociates to form H+ in solution Tastes sour Feels sticky Conductor of electricity Turns blue litmus red Example: HCl Base: substance dissociates to form OH- (hydroxide) in solution Tastes bitter Feels slippery Turns red litmus blue Example: NaOH

Arrhenius Model Arrhenius model: HCl ionizes to produce H+ ions. Traditional definition for acids and bases States that an acid is a substance that contains hydrogen and ionizes to produce hydrogen ions in aqueous solution, and a base is a substance that contains a hydroxide group and dissociates to produce a hydroxide ion in solution. HCl ionizes to produce H+ ions. HCl(g) → H+(aq) + Cl–(aq) NaOH dissociates to produce OH– ions. NaOH(s) → Na+(aq) + OH–(aq) * Some solutions produce hydroxide ions even though they do not contain a hydroxide group.

The usual solvent for acids and bases is water— water produces equal numbers of hydrogen and hydroxide ions in a process called self-ionization. H2O(l) ↔ H+(aq) + OH–(aq) In reality: H2O(l) + H2O(l) ↔ H3O+(aq) + OH–(aq) The hydronium ion is H3O+. The symbols H+ and H3O+ can be used interchangeably.

Brønsted-Lowry Model Brønsted-Lowry Model: More general definition for acids and bases States that an acid is a hydrogen ion donor and a base is a hydrogen ion acceptor ** NH3 is a Brønsted-Lowry base, but not an Arrhenius base NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq) - H2O (acting as an acid) donates a proton to ammonia - NH3 (acting as a base) accepts a proton from water Conjugate acid: the species produced when a base accepts a hydrogen ion (NH4+ in the above reaction) Conjugate base: the species produced when an acid donates a hydrogen ion (OH- in example) Conjugate acid-base pair: consists of two substances related to each other by donating and accepting a single hydrogen ion

Hydrogen fluoride—a Brønsted-Lowry acid HF(aq) + H2O(l) ↔ H3O+(aq) + F–(aq) HF = acid, H2O = base, H3O+ = conjugate acid, F– = conjugate base

HNO2 (aq) + H2O (l)  NO2- (aq) + H3O+ (aq) Water and other substances that can act as acids or bases are called amphoteric.

Monoprotic and Polyprotic Acids Monoprotic Acid: an acid that can donate only one hydrogen ion Only ionizable hydrogen atoms can be donated Polyprotic Acid: an acid that can donate more than one hydrogen ion

Strength of Acids Strong Acids: acids that ionize completely Because they produce the maximum number of hydrogen ions, strong acids are good conductors of electricity Conjugate bases are usually weak HCl, HBr, HI, HNO3, HClO3, HClO4, H2SO4 Weak Acids: acids that ionize only partially in dilute solutions

Strength of Bases Strong Base: a base that dissociates completely into metal ions and hydroxide ions Alkali metals or heavier alkaline earth metals Weak base: ionizes only partially in dilute aqueous solution

pH pH scale: measure of the acidity of an aqueous solution Concentrations of H+ ions are often small numbers expressed in exponential notation pH is the negative logarithm of the hydrogen ion concentration of a solution. Changing [H+] by a factor of 10 causes the pH to change by 1 unit pH = -log [H+] - Litmus paper and a pH meter with electrodes can determine the pH of a solution.

pOH pOH of a solution is the negative logarithm of the hydroxide ion concentration pOH = -log [OH-] The sum of pH and pOH equals 14

Practice Calculate the [H+] and [OH-] when pH = 7.40 Calculate the pH and pOH when [OH-] = 4.0x10-3M

Neutralization A neutralization reaction is a reaction in which an acid and a base in an aqueous solution react to produce a salt and water. A salt is an ionic compound made up of a cation from a base and an anion from an acid. Neutralization is a double-replacement reaction.

Titrations Titration is a method for determining the concentration of a solution by reacting a known volume of that solution with a solution of known concentration. In a titration procedure, a measured volume of an acid or base of unknown concentration is placed in a beaker, and initial pH recorded. A buret is filled with the titrating solution of known concentration, called a titrant.

Measured volumes of the standard solution are added slowly and mixed into the solution in the beaker, and the pH is read and recorded after each addition. The process continues until the reaction reaches the equivalence point, which is the point at which moles of H+ ion from the acid equals moles of OH– ion from the base. An abrupt change in pH occurs at the equivalence point.

Chemical dyes whose color are affected by acidic and basic solutions are called acid-base indicators.

An end point is the point at which an indicator used in a titration changes color. An indicator will change color at the equivalence point.

Molarity from Titration ** Must have a balanced equation** Use stoichiometry A volume of 18.28mL of a standard solution of 0.1000M NaOH was required to neutralize 25.00 mL of a solution of methanoic acid (HCOOH). What is the molarity of the acid solution

What is the molarity of nitric acid solution is 43. 33 mL of 0 What is the molarity of nitric acid solution is 43.33 mL of 0.1000M KOH solution is needed to neutralize 20.00 mL of the acid solution.

How many milliliters of 0. 500 M NaOH would neutralize 25. 00 mL of 0 How many milliliters of 0.500 M NaOH would neutralize 25.00 mL of 0.100M H3PO4.