Acids and Bases Chapter 15

I. Properties of Acids & Bases

Example Acids Citric acid Stomach acid (HCl) Vinegar Ascorbic acid

Properties of Acids Taste sour React with metals Often corrosive Turn blue litmus paper red Watch someone eat a lemon Reacts with metals to form hydrogen gas

Common Acids

Example Bases Ammonia Baking soda Antacids Soaps Cleaners

Properties of Bases Taste bitter Feel slippery Can burn Turn red litmus paper blue Taste of vinegar, slippery like soap

Common Bases

II. Acid- Base Theories

Arrhenius Theory of Acids A chemical compound that increases the concentration of hydrogen ions (H+) in aqueous solution Hydronium ion (H3O+) – formed from an aqueous acid solution Example: HCl + H2O  H3O+ + Cl-

Arrhenius Theory of Bases A substance that increases the concentration of hydroxide ions (OH-) in aqueous solution Example: NaOH(aq)  Na+(aq) + OH–(aq)

Brønsted–Lowry Acid–Base Theory Acid: proton (H+) donor HCl + NH3  NH4+ + Cl– HCl donates a H+ to NH3 H+ does not exist by itself

Bases: accept a proton H2O + NH3  NH4+ + OH– NH3 accepts a H+ from H2O

Acid/Base Conjugates Brønsted–Lowry theory defines acids and bases in terms of proton (H+) transfer. A Brønsted–Lowry acid is a proton donor. A Brønsted–Lowry base is a proton acceptor. The conjugate base of an acid is the acid minus the proton it has donated. The conjugate acid of a base is the base plus the accepted proton.

Practice problems Identify the acid, base, conjugate acid, conjugate base, and conjugate acid-base pairs: HC2H3O2(aq) + H2O(l)  C2H3O2–(aq) + H3O+(aq) acid base conjugate base conjugate acid conjugate acid-base pairs OH –(aq) + HCO3–(aq)  CO32–(aq) + H2O(l) base acid conjugate base conjugate acid conjugate acid-base pairs

III. Acids & Bases with Water

Amphiprotic A substance that can either donate or accept a proton H2O most common amphiprotic substance Reacts with base: H2O + NH3  NH4+ + OH– Reacts with acid: H2O + HCl  Cl– + H3O+

Aqueous Acids and Bases Water as both an acid and a base Arrhenius H2O  H+ + OH- Brønsted-Lowry H2O + H2O  H3O+ + OH-

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, HNO3, H2SO4 Most other acids are weak acids Strong acids are strong electrolytes

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: NH3

Aqueous Acids and Bases Acid in water Ionic compounds Strength based on concentration of H+ in solution Base in water Strength based on concentration of OH- in solution Ions break apart in water

Aqueous Acids and Bases Pure water is perfectly neutral Not the slightest bit acidic or basic Water ionization is reversible H2O + H2O  H3O+ + OH- There exists a dynamic equilibrium Simultaneous ionization and recombination Rate of the forward reaction = rate of the reverse reaction

Neutralization Reaction of an acid and a base to form water and a salt HCl + NaOH  NaCl + H2O

IV. pH Scale

pH Scale Typical values range from 0 to 14 pH = 7 – neutral pH > 7 – basic pH < 7 – acidic

pH in Common Solutions

Measurement of Acidity H2O + H2O  H3O+ + OH- The self-ionization of water at 25˚C Equal amounts of H+ and OH- are produced Equal concentrations of 1 x 10-7 moles/liter Neutral water: [H+ ] = 0.0000001 M or 1 x 10-7M [OH-] = 0.0000001M or 1 x 10-7M Ionization constant of water – Kw Kw = [H3O+][OH-] = (1 x 10-7M)(1 x 10-7M) = 1 x 10-14M pH represents the power of the Hydrogen Negative logarithm of the H+ concentration pH = -log [H+ ] = -log 10-7 = -(-7) pH = +7

What is the pH of 1 x 10-3M of HCl?

pH = 3

What is the pH for 1 x 10-4M of NaOH

pH = 10

What is the pH of a solution if the [H3O+] is 3.4 x 10-5M?

pH = -log [H3O+] = -log (3.4 x 10-5) = 4.47

Calculating the concentration from the pH [H3O+] = antilog (-pH) What is the hydronium concentration of an acid solution having a pH of 3?

What is the hydronium concentration of an acid solution having a pH of 5.3?

Indicators Compounds whose colors are sensitive to pH Methyl orange – changes colors with a pH between 3.1 and 4.4 Phenolphthalein – changes colors with a pH between 8.0 and 10.0

Nature’s Indicator Many flowers and plants contain acid/base indicators which change color in response to a change in pH Chemical structure changes when pH changes Soil pH and anthocyanins

Titration 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 concetration

Antacids and Acid Indigestion Antacids are used to decrease the production of excess stomach acid Active ingredients Hydroxides or carbonates React with stomach acid (HCl) Too much of a good thing! Acid rebound

If a solution ionizes almost completely to form OH- ions, what is the substance?

What is a conjugate acid/base pair in the following equation What is a conjugate acid/base pair in the following equation? HCN + H2O  H3O + CN-

When the hydroxide ion concentration is greater than the hydronium ion concentration, is the solution an acid or a base?

What is the pH of a 3.2 x 10-6M?

What is the pH of a 1.0 x 10-3M solution of NaOH?