Acids and Bases Dr. Ron Rusay Fall 2001 © Copyright 2001 R.J. Rusay

Introduction to Aqueous Acids Acids: taste sour and cause certain dyes to change color.

Introduction to Aqueous Bases Bases: taste bitter, feel soapy and cause certain dyes to turn color.

Electrolytes Aqueous solutions can be categorized into 3 types: non-electrolytes, strong electrolytes or weak electrolytes based on their ability to conduct electricity. A solution must have ions to conduct. Pure Water does not conduct. Aqueous solutions can be tested for conductivity which will determine the degree of ionization of the solutes. It is possible to have full or partial ionization. © Copyright 1995-2000 R.J. Rusay

Solution Test Apparatus for Electrolytes

Conductivity

Electrolytes / Ionization

Electrolytes Almost all ionic compounds and a few molecular compounds are strong electrolytes. Several molecular compounds are weak conductors, most are non-conductors. Conductivity is directly related to the amount of ionization, i.e. ions in solution. Table salt, sodium chloride, is completely ionized: NaCl(s) + H2O(l) ---> NaCl(aq) ---> 0.10mol Na +(aq) + Cl -(aq) 0.00mol 0.10mol 0.10mol © Copyright 1995-2000 R.J. Rusay

Strong vs. Weak Electrolytes

Models of Acids and Bases Arrhenius Concept: Acids produce H+ in solution, bases produce OH ion. Brønsted-Lowry: Acids are H+ donors, bases are proton acceptors. HCl + H2O  Cl + H3O+ acid base

Lewis Acids and Bases Lewis Acid: electron pair acceptor Lewis Base: electron pair donor

Conjugate Acid/Base Pairs HA(aq) + H2O(l)  H3O+(aq) + A(aq) conj conj acid 1 base 1 acid 2 base 2 conjugate base: everything that remains of the acid molecule after a proton is lost. conjugate acid: formed when the proton is transferred to the base.

Acid Strength Strong Acids: 100% of the acid is ionized. For example nitric acid, HNO3 , produces 100% H+ (aq). Other common strong acids are sulfuric and hydrochloric. Stong acids produce very weak conjugate bases, eg. (NO3)

Dissociation of Strong and Weak Acids

Acid Strength (continued) Weak Acids: A weak acid is not 100% ionized. For example acetic acid, CH3COOH, produces <100% H+ (aq). Most acids, particularly organic acids, are weak acids. Weak acids produce a much stronger conjugate base than water, eg. The acetate ion: (CH3COO)

Aqueous Bases NH3 (g) + H2O(l) NH3 (aq) NH4+(aq)+ OH-(aq) Any compound that accepts a proton is a base. The common bases are group IA & IIA metal hydroxide compounds. “Strong” and “weak” are used in the same sense for bases as for acids. Strong = complete dissociation (100% hydroxide ion is supplied to the solution) An example of a weak base is ammonia. NH3 (g) + H2O(l) NH3 (aq) NH4+(aq)+ OH-(aq) © Copyright 1995-2001 R.J. Rusay

H3CNH2(aq) + H2O(l)  H3CNH3+(aq) + OH(aq) Bases (continued) Weak bases have very little dissociation (or reaction with water), eg. methyl amine has <100% hydroxide ion in aqueous solution. H3CNH2(aq) + H2O(l)  H3CNH3+(aq) + OH(aq) Organic bases are weak bases; for example, dopamine (neurotransmitter), cadaverine (product of cellular decomposition) and morphine (narcotic pain killer) are weak bases.

Water as an Acid and a Base Water is amphoteric (it can behave either as an acid or a base). H2O + H2O  H3O+ + OH conj conj acid 1 base 1 acid 2 base 2 Kw = 1  1014 at 25°C

Water: Self-ionization

pH  log[H+]  log[H3O+] The pH Scale pH  log[H+]  log[H3O+] pH in water ranges from 0 to 14. Kw = 1.00  1014 = [H+] [OH] pKw = 14.00 = pH + pOH As pH rises, pOH falls (sum = 14.00). There are no theoretical limits on the values of pH or pOH. (e.g. pH of 2.0 M HCl is -0.301, the pH at Iron Mountain is ~ -2 to -3)

The pH of Some Familiar Aqueous Solutions [H3O+] [OH-] [OH-] = KW acidic solution neutral solution basic solution [H3O+] = [OH-]

Natural Indicators

The pH Scale

Structure and Acid-Base Properties Bond polarity & bond strength affects acidity. In binary compounds: Bond Polarity (The higher the bond polarity, the stronger the bond, the weaker the acid) Bond Strength (The lower the bond strength, the higher the resulting H+ ionization and the stronger the acid. )

Oxides Acidic Oxides (Acid Anhydrides): OX bond is strong and covalent. SO2, NO2, CrO3 Basic Oxides (Basic Anhydrides): OX bond is ionic. K2O, CaO

Neutralization Reactions

Aqueous Reactions: Neutralization Net Ionic Equations HCl(aq) + NaOH (aq) ---> NaCl (aq) + H2O(l) ___________________________________________________ HCl(aq) ---> H+(aq) + Cl -(aq) NaOH (aq) ---> Na+(aq)+ OH-(aq) NaCl (aq) ---> Na+(aq)+ Cl-(aq) ________________________________________________ Na+(aq)+ OH-(aq) + H+(aq) + Cl -(aq) ---> Na+(aq)+ Cl-(aq) + H2O(l) _______________________________________________________ H+(aq) + OH -(aq) ---> H2O(l) © Copyright 1995-2000 R.J. Rusay

Acid-Base Titration

Stomach Chemistry

Buffers