Acids and Bases Chemistry 2013

Terminology Note H+(aq) = H3O+ or hydrogen ions in water are equivalent to hydronium ions! Draw a hydronium ion

Acid-Base Theories There are three Acid-Base Theories Arrhenius Bronsted-Lowry Lewis Arrhenius—most narrow Bronsted-Lowry—broader than Arrhenius Lewis—most broad

Arrhenius Arrhenius Acid: Arrhenius Base: those species that ionize to produce H+ ions (or hydronium ions) in water. Examples: those species that ionize to produce OH-- ions in water. Examples:

Bronsted-Lowry Bronsted Acid Bronsted Base H+ (proton) donors Examples: H+ (proton) acceptors Examples:

Lewis Lewis Acid Lewis Base Electron Pair Acceptor Electron Pair Donor This is an extremely wide definition of acids and bases. This definition of acids and bases gets a new name and purpose in organic chemistry . It is beyond the scope of high school chemistry.

Relationship Between Definitions If a species is an acid under the Arrhenius definition, then it is an acid under Bronsted-Lowry, but the reverse is not automatically true.

Bronsted-Lowry Acids & Bases Let’s take a closer look:

Terms Associated with Bronsted-Lowry A/B _________________ —is the specie that is formed once the proton (H+) has been accepted. It is often abbreviated as CA. It is what the reactant base becomes in the products. _________________ —is the specie that remains after the proton (H+) has been donated. It is often abbreviated as CB. It is what the reactant acid becomes in the products. An acid and its conjugate base can be referred to as a conjugate acid-base pair.

Generic Weak Acid Example HA + H2O   H3O+ + A-

Example 1 Identify the acid, base, conjugate acid, and conjugate base for the following reactions: HI + H2O   I- + H3O+ H2O + HONH3+   HONH2 + H3O+ NH3 + H2O   NH4+ + OH-

Strong Acids There are 5 strong acids: All other acids are weak acids at some level. There are varying degrees of weakness. Strong acids have weak conjugate bases, meaning that as a base, they have a ___________________ for a proton (H+). Weak acids ionize into strong conjugate bases, meaning that as a base, they have a _______________ for a proton. The strength of an acid is _______________________ to the strength of its conjugate base.

Protic & Oxyacids Acids can be monoprotic, diprotic, triprotic (or just polyprotic in general). Write two diprotic acids.  Most acids are oxyacids, meaning that the acidic proton is attached to an oxygen atom. Oxyacids can be strong or weak acids, but most are weak acids. Write two oxyacids.

R = generic hydrocarbon Organic Acids Organic acids are acids that are primary hydrocarbon chains and contain the carboxyl group, usually at the end of the chain: Write one organic acid. O R-C-O-H This is the acidic H R = generic hydrocarbon

Amphoterism Water is an ______________ substance, meaning it can act as either an acid or a base. The autoionization of water reaction is: H2O + H2O   H3O+ + OH- or H2O   H+ + OH-

Equilibrium Expression for Water The equilibrium expression for water is called a Kw Kw = [H+][OH-] = 1.0 x 10-14 [H+] means the concentration of H+ ions in _________ [OH-] means the concentration of OH- ions in ____________

Acidity of Solutions A neutral solution is defined as one where An acidic solution is defined as one where A basic solution is defined as one where

Example 2 Are the following solutions acidic, basic, or neutral? 0.01 M HCl 0.001 M NaOH 1.00 x 10-7 M H3O+

pH Scale The pH scale is simply a specific application of a function. f(x) here is p(x). The “p” function means to take the –log of whatever comes after the “p”. For instance: pH = -log [H+] = -log [H3O+] pOH = -log [OH-]  pKw = -log Kw   Note that since the p function is logarithmic, every whole number change in the pH means a __________________ in the concentration.

Example 3 Calculate the pH of: 1.0 x 10-3 M HCl 2.5 x 10-4 M HClO4 5.9 x 10-2 M NaOH

Example 4 If the pH of the acid is 6.9, calculate the concentration of the hydronium ion.

Strong Bases Strong bases are hydroxide bases such as NaOH, or KOH because they ionize (or dissociate) completely in water. Bases do not have to contain hydroxide ions; they can be species that will remove a proton from water, producing the hydroxide ion from the water molecule. Other important relationships to note: pH + pOH = pKw pH + pOH = 14

Example 5 Calculate the pH for a 1.50 M solution of KOH.

Strong and Weak

Concentration versus Strength

Strength Strong Weak

Comparing Concentration and Strength of Acids Quantitative (Molarity) Acidic Solution Concentration Strength Quantitative (Molarity) Relative Hydrochloric Acid 12 M HCl Gastric juice 0.08 M HCl Ethanoic Acid 17 M CH3COOH Vinegar 0.2 M CH3COOH

Strengths of Weak Acids (& Bases)

Acid Dissociation

Dissociation Constants For any reversible reaction, you can use a balanced equation to express the equilibrium constant. The equilibrium constant (Keq) is the ratio of product concentrations to reactant concentrations at equilibrium. The size of the equilibrium constant indicates whether reactants or products are more common at equilibrium. When Keq has a large value, the reaction mixture at equilibrium will consist mainly of product. When Keq has a small value, the mixture at equilibrium will consist mainly of reactant. When Keq has an intermediate value, the mixture will have significant amounts of both reactant and product.

Acid Dissociation (Ka) For dilute aqueous solutions, the concentration of water is a constant. This constant can be combined with Keq to give an acid dissociation constant. An acid dissociation constant (Ka) is the ratio of the concentration of the substances in the reaction. The acid dissociation constant (Ka) reflects the fraction of an acid that is ionized. For this reason, dissociation constants are sometimes called ionization constants. If the degree of dissociation or ionization of the acid in a solution is small, the value of the dissociation constant will be small. Weak acids have small Ka values. If the degree of ionization of an acid is more complete, the value of Ka will be larger. ________________________________________________________ For example, nitrous acid (HNO2) has a Ka of 4.4 × 10−4, but ethanoic acid (CH3COOH) has a Ka of 1.8 × 10−5. This means that nitrous acid is more ionized in solution than ethanoic acid. Therefore, nitrous acid is a stronger acid than ethanoic acid.

Ka Table

Base Dissociation (Kb) Base Dissociation Constant Just as there are strong acids and weak acids, there are strong bases and weak bases. A strong base dissociates completely into metal ions and hydroxide ions in aqueous solution. Some strong bases, such as calcium hydroxide and magnesium hydroxide, are not very soluble in water. The small amounts of these bases that dissolve in water dissociate completely. A weak base reacts with water to form the conjugate acid of the base and hydroxide ions. For a weak base, the amount of dissociation is relatively small. Ammonia is an example of a weak base. In general, the base dissociation constant (Kb) is the ratio of the concentration of the conjugate acid times the concentration of the hydroxide ion to the concentration of the base. __________________________________________________