SURVEY OF CHEMISTRY I CHEM 1151 CHAPTER 9 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university

CHAPTER 9 ACIDS, BASES, AND SALTS

ARRHENIUS ACIDS - Acids are substances that ionize in aqueous solutions to produce hydrogen ions (proton, H + ) HCl, HNO 3, H 2 SO 4 HCl(aq) → H + (aq) + Cl - (aq) HNO 3 (aq) → H + (aq) + NO 3 - (aq) Ionize: dissolving in solution (water) to form ions - Arrhenius acids are covalent compounds in the pure state Properties sour taste, change blue litmus paper to red, corrosive

ARRHENIUS BASES - Bases are substances that ionize in aqueous solutions to produce hydroxide ions (OH - ) NaOH, KOH, Ca(OH) 2 NaOH → Na + (aq) + OH - (aq) Ca(OH) 2 → Ca 2+ (aq) + 2OH - (aq) - Arrhenius bases are ionic compounds in the pure state Properties bitter taste, change red litmus paper to blue, slippery to touch

BRONSTED-LOWRY ACIDS - Acids are proton (H + ) donors - Not restricted to aqueous solutions HCl, HNO 3, H 2 SO 4

- Bases are proton acceptors - Not restricted to aqueous solutions NH 3, dimethyl sulfoxide (DMSO) - Proton donation cannot occur unless an acceptor is present BRONSTED-LOWRY BASES

LEWIS ACIDS - Acids are electron pair acceptors - Not restricted to protons or aqueous solutions BF 3, B 2 H 6, Al 2 Cl 6, AlF 3, PCl 5

- Bases are electron pair donors - Not restricted to protons or aqueous solutions NH 3, ethers, ketones, carbon monoxide, sulfoxides LEWIS BASES

ACIDS Monoprotic Acid - Donates one proton per molecule (HNO 3, HCl) Diprotic Acid - Donates two protons per molecule (H 2 SO 4, H 2 CO 3 ) Triprotic Acid - Donates three proton per molecule (H 3 PO 4, H 3 AsO 4 ) Polyprotic Acid - Donates two or more protons per molecule

CONJUGATE ACID BASE PAIRS - Most Bronsted-Lowry acid-base reactions do not undergo 100% conversion - Acid-base equilibrium is established - Every acid has a conjugate base associated with it (by removing H + ) - Every base has a conjugate acid associated with it (by adding H + )

CONJUGATE ACID BASE PAIRS HX(aq) + H 2 O(l)X - (aq) + H 3 O + (aq) - HX donates a proton to H 2 O to form X - HX is the acid and X - is its conjugate base - H 2 O accepts a proton from HX H 2 O acts as a base and H 3 O + is its conjugate acid

CONJUGATE ACID BASE PAIRS NH 3 (aq) + H 2 O(l)NH 4 + (aq) + OH - (aq) HF(aq) + H 2 O(l)H 3 O + (aq) + F - (aq) HNO 3 (aq) + H 2 O(l)H 3 O + (aq) + NO 3 - (aq)

AMPHOTERIC SUBSTANCES - A substance that can lose or accept a proton - A substance that can function as either Bronsted-Lowry acid or Bronsted-Lowry base - H 2 O is the most common (refer to previous slide for examples)

AUTOPROTOLYSIS OF WATER H 2 O(l) + H 2 O(l)H 3 O + (aq) + OH - (aq) KwKw - Self ionization - Pure water molecules (small percentage) interact with one another to form equal amounts of H 3 O + and OH - ions reduces to H + (aq) + OH - (aq)H 2 O(l) KwKw

- The number of H 3 O + and OH - ions present in a sample of pure water at any given time is small - At equilibrium (25 o C) [H 3 O + ] = [OH - ] = 1.00 x M - [H 3 O + ] = hydronium ion concentration - [OH - ] = hydroxide ion concentration AUTOPROTOLYSIS OF WATER

- The ion product constant of water = [H 3 O + ] x [OH - ] = (1.00 x ) x (1.00 x ) = 1.00 x Valid in all solutions (pure water and water with solutes) AUTOPROTOLYSIS OF WATER

Addition of Acidic Solute - increases [H 3 O + ] - [OH - ] decreases by the same factor to make product 1.00 x Addition of Basic Solute - increases [OH - ] - [H 3 O + ] decreases by the same factor to make product 1.00 x AUTOPROTOLYSIS OF WATER

STRENGTH OF ACIDS Strong Acids - Transfer 100% (or very nearly 100%) of their protons to H 2 O in aqueous solution - Completely or nearly completely ionize in aqueous solution - Strong electrolytes HCl, HNO 3, H 2 SO 4 Weak Acids - Transfer only a small percentage (< 5%) of their protons to H 2 O in aqueous solution Organic acids: acetic acid, citric acid

Strong Bases - Completely or nearly completely ionize in aqueous solution - Strong electrolytes Hydroxides of Groups IA and IIA are strong bases LiOH, CsOH, Ba(OH) 2, Ca(OH) 2 most common in lab: NaOH and KOH Weak bases - produce small amounts of OH - ions in aqueous solution methylamine, cocaine, morphine most common: NH 3 STRENGTH OF BASES

THE pH CONCEPT pH - Negative logarithm of the hydronium ion concentration [H 3 O + ] in an aqueous solution - [H 3 O + ] and [H + ] are used interchangeably pH = - log[H 3 O + ] Or pH = - log[H + ]

Acidic Solution - An aqueous solution in which [H 3 O + ] is higher than [OH - ] (pH is less than 7.0) Basic Solution - An aqueous solution in which [OH - ] is higher than [H 3 O + ] (pH is greater than 7.0) Neutral Solution - An aqueous solution in which [H 3 O + ] is equal to [OH - ] (pH is equal to 7.0) - Increasing [H 3 O + ] lowers the pH THE pH CONCEPT

- For [H 3 O + ] coefficient of Expressed in exponential notation - The pH is the negative of the exponent value [H 3 O + ] = 1.0 x M, then pH = 5.0 [H 3 O + ] = 1.0 x M, then pH = 3.0 [H 3 O + ] = 1.0 x M, then pH = 11.0

- A change of 1 unit in pH corresponds to a tenfold change in [H 3 O + ] pH = 3.0 implies [H 3 O + ] = 1.0 x M = M pH = 2.0 implies [H 3 O + ] = 1.0 x M = M which is tenfold - The pH meter and the litmus paper are used to determine pH values of solutions THE pH CONCEPT

ACID-BASE REACTIONS - Neutralization reactions - Occurs when a solution of an acid is mixed with a solution of a base - The products are salt and water when the base is a metal hydroxide HCl(aq) + NaOH(aq) → NaCl(aq) + H 2 O(l) HNO 3 (aq) + KOH(aq) → KNO 3 (aq) + H 2 O(l) - These reactions can be viewed as proton transfer reactions or double-replacement reactions - H + and OH - ions always react in a one-to-one ratio to form water

SALTS - A cation from a base combines with an anion from an acid to form a salt (A reaction between an acid and a hydroxide base) - Salts are ionic compounds - The positive ion is a metal or polyatomic ion - The negative ion is a nonmetal or polyatomic ion [exception is the hydroxide ion (OH - )] - Salts dissociate completely into ions in solution (strong electrolytes)

BUFFER SOLUTION - A mixture of a conjugate acid-base pair - Tends to resist changes in pH upon addition of an acid or a base - The resistive action is the result of equilibrium between the weak acid (HA) and its conjugate base (A - ) HA(aq) + H 2 O(l) → H 3 O + (aq) + A - (aq) - Commonly used in biological systems - Enzyme-catalyzed reactions depend on pH

BUFFER SOLUTION Examples HC 2 H 3 O 2 /C 2 H 3 O 2 - HF/F - NH 3 /NH 4 + H 2 CO 3 /HCO 3 -