Acids and Bases Properties of Acids u Sour

Acids and Bases

Properties of Acids u Sour taste u React w/ metals to form H 2 u Most contain hydrogen u Are electrolytes u Change color in the presence of indicators (turns litmus red) u Has a p. H lower than 7

Two Types of Acids u Strong acids – Any acid that dissociates completely in aqueous sol’n u Weak acids – Any acid that partially dissociates in aqueous sol’n

Properties of Bases u Bitter taste u Slippery feel u Are electrolytes u Change color in the presence of indicators (turns litmus blue) u Has a p. H higher than 7

Types of Bases u Strong Base – Any base that dissociates completely in aqueous sol’n u Weak Base – Any base that partially dissociates in aqueous sol’n

Neutralization u Neutralization rxn: a rxn of an acid and a base in aqueous sol’n to produce a salt and water u Salt: compound formed from the positive ion of a base and a negative ion of an acid u Properties of the acid and base cancel each other

Arrhenius Model of Acids and Bases Proposed the model in 1887 u Acid: any compound that produces H+ ions in aqueous (water) sol’n u Base: any compound that produces OH (hydroxide) ion in aqueous sol’n u Offers an explanation of why acids and bases neutralize each other (H+ + OH = H 2 O) u

Problems with Model u Restricts acids and bases to water sol’ns (similar reactions occur in the gas phase) u Does not include certain compounds that have characteristics of bases (e. g. , ammonia)

Brønsted-Lowry Model of Acids and Bases Brønsted acid: a hydrogen ion donor (H+, or proton) u Brønsted base: a hydrogen ion acceptor u Defines acids and bases independently of how they behave in water u Amphiprotic: having the property of behaving as an acid and a base u – Also called amphoteric, e. g. , water

Lewis Model u More general definition than either Arrhenius or Bronsted Lowry u Includes substances that are not classified as acids or bases under the other definitions u Acid a substance that can accept a pair of electrons to form a covalent bond

Conjugate Acid-Base Pairs The rxn between Brønsted Lowry acids and bases can proceed in the reverse direction (reversible reactions) HX (aq) + H 2 O (l) H 3 O+ (aq) + X (aq) u The water molecule becomes a hydronium ion (H 3 O+), and is an acid because it has an extra H+ to donate u The acid HX, after donating the H+, becomes a base X u

Conjugate Acids and Bases Base Conjugate Acid HX (aq) + H 2 O (l) H 3 O+ (aq) + X Acid(aq) Conjugate Base Forward reaction: Acid and base Reverse reaction: Conjugate acid and conjugate base

Hydronium Ions - H 3 O+ ion u In water H+ ions are strongly attracted to the electrons surrounding water molecules u When one compound in a reaction acts as an acid (donate an H+ ion) the other acts as a base (accepts an H+ ion)

Types of Acids u Monoprotic acids: acids that contain only 1 hydrogen; e. g. , HCl u Diprotic acids: acids that contain 2 hydrogens; e. g. H 2 CO 3 u Triprotic acids: acids that contain 3 hydrogens; e. g. H 3 PO 4

Strengths of Acids and Bases u Strong acid/base – acids/bases that dissociate completely in water u Strong Acids HCl, H 2 SO 4, HBr , HNO 3 HI , HCl. O 4 u Strong Bases – Li. OH, Na. OH, Ca(OH) 2 KOH, Sr(OH)2 , Rb. OH, Ba(OH)2 ,

Strengths of Acids and Bases u Weak acid/base – acids/bases that dissociate only partly in water u There is an inverse relationship between the strengths of conjugate acid base pairs u The stronger the acid, the weaker the conjugate base and vise versa u The stronger the base, the weaker the conjugate acid and vise versa

Dissociation Constants Acid dissociation constant: (Ka): the equilibrium constant for the rxn of an aqueous weak acid and water u Base dissociation constant: (Kb): the equilibrium constant for the rxn of an aqueous weak base w/ water u Both are derived from the ratio of the concentration of the products and reactants at equilibrium u

Acid Dissociation Constant Ka = [H 3 O+] [A-] [HA] u Ka is a measure of the strength of an acid u Ka values for weak acids are always less than one u Used mostly w/ weak acids because the Ka values for strong acids approach infinity

Ka Example u Complete the reaction and write the equilibrium constant expression for the following reactions u HCl (aq) + H 2 O (l) u HCO 3 (aq) + H 2 O (l)

Ka Example u. A monoprotic weak acid has a concentration of 0. 092 M. At equilibrium, the concentration of hydronium is 0. 0024 M. What is the Ka for this acid?

Example Assume that enough lactic acid is dissolved in sour milk to give a solution concentration of 0. 100 M lactic acid. A p. H meter shows that the p. H of the sour milk is 2. 43. Calculate Ka for the lactic acid equilibrium system.

Base Dissociation Constant Kb = [HB+] [OH ] [B] u Kb is a measure of the strength of a base u Kb values for weak bases are always less than 1 u Kb values for strong bases approach infinity

Kb Example u Complete the reaction and write the equilibrium constant expression for the following reactions u CH 3 NH 2 (aq) + H 2 O (l) u NH 4+ (aq) + H 2 O (l)

Kb Example u 0. 23 mol of a weak base is mixed with 1. 5 L of water. At equilibrium, the concentration of OH is 0. 0040 M. What is the Kb for the base?

Salt Hydrolysis Reactions u Reactions of ions from salts to form H 3 O+ or OH ions u Can predict whether a salt hydrolysis rxn produces an acidic sol’n or basic sol’n u Consider the acid and base the salt is formed from – see next slide

Salt Hydrolysis Reactions u 4 possibilities – strong acid + strong base = neutral salt – strong acid + weak base = acidic salt – weak acid + strong base = basic salt – weak acid + weak base = acidic, basic, or neutral salt, depending on

Identifying Acids u Acidic hydrogen – hydrogens that can be donated u Not every hydrogen is acidic HC 2 H 3 O 2 u Acidic hydrogens have a slight positive charge while it is still a part of a molecule u the acidic hydrogen is on the

3 Categories of Acids u binary acids – contains hydrogen and 1 other element u oxy acids – contains hydrogen, oxygen, and 1 other element (polyatomic ions) u carboxylic acids – organic acids; acids that contain carbon atoms

Identifying Bases u Bronsted Lowry base always contains an unshared pair of electrons u 2 categories of bases – anions – includes monatomic and polyatomic anions – amines – contains nitrogen atoms that has an unshared pair of electrons; ammonia derivative

u Ex. What is the concentration of hydroxide ions in blood, if the hydronium ion concentration is 4. 5 x 10 8? Is blood acidic, basic, or neutral?

Water u Water can dissociate into its component ions, H+ and OH– 2 H 2 O (l) H 3 O+ (aq) + OH- (aq) One water molecule acts as a weak acid, and the other acts as a weak base u The ions are present in such small amounts they can’t be detected by a conductivity apparatus u In pure water, [H 3 O+] =1. 0 x 10 – 7 M and [OH-] = 1. 0 x 10 -7 M u

Dissociation Constant for Water It is defined as Kw: the ion product constant for water u Kw = [H 3 O+] [OH ] u Kw = (1. 0 x 10 7) u Kw = 1. 0 x 10 14 u The value of Kw can always be used to find the concentration of either H 3 O+ or OH given the concentration of the other u

p. H and [H 3 O+] u p. H: number that is derived from the concentration of hydronium ions ([H 3 O+]) in sol’n – p. H = -log [H 3 O+] – As p. H increases, [H 3 O+] decreases u Scale ranges from 0 – 14 – p. H = 7 is neutral – p. H < 7 is acidic – p. H > 7 is basic

Measuring p. H u 2 ways to measure p. H u indicators u p. H meters u both detect the presence of H 3 O+ ions u indicators change color based on the H 3 O+ ions u common indicators: litmus

p[OH] u p. OH = log [OH ] u p. H + p. OH = 14. 00 u Calculating ion concentrations from p. H [H+] = antilog ( p. H) t [OH ] = antilog ( p. OH) t

Examples What is the p. H of a 0. 001 M sol’n of HCl, a strong acid?

Examples What is the p. H of a sol’n if [H 3 O+] = 3. 4 x 10 5 M?

Examples The p. H of a sol’n is measured with a p. H meter and determined to be 9. 00. What is the [H 3 O+]?

Examples The p. H o f a sol’n is measured with a p. H meter and determined to be 7. 52. What is [H 3 O+]?

Buffers u. A mixture that is able to release or absorb H+ ion, keeping a solutions p. H constant u Can control the p. H within very narrow limits u Most common buffers are mixtures of weak acids and their conjugate bases

Buffer Capacity u The amount of acids or base that a buffer can neutralize u All buffers have a limited capacity to neutralize H 3 O+ ions or OH ions u If you add H 3 O+ ions or OH‑ ions beyond the buffer capacity, the ions will remain in the solution, changing the p. H u The greater concentration of buffer

Titrations An analytical procedure used to determine the concentration of a sample by reacting it with a standard sol’n u In a titration, an indicator is used to determine the end point u Standard sol’n: a sol’n of precisely known concentration u Indicator: any substance in sol’n that changes color as it reacts with either an acid or a base u

Titrations Each indicator changes its color over a particular range of p. H values (transition interval) u An unknown acid sol’n will be titrated with a standard sol’n that is a strong base u An unknown base sol’n will be titrated with a standard sol’n that is a strong acid u

Titrations u Equivalence point: point at which the concentration of H 3 O+ ions is the same as the concentration of OH ions; [H 3 O+ ] = [OH ] u Endpoint: the point at which the indicator changes color u Titration curve: graph that shows how p. H changes in a titration

Titrations u The equivalence point is at the center of the steep, vertical region of the titration curve u At the equivalence point, p. H increases greatly w/ only a few drops

Example Problem 1 What is the molarity of a Cs. OH solution if 20. 0 m. L of the solution is neutralized by 26. 4 m. L of 0. 250 M HBr solution? HBr + Cs. OH → H 2 O + Cs. Br

Example Problem 2 What is the molarity of a nitric acid solution if 43. 33 m. L 0. 200 M KOH solution is needed to neutralize 20. 00 m. L of unknown solution?

Example Problem 3 What is the concentration of a household ammonia cleaning solution (NH 4 OH) if 49. 90 m. L of 0. 5900 M H 2 SO 4 is required to neutralize 25. 00 m. L solution?