2 Section 20.1 Describing Acids and Bases OBJECTIVES:List the properties of acids and bases.
3 Section 20.1 Describing Acids and Bases OBJECTIVES:Name an acid or base, when given the formula.
4 Properties of acids Taste sour (don’t try this at home). Conduct electricity.Some are strong, others are weak electrolytes.React with metals to form hydrogen gas.Change indicators (blue litmus to red).React with hydroxides to form water and a salt.
5 Properties of bases React with acids to form water and a salt. Taste bitter.Feel slippery (don’t try this either).Can be strong or weak electrolytes.Change indicators (red litmus turns blue).
6 Names and Formulas of Acids An acid is a chemical that produces hydrogen ions (H1+) when dissolved in waterThus, general formula = HA, where A is a monatomic or polyatomic anionHCl(g) is hydrogen monochlorideHCl(aq) is named as an acidName focuses on the anion present
7 Names and Formulas of Acids 1. BINARY - When anion ends with -ide, the acid starts with hydro-, and the stem of the anion has the suffix -ic followed by the word acid2. TERNARY - When anion ends with -ite, the anion has the suffix -ous, then acid3. TERNARY - When anion ends with -ate, the anion suffix is -ic and then acid
8 Names and Formulas of Bases A base produces hydroxide ions (OH1-) when dissolved in water.Named the same way as any other ionic compoundname the cation, followed by anionTo write the formula: write symbols; write charges; then cross (if needed)Sample Problem 20-1, p. 579
9 Section 20.2 Hydrogen Ions and Acidity OBJECTIVES:Given the hydrogen-ion or hydroxide-ion concentration, classify a solution as neutral, acidic, or basic.
10 Section 20.2 Hydrogen Ions and Acidity OBJECTIVES:Convert hydrogen-ion concentrations into values of pH, and hydroxide-ion concentrations into values of pOH.
11 Hydrogen Ions from Water Water ionizes, or falls apart into ions:H2O ® H1+ + OH1-Called the “self ionization” of waterOccurs to a very small extent:[H1+ ] = [OH1-] = 1 x 10-7 MSince they are equal, a neutral solution results from waterKw = [H1+ ] x [OH1-] = 1 x 10-14Kw is called the “ion product constant”
12 Ion Product Constant H2O H+ + OH- Kw is constant in every aqueous solution: [H+] x [OH-] = 1 x 10-14If [H+] > 10-7 then [OH-] < 10-7If [H+] < 10-7 then [OH-] > 10-7If we know one, other can be determinedIf [H+] > 10-7 , it is acidic and [OH-] < 10-7If [H+] < 10-7 , it is basic and [OH-] > 10-7Basic solutions also called “alkaline”Sample problem 20-2, p. 582
13 Logarithms and the pH concept Logarithms are powers of ten.Review from earlier lessons, and p. 585definition: pH = -log[H+]in neutral pH = -log(1 x 10-7) = 7in acidic solution [H+] > 10-7pH < -log(10-7)pH < 7 (from 0 to 7 is the acid range)in base, pH > 7 (7 to 14 is base range)
14 pH and pOH pH = -log[H+] pOH = -log [OH-] Kw = [H+] x [OH-] = 1 x 10-14pH + pOH = 14Thus, a solution with a pH less than 7 is an acid; a pH greater than 7 is a base; 7 is neutral
17 Measuring pH Why measure pH? Everything from swimming pools, soil conditions for plants, medical diagnosis, soaps and shampoos, etc.Sometimes we can use indicators, other times we might need a pH meter
18 Acid-Base IndicatorsAn indicator is an acid or base that undergoes dissociation in a known pH range, and has different colors in solution (more later in chapter)Examples: litmus, phenolphthalein, bromthymol blue: Fig 20.8, p.590
19 Acid-Base IndicatorsAlthough useful, there are limitations to indicators:usually given for a certain temperature (25 oC), thus may change at different temperatureswhat if the solution already has color?ability of human eye to distinguish colors
20 Acid-Base Indicators A pH meter may give more definitive results some are large, others portableworks by measuring the voltage between two electrodesneeds to be calibratedFig , p.591
21 Section 20.3 Acid-Base Theories OBJECTIVES:Compare and contrast acids and bases as defined by the theories of Arrhenius, Brønsted-Lowry, and Lewis
23 Svante ArrheniusSwedish chemist ( ) - Nobel prize winner in chemistry (1903)one of the first chemists to explain the chemical theory of the behavior of acids and basesDr. Hubert Alyea-last graduate student of Arrhenius. (link below)
26 1. Arrhenius DefinitionAcids produce hydrogen ions (H1+) in aqueous solution.Bases produce hydroxide ions (OH1-) when dissolved in water.Limited to aqueous solutions.Only one kind of base (hydroxides)NH3 (ammonia) could not be an Arrhenius base.
27 Polyprotic Acids Some compounds have more than 1 ionizable hydrogen. HNO3 nitric acid - monoproticH2SO4 sulfuric acid - diprotic - 2 H+H3PO4 phosphoric acid - triprotic - 3 H+Having more than one ionizable hydrogen does not mean stronger!
28 Polyprotic AcidsHowever, not all compounds that have hydrogen are acidsAlso, not all the hydrogen in an acid may be released as ionsonly those that have very polar bonds are ionizable - this is when the hydrogen is joined to a very electronegative element
29 Arrhenius examples... Consider HCl What about CH4 (methane)? CH3COOH (ethanoic acid, or acetic acid) - it has 4 hydrogens like methane does…?Table 20.4, p. 595 for bases
30 2. Brønsted-Lowry Definitions Broader definition than ArrheniusAcid is hydrogen-ion donor (H+ or proton); base is hydrogen-ion acceptor.Acids and bases always come in pairs.HCl is an acid.When it dissolves in water, it gives it’s proton to water.HCl(g) + H2O(l) H3O+ + Cl-Water is a base; makes hydronium ion.
31 Johannes Bronsted / Thomas Lowry (1879-1947) (1874-1936)
32 Acids and bases come in pairs... A conjugate base is the remainder of the original acid, after it donates it’s hydrogen ionA conjugate acid is the particle formed when the original base gains a hydrogen ionIndicators are weak acids or bases that have a different color from their original acid and base
33 Acids and bases come in pairs... General equation is:HA(aq) + H2O(l) H3O+(aq) + A-(aq)Acid + Base Conjugate acid Conjugate baseNH3 + H2O NH41+ + OH1-base acid c.a c.b.HCl + H2O H3O1+ + Cl1-acid base c.a c.b.Amphoteric - acts as acid or base
34 3. Lewis Acids and BasesGilbert Lewis focused on the donation or acceptance of a pair of electrons during a reactionLewis Acid - electron pair acceptorLewis Base - electron pair donorMost general of all 3 definitions; acids don’t even need hydrogen!Sample Problem 20-7, p.599
36 Section 20.4 Strengths of Acids and Bases OBJECTIVES:Define strong acids and weak acids.
37 Section 20.4 Strengths of Acids and Bases OBJECTIVES:Calculate an acid dissociation constant (Ka) from concentration and pH measurements.
38 Section 20.4 Strengths of Acids and Bases OBJECTIVES:Arrange acids by strength according to their acid dissociation constants (Ka).
39 Section 20.4 Strengths of Acids and Bases OBJECTIVES:Arrange bases by strength according to their base dissociation constants (Kb).
40 Strength Strong acids and bases are strong electrolytes They fall apart (ionize) completely.Weak acids don’t completely ionize.Strength different from concentrationStrong-forms many ions when dissolvedMg(OH)2 is a strong base- it falls completely apart when dissolved.But, not much dissolves- not concentrated
41 Measuring strength Ionization is reversible. HA H+ + A- This makes an equilibriumAcid dissociation constant = KaKa = [H+ ][A- ] (water is constant) [HA]Stronger acid = more products (ions), thus a larger Ka (Table 20.8, p.602)
42 What about bases? Strong bases dissociate completely. B + H2O BH+ + OH-Base dissociation constant = KbKb = [BH+ ][OH-] [B] (we ignore the water)Stronger base = more dissociated, thus a larger Kb.
43 Strength vs. Concentration The words concentrated and dilute tell how much of an acid or base is dissolved in solution - refers to the number of moles of acid or base in a given volumeThe words strong and weak refer to the extent of ionization of an acid or baseIs concentrated weak acid possible?
44 Practice Write the expression for HNO2 Write the Kb for NH3 Sample 20-8, p. 604Carefully study Key Terms and equations, p. 608Be sure to do the ChemASAP programs, and take all the self-tests that are available!