Presentation on theme: "Acids and Bases Acid-Base chemistry important in our everyday lives"— Presentation transcript:
1Acids and Bases Acid-Base chemistry important in our everyday lives acidity of our blood is carefully controlledmaking sulfuric acid is an important industrysulfuric acid is needed to make fertilizers, polymers, steel, etc.environmental impact of acid rain
2Acids and Bases Properties of acids taste sour e.g. vinegar, citric acid in soda and candiesreact with many metals (to form the metal ion and hydrogen gas2H+ + Zn --> H2 + Zn+2react with carbonates (to form CO2)2H+ + CaCO3 --> Ca+2 + H2O + CO2
3Acids and BasesProperties of basesbitter tasteslippery feel
4Acids and Bases Definitions of Acids Arrhenius - an acid produces H+ in aqueous solutionsBronsted-Lowry - an acid is an H+ donorLewis - e- pair acceptor
5Acids and Bases Definitions of Bases Arrhenius - a base produces hydroxide ions in solutionBronsted-Lowry - a base is an H+ acceptorLewis - a base is an e- pair donor
6Acids and Bases Acid-Base Definitions Arrhenius definition is limited to aqueous solutions, and only allows for one kind of base, those with hydroxide ions.Bronsted-Lowry is a more general definitions, water can now act as a base.Lewis - the most general definition
7Acids and Bases Terms to know: proton - H+ ion hydronium ion - H3O+; results from water reacting with H+H2O + H+ --> H3O+conjugate base - whatever is left from an acid after a proton has been donatedconjugate acid - whatever has been formed when a proton has been accepted by a base
8Acids and Bases Acid Dissociation Constant an equilibrium constant for the ionization or dissociation of an acidKafor HA(aq) + H2O(l) <==> H3O+(aq) + A-(aq)Ka = [H3O+][A-] / [HA] which is equivalent toKa = [H+][A-] / [HA]remember, pure liquids, like water, are not included in the equilibrium expression
9Acids and Bases Acid Strength defined by the equilibrium position of the dissociation reactionStrong Acid - equilibrium lies far to the right, i.e. the acid is 100% ionizedthe conjugate base of a strong acid is a much weaker base than water, i.e. the conjugate base will not accept the H+the stronger the acid the weaker the conjugate base
10Acids and Bases Weak acid not 100% ionized equilibrium lies to the leftvery little HA is ionizedthe conjugate base of a weak acid is a stronger base than water, the conjugate base is more likely to accept an H+ than waterthe weaker the acid, the stronger the conjugate base
11Acids and BasesThe Six Strong AcidsHClH2SO4HNO3HBrHIHClO4
12Acids and Bases Oxyacids Organic acids Most acids are oxyacids The acidic proton is attached to an oxygen atomOrganic acidsGenerally weak acidsContain the -COOH (carboxyl) groupex: CH3COOH - acetic acid
13Acids and Bases Water The most common amphoteric substance water can act as both an acid and a basewater can autoionize:H2O + H2O <==> H3O+ + OH-one water molecule acts as an acid (H+ donor), the other acts as an acid (an H+ acceptor)Kw = [H3O+][OH-] = 1.00 x @ 25oCdissociation or ion product constant for water
14Acids and BasesKwIn any aqueous solution at 25oC, the product of [H+] and [OH-] will be 1.0 x 10-14So if you know the [H+], you can figure out the [OH-] and vice versaIf [H+] = [OH-], the solution is neutralIf [H+] > [OH-], the solution is acidicIf [H+] < [OH-], the solution is basic
15Acids and Bases pH scale Because the [H+] in any solution is generally quite small, it is easier to use the pH scale to represent a solution’s acidity.pH comes from the Danish…potenz or strength of the H+ ionpH = - log[H+]pOH = - log [OH-]
16Acids and Bases pH is a log scale when the pH changes by one, the [H+] concentration changes by a power of 10.A solution with a pH of 3 has 10 times more H+ than a solution with a pH of 4, and 100 times more H+ than a solution with a pH of 5.As pH decreases, the [H+] increases.Rule for significant figures for logarithms - the number of places after the decimal point is equal to the number of significant figures in the original numberpH = - log 1.0 x 10-9 M (2 significant figures in 1.0 x 10-9)pH = 9.00 ( 2 places after the decimal point for significant figures)
17Acids and Bases Acid-Base Equilibria…or doing acid-base problems the aqueous solutions contain many componentsyou must be able to determine which components are most significant and which can be ignoredyou must be able to determine which reaction is most important of all the possible reactions
18Acids and Bases Ex: Calculate the pH of 1.0 M HCl First: Determine the major speciesHCl is a strong acid, 100% ionizedMajor species then will be H+, Cl- and H2OSince this is an acidic solution with “lots” of H+, the [OH-] will be insignificant.What about H2O <==> H+ + OH- ? Will any H+ come from this reaction?Le Chatelier’s principle tells us that the reverse reaction will be favored because of the high concentration of H+ from the HCl, so we can ignore this reaction as a source of H+.Thus the pH = - log (1.0) = 0.00
19Acids and Bases pH of Weak Acid Solutions Calculate the pH of 1.0 M HF Determine the major species: HF (because it is a weak acid, very little of it ionizes), and H2OWhich of these major species will provide the H+ ions?Consider the Ka and Kw
20Acids and Bases HF <==> H+ + F- Ka = 7.2 x 10-4 H2O <==> H+ + OH- Kw = 1.0 x 10-14Because the Ka of HF is greater than the Kw, HF is a stronger acid than H2O, and will be the primary contributor of H+ in solution.Use the Ka for HF to determine the [H+] at equilibrium, and then determine the pH.
21Acids and Bases Simplifying acid-base equilibrium calculations If Ka is small, then “x” is small relative to the original concentrations.x becomes essentially 1.00 in these calculationsIs this a valid assumption? Please check at the end!Compare x to [HAo], if x < 5% of [HAo], then the assumption is valid
22Acids and Bases The pH of a mixture of weak acids Calculate the pH of a solution that contains 1.00 M HCN (Ka = 6.2 x 10-10), and 5.00 M HNO2 (Ka = 4.0 x 10-4).Determine the major species: HCN, HNO2, and H2OCompare Ka’s and Kw.The Ka for HNO2 is larger than Kw and the Ka for HCNHNO2 <==> H+ + NO2- is the reaction of interest.
23Acids and Bases % dissociation or % ionization gives us an idea of the amount of weak acid that has dissociated% dissociation = ([H+] /[HAo]) x 100For a weak acid, the % ionization increases as the concentration gets more dilute.
24Acids and Bases Bases Strong Bases 100% ionizedGroup I hydroxides (e.g. NaOH, KOH, etc)heavy Group II hydroxides (Ca(OH)2, Ba(OH)2, Sr(OH)2)A base does not have to contain OH- ion to be a base
25Acids and Bases Bases Ammonia: NH3 + H2O <==> NH4+ + OH- The [OH-] increases due to the reaction of ammonia with water, so ammonia is a base, specifically, a Bronsted-Lowry base, because it is a proton acceptor.Many bases are like ammonia with a lone pair of electrons on the nitrogen that can accept H+. Consider these bases as substituted ammonia moleculese.g. CH3NH2, (CH3)2NH, (CH3)3N, C2H5NH2
26Acids and Bases General reaction between a base and water B + H2O <==> BH+ + OH-Kb = [BH+][OH-]/[B]In equilibrium problems involving weak bases, if Kb < Kw, then the base will be the primary source of OH-.
27Acids and Bases Polyprotic Acids (many proton acids) Some acids can donate more than one H+e.g. H2SO4 and H3PO4Polyprotic acids lose 1 H+ at a time.H2SO4--> H+ + HSO Ka1 >> 0HSO4- <==> H+ + SO4-2 Ka2 = 1.2 x 10-2The decreasing Ka as protons are lost indicate it is less favorable (as the negative charge on the acid increases) to lose the second (or third) proton
28Acids and Bases Acid-Base Properties of Salts Salt = an ionic compound salts dissolve in water to form ionssometimes these ions can react with water to form weak acids or weak bases
29Acids and Bases Neutral Salts When these salts dissolve in water, the pH does not change.There is no reaction of the ions from the salt with water.The anion and cation are derived from a strong acid and a strong base, respectively.
30Acids and BasesEx. NaCl is a neutral salt. Na+, the cation, can be considered as coming from NaOH, a strong base. Cl-, , the anion, can be considered as coming from HCl, a strong acid. Remember the conjugates of strong acids and strong bases are very weak, weaker than water, and so are unlikely to react with water to reform the acid or base.
31Acids and BasesNa+ + H2O cannot form NaOH + H+…strong bases do not re-form in solution.Cl- + H2O cannot HCl + OH- …strong acids do not re-form in solution.
32Acids and Bases Basic Salts The anion from this salt will react with water to form a weak acid and OH-.The anion must have come from a weak acid originally.Ex: NaCH3COO. The Na+ comes from a strong base, NaOH. CH3COO- comes from a weak acid, CH3COOH.Na+ + H2O cannot form NaOH + H+CH3COO- + H2O <==> CH3COOH + OH-
33Acids and Bases Acidic Salts The cation from an acidic salt will react with water to form a weak base and H+.The cation must have come from a weak base.Ex: NH4Cl. The NH4+ comes from a weak base, and will react with water to re-form the weak base and H+ (H3O+). The Cl- comes from a strong acid, and cannot react with water to re-form HCl.NH4+ + H2O <==> NH3 + H3O+
34Acids and BasesHow do we know the Kb for the conjugate base of a weak acid? How do we know the Ka for the conjugate acid of a weak base?CH3COOH + H2O <==> CH3COO- + OH- Ka = 1.8 x 10-5CH3COO- + H2O <==> CH3COOH + OH-Kb = [CH3COOH][OH-]/[CH3COO-] = ?Ka. Kb = Kw
35Acids and BasesSalts that are derived from both a weak acid and a weak baseWe can predict whether the solution will be acidic or basic based on the Ka of the acidic ion and the Kb of the basic ion.Ka > Kb pH < 7, acidic solutionKb > Ka pH > 7, basic solutionKb = Ka pH = 7, neutral solution
36Acids and Bases The effect of structure on acid-base properties Any molecule containing hydrogen could theoretically act as an acid. However, most of these molecules don’t.Organic molecules with lots of C-H bonds are not acidic because the C-H is both strong, and relatively nonpolar, so there is no tendency to lose H+However, while the H-Cl bond is stronger than the C-H bond, the H-Cl bond is very polar, and there is a strong tendency to lose H+.
37Acids and BasesTwo Factors that determine the acidity of a molecule containing X-Hstrength of the X-H bondpolarity of the X-H bondConsider polarity of the X-H bondH-F > H-Cl > HBr > HI in terms of polarityH-F > H-Cl > H-Br > HI in terms of bond strengthHF is a weak acid, HCl, HBr, and HI are strong acidseven though the H-F bond is very polar, the H-F bond is very strong, so HF is a weak acid
38Acids and Bases Oxyacids contains the grouping H-O-X HClO4 Ka >>> 0HClO3 Ka = 1HClO2 Ka = 1.2 x 10-2HClO Ka = 3.5 x 10-8As the number of oxygen atoms increase, the acid strength increases
39Acids and BasesOxygen is very electrognegative. With increasing number of oxygen atoms, the oxygen atom draw more and more of the electron density towards themselves, thus polarizing and weakening the O-H bond,
40Acids and Bases This behavior also occurs with hydrated metal ions. Al+3 is strongly attracted to water, weakening the O-H bond. The greater the charge on the metal ion, the stronger the acidity.
41Acids and BasesThe greater the electronegativity of X in the H-O-X grouping, the stronger the acid.X is able to withdraw electron density from the H-O bond, thus weakening and polarizing the H-O bond, resulting in a stronger acid
42Acids and BasesComparison of Electronegativity of X and Ka
43Acids and Bases Acid-Base Properties of Oxides An H-O-X grouping in a molecule may result in a molecule that behaves as a base or an acid.The nature of the O-X bond will determine the acidic or basic behavior of the molecule
44Acids and BasesIf X is very electronegative, the O - X bond will be covalent (not as polar because of similar E.N.’s), and strong. The O - X bond will remain intact in water, while the polar and weak H - O bond will break, resulting in acidic behavior.e.g. HClO2 or H2SO4If X is not very electronegative, the O - X bond will be ionic, and can be broken in water, resulting in basic behavior.e.g. NaOH or KOH
45Acids and Bases Basically… Nonmetal oxides will form acids in water SO2 + H2O <==> H2SO3CO2 + H2O <==> H2CO3Metal oxides will form bases in waterNa2O + H2O --> 2 NaOHCaO + H2O --> Ca(OH)2(the oxide ion has a high affinity for protons and reacts with water to form hydroxide ions)
46Acids and BasesLewis acids and bases are electron pair acceptors and electron pair donors, respectivelyA very general model for acids-base reactions…see Lewis structures for explanations...