2Acids, Bases and Equilibrium When an acid is dissolved in water, the H+ ion (proton) produced by the acid combines with water to produce the hydronium ion, H3O+HCl and other strong electrolytes ionize completely in waterWeak acids like acetic acid ionize only to a very small extent in water
3Acids, Bases and Equilibrium The equilibrium concept is used to describe to what extent an acid or base ionizes in waterIonization constants: K>1 indicates a strong acid or base, K<1 refers to a weak acid or base
5Arrhenius Acids and Bases An acid is a substance that, when dissolved in water, increases the concentration of hydrogen ions, H+, in the waterA base is a substance that, when dissolved in water increases the concentration of hydroxide ion, OH-, in the water
6Bronsted-Lowry Acids and Bases An acid is any substance that can donate a proton to any other substanceExamples of Bronsted acids – molecular compounds (HNO3), cations (NH4+), hydrated metals (Fe(H2O)63+), or anions (H2PO4-)Theory is not restricted to compounds in water
7Bronsted-Lowry Acids and Bases A Bronsted base is a substance that can accept a proton from any other substanceBronsted bases can be molecular compound (NH3), anions (CO32-), or cations (Al(H2O)5(OH)2+)
9Conjugate Acid-Base Pairs A pair of compounds or ions that differ by the presence of one H+ ion is called a conjugate acid-base pairEvery reaction between a Bronsted acid and Bronsted base involves H+ transfer and has two conjugate acid-base pairs
20Water and the pH scaleWater autoionizes to a small extent, producing low concentrations of H3O+ and OH- ions (water conducts electricity)The equilibrium for autoionization of water lies far to the leftAt 25oC, Kw=1.0 x (the ionization constant for water)
21Water and the pH scaleKw increases with temperature because the autoionization of water is endothermicKw is valid in pure water and any aqueous solutionIn pure water and dilute aqueous solutions, the concentration of water is considered to be constant at 55.5 M
22Water and the pH scale[H3O+] = [OH-] = 1.0 x 10-7 M in pure water, a neutral solutionIn acidic solution, [H3O+]>[OH-]In basic solution, [H3O+]<[OH-]
23Water and the pH scale pH = -log[H3O+] pOH = -log[OH-] pKw = pH + pOH = 14.00
24Relationship between hydronium and hydroxide ion concentrations, pH and pOH
25Equilibrium Constants for Acid and Bases The strength of acids and bases of the same concentration can be compared by measuring pHThe relative strength of an acid can be expressed with an equilibrium constantKa = [H3O+][A-]/[HA]
26Equilibrium Constants for Acid and Bases Kb = [BH+][OH-]/[B]Weak acid, Ka<1, pH>2, small [H3O+]Weak base, Kb<1, pH<12, small [OH-]A large value of K indicates ionization products are strongly favored; small K value indicates reactants are favored
27Equilibrium Constants for Acid and Bases Weak acids have strong conjugate bases; small Ka corresponds with large KbConsider the acids and conjugate bases in table 15.2 on page 668Notice trends: as acid strength declines in a series, the relative conjugate base strength increases
28Equilibrium Constants for Acid and Bases Use Tables 15.3 on page 671, 15.4 on page 679, and 15.5 on page 683Which is the stronger acid, H2SO4 or H2SO3?Is benzoic acid stronger or weaker than acetic acid?Which has the stronger conjugate base, acetic acid or formic acid?Which is the stronger base, ammonia or methylamine?Which has the stronger conjugate acid, ammonia or methylamine?
29Calculations with Equilibrium Constants The principles of the equilibria can be applied to aqueous solutions of weak acids and bases. The equilbrium constants Ka and Kb can be determined if the concentrations of the various species present in the solution are known. These are often determined by measuring pH.
30Calculations with Equilibrium Constants If the acid or base is weak, and the initial concentration of acid (or base) is at least 100x Ka (or Kb), then the approximation that[acid]initial = [acid]equilibrium is valid. Otherwise the quadratic equation must be solved.
31Calculations with Equilibrium Constants (ex) For the weak acid:HA (aq) + H2O (l) H3O+ (aq) + A- (aq) Ka = [H3O+ ][A-]/[HA] and because [H3O+ ] = [A-], Ka = [H3O+ ]2/[HA]The assumption that [HA]equil = [HA]initial is valid if [HA]initial > 100 x Ka
32Calculations with Equilibrium Constants Calculating a Ka value from a measured pHA solution prepared from mol butanoic acid dissolved in sufficient water to give 1.0 L of solution has a pH of Determine Ka for butanoic acid. The acid ionizes according to the balanced equation:CH3CH2CH2CO2H + H2O H3O+ + H3CH2CH2CO2-
33CH3CH2CH2CO2H + H2O H3O+ + H3CH2CH2CO2- Calculate initial molarity of butanoic acidCalculate equilibrium molarity of hydronium ion from pHConstruct ice tableWrite equilibrium constant expression and substitute in values from ice tableCalculate Ka
34Calculating Equilibrium Concentrations and pH from Ka What are the equilibrium concentrations of acetic acid, acetate ion, and hydronium ion for a 0.10 M solution of acetic acid (Ka = 1.8 x 10-5)? What is the pH of the solution?Write chemical equation for ionization of acetic acid in waterICE it upWrite equilibrium constant expression, substitute values from ice table, solve for xUse x to find equilibrium concentrationsUse concentration of hydronium ion to solve for pH
35Calculating the pH of a salt solution What is the pH of a M solution of sodium acetate?Write equation for ionization reaction of acetate ion in water (it is a base!)Put it on ICEWrite equilibrium constant expression and substitute ICE valuesSolve for x, solve for concentration of hydroxide ionSolve for concentration of hydronium ion, solve for pH
36Calculating the pH after the reaction of an acid with a base Calculate the pH after mixing 15 mL of 0.12 M acetic acid with 15 mL of 0.12 M NaOH. What are the major species in solution at equilibrium (besides water) and what are their concentrations?Write balanced equationsStoichiometry problem to solve for “initial” concentration of acetate anionICE, equilibrium constant express Kb, pOH and such
37Polyprotic acids and bases The pH of many inorganic polyprotic acids depends primarily on the hydronium ion generated in the first ionization stepEach successive loss of a proton is about more difficult than the previous stepThe hydronium ion produced in the second step can be neglected; calculate using the k of the first ionization
38Molecular Structure, Bonding, and Acid Strength In a series of acids, as bond strength decreases, acid strength increasesAdjacent electronegative atoms that pull electrons from a hydrogen increase the strength of an acid (inductive effect)Acids that have resonance structures are stronger because they are more stable after they lose a proton than acids without resonance
39Lewis Acids and BasesA Lewis acid is a substance that can accept a pair of electrons from another atom to form a new bondA Lewis base is a substance that can donate a pair of electrons to another atom to form a new bondThis is also know as coordinate covalent chemistry
40Lewis Acids and Bases A + B: B:A acid + base acid-base adduct The acid must have an empty orbital available or be able to make one availableA base must have a pair of nonbonding electronsThe formation of hydronium ion is an example of this type of reaction
41Lewis Acids and BasesGood Lewis bases include hydroxide ion and ammonia and waterMetal cations are good Lewis acids; transition metals form complexes known as coordination complexesNonmetal oxides such as carbon dioxide are Lewis acidsSome metal hydroxides are amphoteric – acting like an acid in the presence of a base and a base in the presence of an acid (ex. Al(OH)3)
42Acid and Base Properties of Some Ions in Aqueous Sol’n NeutralBasicAcidicAnionsCl- NO3-Br- ClO4-I-CH3CO2- CN- OCl-SO HPO42-HCO2- PO43- NO2-CO32- HCO3- F-S2- HS- SO32-HSO4-H2PO4-HSO3-CationsLi+Na+ Ca2+K+ Ba2+Al(H2O)5(OH)2+ and analogous ionsAl(H2O)63+ and hydrated transition metal cationsNH4+
43Acid and Base Properties of Some Ions in Aqueous Sol’n Anions that are conjugate bases of strong acids are such weak bases that they have no effect on pHThere are numerous basic anions that are conjugate bases of weak acidsAcidic anions arise from polyprotic acids
44Acid and Base Properties of Some Ions in Aqueous Sol’n Alkali metal and alkaline earth metal cations have no effect on pH (conj. acid of strong base)All metal cations are hydrated in water, M(H2O)6n+, but when M is +2 or +3 and particularly a transition metal, the ion acts as an acid
45Aqueous Solutions of Salts CationAnionpH of SolutionFrom strong base (Na+)From strong acid (Cl-)=7 neutralFrom strong base (K+)From weak acid (CH3CO2-)>7 basicFrom weak base (NH4+)<7 acidicFrom any weak base (BH+)From any weak acid (A-)Depends on relative strengths of acid and base
46For each of the following salts in water, predict whether the pH will be greater than, less than, or equal to 7KBrNH4NO3AlCl3Na2HPO4
47pKa = -log KaA logarithmic scale is used to report and compare acid strengthsThe pKa value becomes smaller as the acid strength increasesKaKb = Kw for an acid and its conjugate baseAlso, pKw = pKa + pKb
48Equilibrium Constants and Acid-Base Reactions All proton transfer reactions proceed from the stronger acid and base to the weaker acid and base.Write the net ionic equation for the possible reaction between acetic acid and sodium hydrogen sulfate, NaHSO4. Does the equilibrium lie to the left or right?
49Types of Acid-Base Reactions Reaction of a Strong acid with a Strong BaseNet ionic equation:H3O+ + OH- 2H2OK = 1/Kw = 1.0 x 1014Mixing equal molar quantities of a strong base with a strong acid produces a neutral solution (pH=7, at 25o )
50Types of Acid-Base Reactions Unless both the acid and base involved in the neutralization reaction are strong, the pH of the solution that results will not be neutral.
51Types of Acid-Base Reactions Reaction of a Weak Acid with a Strong Base(ex.) net ionic equation:CH3CO2H + OH- H2O + CH3CO2-Adding these two reactions give the net ionic equation above;CH3CO2H + H2O H3O+ + CH3CO2- Ka=1.8x10-5H3O+ + OH- 2 H2O Ka=1/Kw=1.0x1014The K for the net reaction is the product of the two equilibrium constants Kneut=Ka x 1/Kw=1.8x109
52Types of Acid-Base Reactions Mixing equal molar quantities of a strong base with a weak acid produces a salt whose anion is the conjugate base of the weak acid. The solution is basic, with the pH depending on Kb for the anion.In the example above, the salt that results from the reaction is acetate, the conjugate base of a weak acid. Therefore the solution will be basic at the equivalence point.
53Types of Acid-Base Reactions Reaction of a Strong Acid with a Weak Base(ex.) net ionic equation:H3O+ + NH3 NH4+ + H2OAdding these two reactions give the net ionic equation above:NH3 + H2O NH4 + OH- Kb = 1.8x10-5H3O+ + OH- 2 H2O K = 1/Kw = 1.0x1014The K for the net reaction is the product of the two equilibrium constants K = Kb x 1/Kw = 1.8x109
54Types of Acid-Base Reactions Mixing equal molar quantities of a strong acid and a weak base produces a salt whose cation is the conjugate acid of the weak base. The solution is acidic, with the pH depending on the Ka for the cation.In the above example, the solution at the equivalence point contains the ammonium ion, the conjugate acid of a weak base, and the solution is acidic.
55Types of Acid-Base Reactions Reaction of a Weak Acid with a Weak Base(ex.) net ionic equation:CH3CO2H + NH3 NH4+ + CH3CO2-The reaction is product-favored because acetic acid is stronger than ammonium ion and ammonia is a stronger base than acetate ion (use table)K = Ka x Kb / Kw (what is the K for this ex?)
56Types of Acid-Base Reactions When a weak acid reacts with a weak base, the pH of the solution at the equivalence point depends upon which is the stronger, the acid or the baseIf equal molar solutions are mixed the resulting solution contains ammonium acetate; is it acidic or basic? (use table)
57Types of Acid-Base Reactions Mixing equal molar quantities of a weak acid and a weak base produces a salt whose cation is the conjugate acid of the weak base and whose anion is the conjugate base of the weak acid. The solution pH depends on the relative Ka and Kb values.