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Chapter 11. Acids, Bases, & Salts Acid/Base Equilibrium. Properties of Acids. sour or tart taste strong acids burn; weak acids feel similar to H 2 O acid solutions are electrolytes acids react with most metals to release H 2 acids cause indicators to change color Acids turn litmus red.

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chapter 11

Chapter 11

Acids, Bases, & Salts

Acid/Base Equilibrium

properties of acids
Properties of Acids
  • sour or tart taste
  • strong acids burn; weak acids feel similar to H2O
  • acid solutions are electrolytes
  • acids react with most metals to release H2
  • acids cause indicators to change color
    • Acids turn litmus red
properties of bases
Properties of Bases
  • bases taste bitter
  • basic solutions do not burn
  • basic solutions feel smooth and slippery
  • basic solutions are also electrolytes
  • bases usually do not react with metals
  • bases also cause indicators to change color
    • Bases turn litmus blue
three definitions
Three definitions
  • Arrhenius
    • Deals with H+ and OH-
  • Bronsted Lowry
    • Deals with protons
  • Lewis
    • Deals with pairs of electrons
arrhenius definition
Arrhenius Definition
  • Arrhenius defined an acid as a substance that ionizes in water to produce H+ ions
    • Ex. HCl
    • Any of our acids with H+ in the beginning
  • Arrhenius defined a base as a substance that ionizes in water to produce OH- ions
    • Ex. NaOH
    • Any of the metal hydroxides
bronsted lowry definition
Bronsted Lowry Definition
  • NH3 is known to turn litmus blue, but it does not have hydroxide ion
    • Needed a new definition
    • Substances do not need to be in water
  • A B/L acid is any substance that can donate a proton (H+)
  • A B/L base is any substance that can accept a proton (H+)
conjugate pairs
Conjugate pairs

HC2H3O2 + H2O → C2H3O2- + H3O+

NH3 + H2O → NH4+ + OH-

amphoteric substances
Amphoteric Substances
  • Notice in the examples, that H2O can act as a B/L acid and as a B/L base.
  • It is know as an amphoteric substance.
  • Additional examples include
    • HSO3-
    • HCO3-
    • H2PO4-
    • HPO42-

These are the ions that result when polyprotic acids release H+ ions one at a time. Notice that they can accept an H+ or they can release an H+.

lewis definition
Lewis definition
  • This is the most broad of the definitions
  • A Lewis acid is an electron pair acceptor.
  • A Lewis base is an electron pair donor.
  • Example Lewis acid/base reaction….
ion product constant for h 2 o
Ion product constant for H2O
  • Remember [H+][OH-] = 1.0 x 10-14 = Kw
  • So if you know [H+] you can find [OH-] and vice versa
  • If [H+] = 9.3 x 10-4 M, what is [OH-]
ph or poh
pH or pOH
  • pH = -log[H+] or pOH = -log[OH-]
  • pH + pOH = 14
  • What is the pH of a solution that contains 4.9 x 10-9 M OH-?
slide12
What is the [H+] of a solution that is pH4.82?

What is the [OH-] of a solution that is pH8.56?

you must understand that
You must understand that…
  • When [H+] = [OH-], the solution is neutral, and the pH = 7
  • When [H+] > [OH-], the solution is acidic, and the pH <7
  • When [H+] < [OH-], the solution is basic, and the pH > 7.
  • Increasing pH means decreasing [H+]…fewer H+ ions floating in solution….less acidic.
  • Decreasing pH means increasing [H+]…more H+ ions floating in solution….more acidic.
weak acids
Weak Acids
  • When a weak acid is placed in water, a small fraction of its molecules will dissociate into hydrogen ions and conjugate base ions.
    • Most of the acid molecules will remain in solution as undissociated aqueous particles.
  • The dissociation constant, Ka, is a measure of the strength of weak acids.
  • For the reaction…HA ↔ H+ + A-

Ka =

weak bases
Weak Bases
  • When a weak base is placed in water, a small fraction of its molecules will dissociate into hydroxide ions and conjugate acid ions.
    • Most of the base molecules will remain in solution as undissociated aqueous particles.
  • The dissociation constant, Kb, is a measure of the strength of weak bases.
  • For the reaction…B ↔ HB+ + OH-

Kb =

acid equilibrium problems
Acid Equilibrium problems
  • Things to know
    • All of our acids will be monoprotic, that is, give off only one hydrogen ion.
    • x will represent the amount of acid that dissociates.
      • Therefore x also represents the [H+]
    • We will use ICE to determine concentrations for the Ka expression
    • We can use HA to represent the acid and A- to represent the conjugate base.
k a example
Ka example

What is the pH of a 0.20M HC2H3O2, with Ka = 1.8 x 10-5?

(also find % ionization)

polyprotic acids
Polyprotic acids
  • Polyprotic acids are weak acids (except for H2SO4 which is a strong acid).
    • They release their H+ ions one at a time.
    • Each has its own Ka
    • Ex. H3PO4

H3PO4 → H+ + H2PO4- Ka = 7.5 x 10-3

H2PO4- → H+ + HPO42- Ka = 6.2 x 10-8

HPO42- → H+ + PO43- Ka = 4.8 x 10-13

k w revisited
Kw revisited
  • Water comes to equilibrium with its ions according to the following reaction…

H2O(l) ↔ H+(aq) + OH-(aq)

Kw = [H+][OH-] = 1.0 x 10-14

Kw = KaKb pKa + pKb = 14

strong acids bases
Strong Acids/Bases

Strong Acids Strong Bases

HCl LiOH

HBr NaOH

HI KOH

HNO3 RbOH

HClO4 CsOH

H2SO4 Ba(OH)2

Sr(OH)2

strong acids base description
Strong Acids/Base description
  • Strong acids and bases completely dissociate in water, therefore no Ka or Kb
    • The dissociations do not reverse.
  • Oxoacids are acids that contain oxygen.
    • The greater the number of oxygen atoms attached to the central atom in an oxoacid, the stronger the acid.
      • That’s because increasing the number of oxygen atoms that are attached to the central atom weakens the attraction that the central atom has for the H+ ion.
strong acid base calculations
Strong Acid/Base calculations
  • Since these acids and bases completely dissociate in water, the final concentration of H+ ions is the same as the original concentration.
  • So you can always find the pH of a strong acid solution directly from its concentration.
    • What is the pH of .20 M HCl?

pH = -log(.20) =

titration
Titration
  • When an acid and a base are mixed, a neutralization reaction occurs.
    • Acid + base → salt + water
  • Neutralizations reactions are generally performed by titration, where a base of known concentration is slowly added to an acid (or vice versa)
  • The progress of a neutralization reaction can be shown in a titration curve.
  • The equivalence point is the point in the titration when exactly enough base has been added to neutralize all the acid that was initially present.
  • An indicator will be used to mark the equiv. pt.
slide24
Strong acid/strong base titration.
    • Equivalence point at pH7
slide25
Notice the shape of the curve if a strong acid is added to a strong base
    • The equivalence point is also pH7
slide26
Weak acid vs strong base
    • The equivalence point is > pH7
slide27
Weak base vs strong acid
    • Notice the equivalence point is < pH7
which indicator to use
Which indicator to use?
  • The indicator needs to change color close to the equivalence point.
ph of dissolved salts
pH of dissolved salts
  • If a salt is composed of the conjugates of a strong base and a strong acid, its solution will be neutral. (NaCl)
  • If a salt is composed of the conjugates of a weak base and a strong acid, its solutions will be acidic. (NH4Cl)
  • If a salt is composed of the conjugates of a strong base and a weak acid, its solution will be basic. (NaC2H3O2)
  • If a salt is composed of the conjugates of a weak base and a weak acid, the pH of its solution will depend on the relative strengths of the conjugate acid and base of the specific ions in the salt. (NH4C2H3O2)
anhydrides
Anhydrides
  • An acid anhydride is a substance that combines with water to form an acid.
    • Generally nonmetal oxides.
      • Ex. CO2 + H2O→ H2CO3
  • A basic anhydride is a substance that combines with water to form a base.
    • Generally metal oxides.
      • Ex. Na2O + H2O→ 2NaOH
what to know for the test
What to know for the test?
  • Find pH from either [H+] or [OH-]
  • Find [H+] from pH or pOH
  • Find the # of grams of a base to make a solution of a certain pH
  • Use titration info to find molar mass acid
  • Ka problems
    • Find pH - find Ka from pH
    • Find concentrations
    • Find % ionization