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AP Ch 15 Acid-Base

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					Acids and Bases
   Chapter 15
       Some Properties of Acids
 Produce H+ (as H3O+) ions in water (the hydronium ion is a
  hydrogen ion attached to a water molecule)

 Taste sour
 Corrode metals
 Electrolytes
 React with bases to form a salt and water
 pH is less than 7
 Turns blue litmus paper to red “Blue to Red A-CID”
    Some Properties of Bases
 Produce OH- ions in water

 Taste bitter, chalky

 Are electrolytes

 Feel soapy, slippery

 React with acids to form salts and water

 pH greater than 7

 Turns red litmus paper to blue   “Basic Blue”
     Acid Nomenclature Review
             Anion
             Ending    Acid Name
No Oxygen    -ide     hydro-(stem)-ic acid

              -ate     (stem)-ic acid
w/Oxygen
              -ite     (stem)-ous acid
An easy way to remember which goes with which…
   “In the cafeteria, you ATE something ICky”
                  Acid/Base definitions
                     Definition 1: Arrhenius

Arrhenius acid is a substance that produces H+ (H3O+) in water




 Arrhenius base is a substance that produces OH- in water




                                                            4.3
           Acid/Base Definitions
• Definition #2: Brønsted – Lowry

   Acids – proton donor

   Bases – proton acceptor

   A ―proton‖ is really just a hydrogen
     atom that has lost it’s electron!
       A Brønsted-Lowry acid is a proton donor
       A Brønsted-Lowry base is a proton acceptor




                                conjugate   conjugate
base            acid
                                  acid        base
   ACID-BASE THEORIES
The Brønsted definition means NH3 is a BASE in
  water — and water is itself an ACID

    NH3    +   H2O          NH4+ + OH-
   Base        Acid        Acid    Base
Conjugate Pairs
         Learning Check!

Label the acid, base, conjugate acid, and
  conjugate base in each reaction:
      HCl + OH-  Cl- + H2O
      Acid   Base    Conj.   Conj.
                     Base    Acid

      H2O + H2SO4  HSO4- + H3O+
                       Conj.     Conj.
     Base    Acid      Base      Acid
      Acids & Base Definitions
Definition #3 – Lewis

 Lewis acid - a substance that
   accepts an electron pair



 Lewis base - a substance
 that donates an electron
 pair
    Lewis Acids & Bases
Formation of hydronium ion is also
 an excellent example.
              ••
            • O—H
            •              ••
        +                H O—H
    H
             H
                            H
    ACID    BASE

•Electron pair of the new O-H bond
originates on the Lewis base.
Lewis Acid/Base Reaction
The pH scale is a way of
expressing the strength of
acids and bases. Instead of
using very small numbers,
we just use the NEGATIVE
power of 10 on the Molarity
of the H+ (or OH-) ion.


Under 7 = acid
      7 = neutral
 Over 7 = base
           Calculating the pH
                  pH = - log [H+]
         (Remember that the [ ] mean Molarity)

Example: If [H+] = 1 X 10-10
  pH = - log 1 X 10-10
  pH = - (- 10)
  pH = 10
Example: If [H+] = 1.8 X 10-5
  pH = - log 1.8 X 10-5
  pH = - (- 4.74)
  pH = 4.74
                 Try These!
                            pH = - log [H+]
Find the pH of these:
                        pH = - log 0.15
1) A 0.15 M solution of
   Hydrochloric acid    pH = - (- 0.82)
                        pH = 0.82

                         pH = - log 3 X 10-7
2) A 3.00 X 10-7 M
    solution of Nitric   pH = - (- 6.52)
    acid                 pH = 6.52
 pH calculations – Solving for H+
If the pH of Coke is 3.12, [H+] = ???
Because pH = - log [H+] then
        - pH = log [H+]
Take antilog (10x) of both
  sides and get
10-pH = [H+]
[H+] = 10-3.12 = 7.6 x 10-4 M
   *** to find antilog on your calculator, look for ―Shift‖
  or ―2nd function‖ and then the log button
        More About Water
  H2O can function as both an ACID and a BASE.
  In pure water there can be AUTOIONIZATION




Equilibrium constant for water = Kw
Kw = [H3O+] [OH-] = 1.00 x 10-14 at 25 oC
       More About Water
    Autoionization            OH-



                                H3O+




Kw = [H3O+] [OH-] = 1.00 x 10-14 at 25 oC
In a neutral solution [H3O+] = [OH-]
and so [H3O+] = [OH-] = 1.00 x 10-7 M
               pOH
• Since acids and bases are
  opposites, pH and pOH are
  opposites!
• pOH does not really exist, but it is
  useful for changing bases to pH.
• pOH looks at the perspective of a
  base
          pOH = - log [OH-]
Since pH and pOH are on opposite
  ends,
          pH + pOH = 14
pH   [H+]   [OH-]   pOH
       [H3O+],       [OH-]         and pH
What is the pH of the
      0.0010 M NaOH solution?
[OH-] = 0.0010 (or 1.0 X 10-3 M)
    pOH = - log 0.0010
    pOH = 3
pH = 14 – 3 = 11


OR Kw = [H3O+] [OH-]
[H3O+] = 1.0 x 10-11 M
pH = - log (1.0 x 10-11) = 11.00
     What is the pH of a 2 x 10-3 M HNO3 solution?
      HNO3 is a strong acid – 100% dissociation.
Start 0.002 M                  0.0 M      0.0 M
       HNO3 (aq) + H2O (l)     H3O+ (aq) + NO3- (aq)
End 0.0 M                     0.002 M 0.002 M

      pH = -log [H+] = -log [H3O+] = -log(0.002) = 2.7

     What is the pH of a 1.8 x 10-2 M Ba(OH)2 solution?
      Ba(OH)2 is a strong base – 100% dissociation.
Start 0.018 M             0.0 M       0.0 M
       Ba(OH)2 (s)       Ba2+ (aq) + 2OH- (aq)
End 0.0 M                0.018 M 0.036 M
      pH = 14.00 – pOH = 14.00 + log(0.036) = 12.56
                                                          15.4
 Strong and Weak Acids/Bases
 The strength of an acid (or base) is
 determined by the amount of
 IONIZATION.




HNO3, HCl, HBr, HI, H2SO4 and HClO4 are
the strong acids.
Strong and Weak Acids/Bases
• Generally divide acids and bases into STRONG or
  WEAK ones.
STRONG ACID: HNO3 (aq) + H2O (l) 
              H3O+ (aq) + NO3- (aq)
HNO3 is about 100% dissociated in water.
   Strong and Weak Acids/Bases
• Weak acids are much less than 100% ionized in
  water.
  *One of the best known is acetic acid = CH3CO2H
 Strong and Weak Acids/Bases
• Strong Base: 100% dissociated in water.
  NaOH (aq)  Na+ (aq) + OH- (aq)
                     Other common strong
                     bases include KOH and
                     Ca(OH)2.
                     CaO (lime) + H2O -->
                           Ca(OH)2 (slaked lime)
                     Strong bases are the group I hydroxides
             CaO     Calcium, strontium, and barium hydroxides are
                     strong, but only soluble in water to 0.01 M
 Strong and Weak Acids/Bases
• Weak base: less than 100% ionized in water


One of the best known weak bases is ammonia
NH3 (aq) + H2O (l) ↔ NH4+ (aq) + OH- (aq)
Weak Bases
            Equilibria Involving
           Weak Acids and Bases
  Consider acetic acid, HC2H3O2 (HOAc)
         HC2H3O2 + H2O ↔ H3O+              +    C2H3O2 -
         Acid                                   Conj. base

               [H3O+ ][OAc - ]           -5
          Ka                   1.8 x 10
                  [HOAc]

(K is designated Ka for ACID)
K gives the ratio of ions (split up) to molecules (don’t split up)
     Ionization Constants for Acids/Bases


 Acids                               Conjugate
                                       Bases
Increase
strength




                                      Increase
                                      strength
  Equilibrium Constants
    for Weak Acids




Weak acid has Ka < 1
Leads to small [H3O+] and a pH of 2 - 7
Equilibria Involving A Weak Acid
You have 1.00 M HOAc. Calc. the
 equilibrium concs. of HOAc, H3O+, OAc-,
 and the pH.
Step 1. Define equilibrium concs. in ICE
  table.
          [HOAc]     [H3O+]     [OAc-]
initial    1.00       0          0
change    -x         +x         +x
equilib    1.00-x     x          x
  Equilibria Involving A Weak Acid
 You have 1.00 M HOAc. Calc. the equilibrium concs.
 of HOAc, H3O+, OAc-, and the pH.

 Step 2. Write Ka expression
                        [H3O+ ][OAc - ]      x2
      Ka  1.8 x 10-5 =                 
                           [HOAc]         1.00 - x


     This is a quadratic. Solve using quadratic
                     formula.

or you can make an approximation if x is very
small! (Rule of thumb: 10-5 or smaller is ok)
Equilibria Involving A Weak Acid
You have 1.00 M HOAc. Calc. the equilibrium concs.
of HOAc, H3O+, OAc-, and the pH.

Step 3. Solve Ka expression
                    [H3O+ ][OAc - ]      x2
  Ka  1.8 x 10-5 =                 
                       [HOAc]         1.00 - x

  First assume x is very small because
  Ka is so small.
                                     x2
                Ka  1.8 x 10-5 =
                                    1.00
   Now we can more easily solve this
   approximate expression.
   Equilibria Involving A Weak Acid
You have 1.00 M HOAc. Calc. the equilibrium concs.
of HOAc, H3O+, OAc-, and the pH.

Step 3. Solve Ka approximate expression
                                   x2
              Ka  1.8 x 10-5 =
                                  1.00

  x = [H3O+] = [OAc-] = 4.2 x 10-3 M
  pH = - log [H3O+] = -log (4.2 x 10-3) = 2.37
Equilibria Involving A Weak Acid
Calculate the pH of a 0.0010 M solution of formic
  acid, HCO2H.
HCO2H + H2O ↔ HCO2- + H3O+
Ka = 1.8 x 10-4
Approximate solution
  [H3O+] = 4.2 x 10-4 M, pH = 3.37
Exact Solution
  [H3O+] = [HCO2-] = 3.4 x 10-4 M
  [HCO2H] = 0.0010 - 3.4 x 10-4 = 0.0007 M
  pH = 3.47
  Equilibrium Constants
    for Weak Bases




Weak base has Kb < 1
Leads to small [OH-] and a pH of 12 - 7
Relation
 of Ka,
  Kb,
[H3O+]
and pH
Equilibria Involving A Weak Base
You have 0.010 M NH3. Calc. the pH.
  NH3 + H2O ↔ NH4+ + OH-
  Kb = 1.8 x 10-5
Step 1. Define equilibrium concs. in ICE table
           [NH3]        [NH4+]     [OH-]

initial     0.010       0          0
change     -x           +x         +x
            0.010 - x   x          x
equilib
Equilibria Involving A Weak Base
You have 0.010 M NH3. Calc. the pH.
  NH3 + H2O  NH4+ + OH-
  Kb = 1.8 x 10-5
Step 2. Solve the equilibrium expression
                             -
                        +
              -5 = [NH4 ][OH ] =    x2
 Kb  1.8 x 10
                      [NH3 ]     0.010 - x

Assume x is small, so
                x = [OH-] = [NH4+] = 4.2 x 10-4 M
and [NH3] = 0.010 - 4.2 x 10-4 ≈ 0.010 M
The approximation is valid !
 Equilibria Involving A Weak Base
You have 0.010 M NH3. Calc. the pH.
  NH3 + H2O  NH4+ + OH-
  Kb = 1.8 x 10-5
Step 3. Calculate pH
[OH-] = 4.2 x 10-4 M
so pOH = - log [OH-] = 3.37
Because pH + pOH = 14,
pH = 10.63
Types of Acid/Base Reactions:
          Summary
Weak Bases are weak electrolytes
    F- (aq) + H2O (l)     OH- (aq) + HF (aq)
    NO2- (aq) + H2O (l)     OH- (aq) + HNO2 (aq)


Conjugate acid-base pairs:
•    The conjugate base of a strong acid has no measurable
     strength.
•    H3O+ is the strongest acid that can exist in aqueous
     solution.
•    The OH- ion is the strongest base that can exist in aqueous
     solution.


                                                             15.4
15.4
Strong Acid   Weak Acid




                          15.4
                     Ionized acid concentration at equilibrium
percent ionization =                                           x 100%
                            Initial concentration of acid

                    For a monoprotic acid HA

                        [H+]
Percent ionization =           x 100%   [HA]0 = initial concentration
                       [HA]0




                                                                 15.5
Ionization Constants of Conjugate Acid-Base Pairs

         HA (aq)        H+ (aq) + A- (aq)      Ka

   A- (aq) + H2O (l)      OH- (aq) + HA (aq)   Kb

        H2O (l)        H+ (aq) + OH- (aq)      Kw


                        KaKb = Kw


           Weak Acid and Its Conjugate Base

                  Kw                     Kw
             Ka =                   Kb =
                  Kb                     Ka

                                                    15.7
         Molecular Structure and Acid Strength
      H X          H+ + X-
                                • Bond strength

    The stronger   The weaker   • Polarity
     the bond       the acid




HF << HCl < HBr < HI

                                                  15.9
        Molecular Structure and Acid Strength
                  d-   d+
           Z     O     H             Z     O- + H+

The O-H bond will be more polar and easier to break if:
•   Z is very electronegative or
•   Z is in a high oxidation state




                                                          15.9
             Molecular Structure and Acid Strength

1. Oxoacids having different central atoms (Z) that are from
the same group and that have the same oxidation number.

  Acid strength increases with increasing electronegativity of Z
        ••                       ••
         ••
         ••




                                  ••
                                  ••
        O                        O
   ••       ••              ••      ••
                ••




                                        ••
 H O Cl O               H O Br O
   •• •• ••                 •• •• ••

       Cl is more electronegative than Br



            HClO3 > HBrO3


15.9
          Molecular Structure and Acid Strength

2. Oxoacids having the same central atom (Z) but different
numbers of attached groups.

Acid strength increases as the oxidation number of Z increases.




                                      HClO4 > HClO3 > HClO2 > HClO




                                                             15.9
              Acid-Base Properties of Salts
Neutral Solutions:
   Salts containing an alkali metal or alkaline earth metal
   ion (except Be2+) and the conjugate base of a strong
   acid (e.g. Cl-, Br-, and NO3-).
                         H2O
              NaCl (s)          Na+ (aq) + Cl- (aq)

Basic Solutions:
   Salts derived from a strong base and a weak acid.
                         H 2O
      NaCH3COO (s)              Na+ (aq) + CH3COO- (aq)

    CH3COO- (aq) + H2O (l)         CH3COOH (aq) + OH- (aq)


                                                              15.10
              Acid-Base Properties of Salts
Acid Solutions:
   Salts derived from a strong acid and a weak base.

                           H 2O
             NH4Cl (s)            NH4+ (aq) + Cl- (aq)

               NH4+ (aq)           NH3 (aq) + H+ (aq)

   Salts with small, highly charged metal cations (e.g. Al3+,
   Cr3+, and Be2+) and the conjugate base of a strong acid.

        Al(H2O)3+(aq)
               6
                                         2+
                              Al(OH)(H2O)5 (aq) + H+ (aq)



                                                                15.10
               Acid-Base Properties of Salts
Solutions in which both the cation and the anion hydrolyze:

 •   Kb for the anion > Ka for the cation, solution will be basic
 •   Kb for the anion < Ka for the cation, solution will be acidic
 •   Kb for the anion  Ka for the cation, solution will be neutral




                                                                15.10

				
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