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Title: Chapter 16 : Acid-Base Equilibira


1
Chapter 16 Acid-Base Equilibira
  • Lauren Querido

2
Table of Contents
  • 16.1 Review
  • 16.2 Brønsted-Lowry Acids and Bases
  • 16.3 Autoionization of Water
  • 16.4 pH Scale
  • 16.5 Strong Acids and Bases
  • 16.6 Weak Acids
  • 16.7 Weak Bases
  • 16.8 Relationship Between Ka and Kb
  • 16.9 Acid-Base Properties of Salt Solutions
  • 16.10 Acid-Base Behavior and Chemical
    Structure
  • 16.11 Lewis Acids and
  • Bases

3
16.1 Review
  • Acids
  • Sour in taste
  • Litmus paper turns red
  • Bases
  • Bitter, slippery
  • Litmus paper turns blue
  • When acids and bases mix, their properties
    disappear!

4
Arrhenius Acids and Bases
  • Svante Arrhenius (1880)
  • In aqueous solutions
  • Acids will increase the concentration of H ions
    when dissolved in water.
  • Bases will increase the concentration of OH-ions
    when dissolved in water.

5
16.2 Brønsted-Lowry Acids and Bases
  • 1923 Brønsted and Lowry made a more general
    definition
  • Brønsted-Lowry Acid is a substance that can
    transfer a proton. It must have a hydrogen atom
    that can be lost as H.
  • Brønsted-Lowry Base is asubstance that can accept
    a proton. Must have a nonbonding pair of
    electrons to gain a H ion.

6
Conjugate Acid-Base Pairs
  • Conjugate base- Removal of proton from the acid
  • Conjugate acid- Addition of proton to the base

7
Relative Strengths of Acids and Bases
  • The stronger the acid, the weaker its conjugate
    base.
  • The stronger the base, the weaker its conjugate
    acid.
  • 1. Strong acids completely transfer protons to
    water.
  • 2. Weak acids partly dissociate in aqueous
    solutions and exist as a mixture of acid
    molecules and component ions.
  • 3. Negligible acidity contain Hydrogen but do
    not demonstrate acidic behavior. Ex CH4
  • Position of equilibrium favors transfer of proton
    from stronger acid to stronger base.

8
16.3 Autoionization of Water
  • Ion product of water
  • 1.0 x 10 14 H OH-
  • This is used to calculate concentrations of H
    and OH- .
  • If H OH-, than neutral equation
  • If H gt OH-, than acidic equation
  • If H lt OH-, than basic equation


9
16.4 The pH Scale
  • pH -log H
  • pH of 7 is neutral
  • Acidic solution 0 lt pH lt 7
  • Basic solution 14 gt pH gt 7
  • Other p scales are
  • pOH -log OH-
  • pOH pH -log Kw 14.0

10
Examples on the pH Scale
11
Measuring pH
  • A pH meter consists of a pair of electrodes
    connected to a meter which pH is generated when
    placed in the solution.
  • An acid-base indicator turns a color if placed
    in acid or base. Ex litmus paper

12
16.5 Strong Acids and Bases
  • Strong Acids
  • 7 most common strong acids are
  • HCl, HBr, HI, HNO3, HClO3, HClO4, and H2SO4
  • In acidic reactions, equilibrium lies entirely to
    the right side.
  • Completely dissociates
  • Example
  • HNO3 gt H NO3-

13
Strong Bases
  • Most common strong bases are ionic hydroxides of
    alkali metals (1A) and heavier alkaline earth
    metals (2A). Examples LiOH, RbOH, CsOH, NaOH,
    KOH, and Ca(OH)2, Sr(OH)2, and Ba(OH)2.
  • Other strong bases react with water to form OH-
    such as Na2O, CaO.
  • Also, anions O2-, H-, and N3- are stronger bases
    than OH- and therefore remove a proton from H2O.
  • Example N3- H2O gt NH3 3OH-

14
16.6 Weak Acids
  • A weak acid only partially ionizes in aqueous
    solutions.
  • General weak acid equation
  • HX ? H X- where H is Hydrogen
  • Many weak acids contain some Hydrogen atoms
    bonded to carbon atoms and oxygen atoms (organic
    compounds).
  • Ka is the acid dissociation constant.
  • The larger the value of Ka , the stronger the
    acid.

15
Calculating Ka from pH
  • Use and ICE box!
  • Sample exercise
  • A student prepared a .10 M solution of formic
    acid and measures its pH which was 2.38.
  • A) calculate Ka for formic acid
  • B) what percentage of the acid is ionized in the
    .10M solution?

16
Answer
  • a) HCHO2 ? H CHO2-
  • Ka HCHO2- HCHO2
  • pH -logH
  • 10 2.38 4.2 X 10-3M
  • Ka 4.2 X 103 4.2 X 103 .10
  • 1.8 X 10-4 4.2 X 103 4.2 X 103
    .10
  • b) Percent Ionization
  • Concentration of H
  • Initial concentration of component
  • 4.2

HCHO2 ? H CHO-
I .10 M 0 M 0 M
C -4.2 X 103 4.2 X 103 4.2 X 103
E .10 - 4.2 X 103 4.2 X 103 4.2 X 103
17
Using Ka to Calculate pH
  • The best way to explain this is by an example.
  • Calculate the pH of a .30 M solution of acetic
    acid at 25o C. (Ka 1.8 X 10-5)
  • So HC2H3O2 ? H C2H3O2-
  • Ka HC2H3O2- 1.8 X 10-5
  • HC2H3O2
  • What now?

18

HC2H3O2 ? H C2H3O2-
I .30 M 0 M 0 M
C -x x x
E .30-x x x

Ka (x)(x) 1.8 X 10-5 (.30
x) Either do the quadratic equation or in this
case you can take out x in the denominator.
H x 2.3 X 10-3 pH -log 2.3 X 10-3
2.64
19
Polyprotic Acids
  • Polyprotic acids have more than one ionizable
    Hydrogen atom.
  • Example
  • H2SO3 ? H HSO4-
  • HSO4- ? H SO32-
  • The second Ka (Ka2) is much smaller than Ka1
    because it is easier to remove the first proton.

20
16.7 Weak Bases
  • Weak base water gt conjugate acid hydroxide
    ion
  • Kb is the base-dissociation constant (equilibrium
    in which base reacts when H2O to form conjugate
    acid and OH- ion).
  • Types of weak bases
  • Neutral substances that have atoms with a
    non-bonding pair of electrons that can serve as a
    proton acceptor.
  • Most of these contain amines, N-H which is
    sometimes replaced with a bond between C or N Ex
    NH2CH3
  • Anions of weak acids
  • Ex ClO- H2O ? HClO H
  • ClO- is the weak base

21
16.8 Relationship Between Ka and Kb
  • Reaction 1 reaction 2 reaction 3
  • Which leads to K1 x K2 K3
  • Which leads to Ka x Kb Kw
  • Kw is the ion-product constant for water
  • Kw 1 x 10-14
  • As the strength of the acid increases, the
    strength of the base decreases and visa-versa.
  • pKa pKb pKw 14.00

22
16.9 Acid-Base Properties of Salt Solutions
  • Hydrolysis is the process at which ions react
    with water and produce H or OH-
  • X- H2O ?HX OH-
  • Anions of strong acids do not influence pH
  • Ex NO3-
  • Anions that still have ionizable protons are
    amphoteric
  • Ex HSO3- from H2SO4
  • Most cations (except 1A elements and Ca2, Sr2.
    Ba2) act as weak acids in solution.

23
Predicting the pH of a Solution
  • 1. Salts derived from a strong acid and a strong
    base makes a neutral pH (pH of 7).
  • NaOH HCl gt NaCl H2O
  • 2. Salts derived from a strong base and a weak
    acid will yield a pH of above 7 because the anion
    hydrolyzes to produce OH- ions and the cation
    does not hydrolyze.
  • NaOH HClO gt NaClO H2
  • 3. Salts derived from a weak base and a strong
    acid will result in a pH that is below 7 because
    the cation hydrolyzes to produce H ions and the
    anion does not hydrolyze.
  • Al(OH)3 3HNO3 gt Al(NO3)3 3H2O

24
  • 4. Salts derived from a weak base and a weak acid
    will yield a pH that is dependant on the constant
    value of the constant dissociations (Ka and Kb).
  • if the base is more basic than the acid is
    acidic, then the solution will have a pH that is
    greater than 7.
  • If the acid is more acidic, than the pH will be
    less than 7.
  • NH4 CN- ? NH4CN
  • NH4 Ka 5.6 X 10-10
  • CN- Kb 2.0 X 10-5
  • Therefore, the pH of NH4CN is greater than 7

25
16.10 Acid-Base Behavior and Chemical Structure
  • Factors that effect acid strength
  • If H-X bond is polarized (X is more
    electronegative) the H acts as a proton acceptor.
  • Non-polar bonds (CH4) produce neutral solutions.
  • Weaker bonds dissociate more easily than very
    strong bonds.
  • HF is a weak acid because of this.
  • The greater the stability of the conjugate base,
    the weaker the acid.
  • Ultimately, there are three factors effecting
    acid strength
  • Polarity of H-X bond
  • Strength of H-X bond
  • Stability of conjugate base, X-

26
Binary Acids
  • Binary acids are composed of Hydrogen and a
    non-metal.
  • Ex HCl, HF, H2S, etc.
  • The more polar the bond,the stronger it is
  • The weaker the bond, the stronger the acid.
  • Strength of the bond decreases (acidity
    increases) as the element increases in size or
    moves down a group.
  • Acid strength increases (acidity decreases)
    moving from left to right

27

Group Group Group Group
4A 5A 6A 7A
Period 2 CH4 No acid or base properties NH3 Weak base H2O ------- HF Weak acid
Period 3 SiH4 No acid or base properties PH3 Weak base H2S Weak acid HCl Strong acid

Increasing base strength
Increasing acid strength
Increasing acid strength
Increasing base strength
28
Oxyacids
  • Oxyacids are acids with an OH group is bound to a
    central atom.
  • Example H2SO4

29
OH- Bonding
  • To determine if an OH group acts as an acid or
    base, consider this
  • If Y is a metal than sources of OH- behave as
    bases.
  • If Y is a non-metal than the compound will not
    readily lose the OH- ion.
  • The electronegativity will increase and so will
    the acidity.
  • The increasing number of Oxygen atoms stabilizes
    the conjugate base and thus increases the
    strength of the acid.

30
Oxyacid Rules of Thumb
  • Oxyacids that have the same number of OH groups
    and the same number of Oxygen atoms, acid
    strength increases with increasing
    elecronegativity of the central atom
  • Example HClO gt HBrO gt HIO (gt more acidic)
  • 2. For oxyacids with the same central atom,
    acid strength increases with increasing number of
    Oxygen atoms that are attached.
  • Example HClO lt HClO2 lt HClO3 lt HClO4 ( lt less
    acidic)

31
Carboxylic Acid
  • Carboxylic acids are organic compounds.
  • -COOH is the
    functional group
  • -R is either a
    Hydrogen
  • or Carbon based group
  • If an extra Oxygen is added than it stabilizes
    the conjugate base and increases the acidity.
  • If conjugate base has resonance structures, it
    spreads the negative charge evenly over the
    compound.
  • Acid strength of carboxylic acid increases as the
    number of electronegative atoms increase.

32
6.11 Lewis Acids and Bases
  • G.N. Lewis proposed this
  • Lewis Acids have an incomplete
  • octet of electrons. Function as
  • electron pair acceptors
  • Lewis Bases act as electron pair
  • donators

33
Hydrolysis of Metal Ions
  • Hydration is a process when when metals attract
    unshared electron pairs of water molecules.
  • The metal acts as Lewis acid
  • The water acts as Lewis base
  • Ex Fe(H2O)63 ? Fe(H2O)5(OH)2 H
  • So, general equation
  • M(H2O)nc ? M(H2O)n-1(OH)c-1 H

34
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