Title: Acids and Bases
1Chapter 16 Acids and Bases
2- Drill
- Determine which strong acid and strong base the
following salts were derived from - LiCl
- Ba3(PO4)2
- CaSO4
- Sr(NO3)2
3Objectives iWBAT Distinguish between Arrhenius,
Bronsted Lowry and Lewis acids and bases. Write
conjugate acid/base pairs.
4Arrhenius Definition
- Definitions
- Acids produce hydrogen ions in aqueous solution.
- Bases produce hydroxide ions when dissolved in
water. - Limited to aqueous solutions.
- Uses only one kind of base (hydroxide).
- NH3 ammonia could not be an Arrhenius base.
5Bronsted-Lowry Definitions
- DefinitionAn acid is a proton (H) donor and a
base is a proton acceptor. - Acids and bases always come in pairs.
- HCl is an acid.
- When it dissolves in water, it gives its proton
to water. - HCl(g) H2O(l) H3O Cl-
- Water is a base that makes a hydronium ion
6Example Reaction
Base
Hydronium Ion
Acid
7Remember
- Strong acids completely dissociate in water.
- HCl H2O ? H3O 1 Cl-1
- This reaction goes to completion and there is no
HCl left in the solution. - Use a single direction arrow.
8Strong Acids
9Remember
- Weak acids only partially dissociate.
- CH3COOH NH3 ? CH3COO-1 NH41
- This is an equilibrium reaction.
- There are significant amounts of reactants and
products in the solution. - Use a double headed arrow. ?
10Hydroxides (and some oxides) are strong bases.
11Weak Bases
- All other common bases are weak.
- Weak bases establish an equilibrium system like
acids. - Some examples of weak bases are on the next
slide.
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13Acid Base Pairs
- General equation
- HA(aq) H2O(l) H3O(aq) A-(aq)
- Acid Base Conjugate acid Conjugate
base - This is an equilibrium situation.
- There is competition for H between H2O and A-
- The stronger base controls direction of the rxn.
- If H2O is a stronger base it takes the H
- Equilibrium would then move to right.
14Practice
- Try the Bronsted Lowry conjugate acid/base
worksheet - Keep in mind
- The acid turns into a base and
- The base turns into an acid.
15Day 2
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18This is an abbreviated chart with Common
Acids/Bases
19Use the following reaction and the conjugate
acid/base chart to determine which direction the
equilibrium will lie.
- CH3COOH NH3 ? CH3COO-1 NH41
- CH3COOH is a stronger acid than NH41
- NH3 is a stronger base than CH3COO-1
- The equilibrium will favor the side in which the
weaker acid and base a present. - Equilibrium will lie to the right.
-
20Acid Dissociation Constant Ka
- HA(aq) H2O(l) H3O(aq) A -1(aq)
- Ka H3O1A-1 HA
- H3O1 is often written H1 ignoring the water in
equation (it is implied). - Since this is the equilibrium constant associated
with weak acid dissociation, this particular Kc
is most commonly called the acid dissociation
constant Ka
21Acid Dissociation Constant Ka
- HA(aq) H(aq) A-(aq)
- Ka HA- HA
- We can write the expression for any acid.
- Strong acids dissociate completely.
- Equilibrium lies far to right.
- Conjugate base must be weak.
22Back to Pairs
- Strong acids
- Ka is large
- H is equal to HA
- A-1 is a weaker base than water
- Weak acids
- Ka is small
- H ltltlt HA
- A-1 is a stronger base than water
23Types of Acids
- Monoprotic Acids have only one hydrogen.
- Polyprotic Acids more than 1 acidic hydrogen
(diprotic, triprotic). - OxyacidsProton is attached to the oxygen of an
ion. - Organic acidscontain the Carboxyl group -COOH
with the H attached to O - Generally very weak.
24- Oxyacid examples
- Sulfuric acid (H2SO4), phosphoric acid (H3PO4),
and nitric acid (HNO3) are all oxyacids. - Organic acid examples
- Lactic acid Uric acidAcetic acid Oxalic
acidFormic acid Citric acid
25Amphoteric
- Amphoteric means that the substance can behave as
both an acid and a base. - Water molecules interact with each other and
ionize. At the same time, the ions in solution
reform molecules of water as shown in the
following reaction. (This means that water
auto-ionizes) - 2H2O(l) H3O1(aq) OH-1 (aq)
- KW H3OOH- HOH-
26- In pure water the concentrations of H3O1 and
OH-1 will always be equal. - H OH- 1.0 x 10-7
- At 25ºC KW 1.0 x10-14
- Therefore
- Neutral solution H OH- 1.0 x10-7
- Acidic solution H gt OH-
- Basic solution H lt OH-
27pH
- In 1909, Danish biochemist S. P. L Sorensen
introduced the pH system. - pH representing power of hydrogen
28pH
- pH -logH
- Used because H is usually very small
- As pH decreases, H increases exponentially
- Other equations
- pOH -logOH-
- pKa -log K
29Sig Figs for pH
- Sig figs the number of sig figs in the lead
number is the number of decimal places for the pH
value. (only the digits after the decimal place
of a pH are significant) - H 1.0 x 10-8 pH 8.00 2 sig figs
30Relationships
- Derivation
- KW HOH-
- -log KW -log(HOH-)
- -log KW -logH -logOH-
- pKW pH pOH
- KW 1.0 x10-14
- 14.00 pH pOH
- H,OH-,pH and pOH
- Given any one of these we can find the other
three.
31Basic
Acidic
Neutral
32Strong Acids
- HBr, HI, HCl, HNO3, H2SO4, HClO4
- These acids completely dissociate
- Therefore, H HA
- 10-14 HOH-
33Weak Acids
- Ka will be small.
- ALWAYS WRITE THE MAJOR SPECIES.
- It will be an equilibrium problem from the start.
- Determine whether most of the H will come from
the acid or the water. - Compare Ka or Kw
- Rest is just like equilibrium chapter.
34Example
- Calculate the pH of 2.0 M acetic acid HC2H3O2
with a Ka 1.8 x10-5 - Calculate pOH, OH-, H
35A Mixture of Weak Acids
- The process is the same.
- Determine the major species.
- The stronger will predominate.
- Bigger Ka if concentrations are comparable
- Calculate the pH of a mixture 1.20 M HF (Ka 7.2
x 10-4) and 3.4 M HOC6H5 (Ka 1.6 x 10-10)
36Percent Dissociation
- amount dissociated x 100 initial
concentration - For a weak acid percent dissociation increases as
acid becomes more dilute. - Calculate the dissociation of 1.00 M and
.00100 M Acetic acid (Ka 1.8 x 10-5 - As HA0 decreases H decreases but
dissociation increases. - Le Chatelier
37The Other Way
- What is the Ka of a weak acid that is 8.1
dissociated as 0.100 M solution?
38Bases
- The OH- is a strong base.
- Hydroxides of the alkali metals are strong bases
because they dissociate completely when
dissolved. - The hydroxides of alkaline earths Ca(OH)2 etc.
are strong dibasic bases, but they dont
dissolve well in water. - Used as antacids because OH- cant build up.
39Bases without OH-
- Bases are proton acceptors.
- NH3 H2O NH4 OH-
- It is the lone pair on nitrogen that accepts the
proton. - Many weak bases contain N
- B(aq) H2O(l) BH(aq) OH- (aq)
- Kb BHOH- B
40Strength of Bases
- Hydroxides are strong.
- Others are weak.
- Smaller Kb weaker base.
- Calculate the pH of a solution of 4.0 M pyridine
(Kb 1.7 x 10-9)
N
41Polyprotic Acids
- Always dissociate stepwise.
- The first H comes of much easier than the
second. - Ka for the first step is much bigger than Ka for
the second. - Denoted Ka1, Ka2, Ka3
42Polyprotic Acids
- What does K stand for?
- Is it easier to remove the first or second
ionizable proton? - Is is easier to remove the first.
- The K values become successively smaller as
successive protons are removed. - You will need to do 2 or more ice boxes.
43Polyprotic Acid
- H2CO3 H HCO3-1 Ka1 4.3 x 10-7
- HCO3-1 H CO3-2 Ka2 4.3 x 10-10
- Base in first step is acid in second.
- In calculations we can normally ignore the second
dissociation.
44Calculate the Concentration
- Of all the ions in a solution of 1.00 M Arsenic
acid H3AsO4 - Ka1 5.0 x 10-3
- Ka2 8.0 x 10-8
- Ka3 6.0 x 10-10
45Sulfuric Acid is Special
- In first step it is a strong acid.
- Ka2 1.2 x 10-2
- Calculate the concentrations in a 2.0 M solution
of H2SO4 - Calculate the concentrations in a 2.0 x 10-3 M
solution of H2SO4
46Salts
- A salt is an ionic compound formed by the
reaction between an acid and a base. - Salts are strong electrolytes that completely
dissociate into ions in water. - Salts of the cation of strong bases and the anion
of strong acids are neutral. - for example NaCl, KNO3
47Salts that Produce Neutral Solutions
48Basic Salts
- If the anion of a salt is the conjugate base of a
weak acid - basic solution. - In an aqueous solution of NaF
- The major species are Na, F-, and H2O
- F- H2O HF OH-
- Kb HFOH- F-
- but Ka HF- HF
49Basic Salts
- Ka x Kb HFOH- x HF- F-
HF
50Basic Salts
- Ka x Kb HFOH- x HF- F-
HF - Ka x Kb OH- H
- Ka x Kb KW
51Ka tells us Kb
- The anion of a weak acid is a weak base.
- Calculate the pH of a solution of 1.00 M NaCN. Ka
of HCN is 6.2 x 10-10 - The CN- ion competes with OH- for the H
52Acidic Salts
- A salt with the cation of a weak base and the
anion of a strong acid will be basic. - The same development as bases leads to
- Ka x Kb KW
- Calculate the pH of a solution of 0.40 M NH4Cl
(the Kb of NH3 1.8 x 10-5). - Other acidic salts are those of highly charged
metal ions.
53Anion of weak acid, cation of weak base
- Ka gt Kb acidic
- Ka lt Kb basic
- Ka Kb Neutral
54Structure and Acid Base Properties
- Any molecule with an H in it is a potential acid.
- The stronger the X-H bond the less acidic
(compare bond dissociation energies). - The more polar the X-H bond the stronger the acid
(use electronegativities). - The more polar H-O-X bond -stronger acid.
55Strength of Oxyacids
- The more oxygen hooked to the central atom, the
more acidic the hydrogen. - HClO4 gt HClO3 gt HClO2 gt HClO
- Remember that the H is attached to an oxygen
atom. - The oxygens are electronegative
- Pull electrons away from hydrogen
56Strength of Oxyacids
Electron Density
57Strength of Oxyacids
Electron Density
O
58Strength of Oxyacids
Electron Density
O
O
59Strength of Oxyacids
Electron Density
O
O
O
60Hydrated Metals
- Highly charged metal ions pull the electrons of
surrounding water molecules toward them. - Make it easier for H to come off.
H
Al3
O
H
61Acid-Base Properties of Oxides
- Non-metal oxides dissolved in water can make
acids. - SO3 (g) H2O(l) H2SO4(aq)
- Ionic oxides dissolve in water to produce bases.
- CaO(s) H2O(l) Ca(OH)2(aq)
62Lewis Acids and Bases
- Most general definition.
- Acids are electron pair acceptors.
- Bases are electron pair donors.
F
H
B
F
N
H
F
H
63Lewis Acids and Bases
- Boron triflouride wants more electrons.
F
H
B
F
N
H
F
H
64Lewis Acids and Bases
- Boron triflouride wants more electrons.
- BF3 is Lewis base NH3 is a Lewis Acid.
F
H
F
H
B
N
F
H
65Lewis Acids and Bases
(
H
Al3
6
O
H
3
(
H
Al
O
H