CHEM, 2nd edition Cengage Learning Chapter 9 Acid-base...
Transcript of CHEM, 2nd edition Cengage Learning Chapter 9 Acid-base...
-
Shawn McDonald
Linn-Benton Community College
CHEM, 2nd edition
Cengage Learning
Chapter 9
Acid-base reactionsAcids and bases are chemical
compounds that occur regularly in
'everyday life'. These two types of
substances have opposite properties.
They often occur in the foods we eat.
-
2
Types of Electrolytes
• Salts are water-soluble ionic compounds.
All are strong electrolytes. Example: NaCl
• Acids form H+1 ions in water solution.
• Bases combine with H+1 ions in water solution.
Bases increase the OH-1 concentration of the solution.
May either directly release OH-1 or pull H+1 off H2O
molecule. If the latter, this forms OH- ion and a different
positively charged ion.
-
3
Properties of Acids• Sour taste. Like biting into a lemon...
• React with “active” metals.
I.e., Al, Zn, Fe, but not Cu, Ag or Au.
2 Al + 6 HCl AlCl3 + 3 H2
Corrosive. To Al (mid-right) and skin!
• React with carbonates, producing CO2.
Marble, baking soda, chalk, limestone.
CaCO3 + 2 HCl CaCl2 + CO2(g) + H2O
• Change color of vegetable dyes.
Blue litmus turns red. Picture at right.
• React with bases to form ionic salts
and water. Called a neutralization.
-
4
Common AcidsChemical name Formula Uses Strength
Nitric acid HNO3 Explosive, fertilizer, dye, glue Strong
Sulfuric acid H2SO4 Explosive, fertilizer, dye, glue,
batteries Strong
Hydrochloric acid HCl Metal cleaning, food prep, ore
refining, stomach acid Strong
Phosphoric acid H3PO4 Fertilizer, plastics and rubber,
food preservation Moderate
Acetic acid HC2H3O2 Plastics and rubber, food
preservation, vinegar Weak
Hydrofluoric acid HF Metal cleaning, glass etching Weak
Carbonic acid H2CO3 Soda water Weak
Boric acid H3BO3 Eye wash Weak
-
5
Structures of Acids
• Binary acids have acid
hydrogens attached to a
nonmetal atom. 2 types
of elements only.
HCl, HF
Write the H atom first,
then the nonmetal atom.
Dissociate in water to
form H+ ions and
nonmetal anions (such as
Cl- or F-)
Hydrofluoric acid
-
6
Structure of Acids• Oxyacids have acid
hydrogens attached to an
oxygen atom.
H2SO4, HNO3
Also write the H atom(s)
first, then the polyatomic
ion group.
Will dissociate when put
into water, to give H+ ions
and a polyatomic anion
(like NO3- or SO4
2-)
-
7
Structure of Acids
• Carboxylic acids have COOH group.
HC2H3O2, H3C6H5O3
• Only the first H in the formula is acidic.
The H is on the COOH.
Lemons and limes Apples and
wine
Component of vinegar
-
8
Properties of Bases• Also known as alkalis.
• Taste bitter.Alkaloids = Plant product that is
alkaline.
Often poisonous. Potato and tomato shoots.
• Solutions feel slippery.
• Change color of vegetable dyes.Different color than acid.
Red litmus turns blue. The dye in the paper reacts with OH- from base.
• React with acids to form ionic salts.Neutralization. Negates taste and metal
dissolving power of acids.
Occurs in hemlock bark,
repels beetles.
-
9
Common BasesChemical
name Formula
Common
name Uses Strength
Sodium
hydroxide NaOH
Lye,
caustic soda
Soap, plastic,
petrol refining Strong
Potassium
hydroxide KOH
Caustic
potash
Soap, cotton,
electroplating Strong
Calcium
hydroxide Ca(OH)2 Slaked lime Cement Strong
Sodium
bicarbonate NaHCO3 Baking soda Cooking, antacid Weak
Magnesium
hydroxide Mg(OH)2
Milk of
magnesia Antacid Weak
Ammonium
hydroxide
NH4OH,
{NH3(aq)}
Ammonia
water
Detergent,
fertilizer,
explosives, fibers
Weak
-
10
Structure of Bases
• Most ionic bases contain
OH ions.
NaOH, Ca(OH)2
• Some contain CO32- ions.
CaCO3 NaHCO3
• Molecular bases contain
structures that react with
H+.
Mostly amine groups (N
atoms).
Caffeine has three amine type
groups with CH2 group
attached.
-
11
9-1b What is an acid or a base?
• An acid–base reaction is any reaction in
which an H+ is transferred. Does not have to take place in aqueous solution.
Broader definition than Arrhenius.
• An acid is a H+ donor; A base is a H+
acceptor. Either can be a molecule or an ion. Since H+ is a proton, acid is a proton donor and
base is a proton acceptor.
Base structure must contain an atom with an
unshared pair of electrons to bond to H+.
• In the reaction, the acid molecule gives an H+
to the base molecule.
H–A + :B :A– + H–B+
-
12
Amphoteric Substances• Amphoteric substances can act as either an acid or
a base.They have both a transferable H atom and an atom with a
lone pair.
• HCl(aq) is acidic because HCl transfers an H+ to H2O, forming H3O
+ ions.Water acts as base, accepting H+.
HCl(aq) + H2O(l) → Cl–(aq) + H3O
+(aq)• NH3(aq) is basic because NH3 accepts an H
+ from H2O, forming OH
–(aq).Water acts as acid, donating H+.
NH3(aq) + H2O(l) NH4+(aq) + OH–(aq)
Thus water is amphoteric, it can act as a base with an acid, or as an acid with a base. Its nature is the opposite of the
compound with which it is interacting.
-
13
An example acid-base reaction
In the reaction H2O + NH3 HO– + NH4
+:
water ammonia hydroxide ion ammonium
ion
H2O and HO– constitute an
acid/conjugate–base pair. If
hydroxide ion accepts a proton it
will revert to a water molecule.
NH3 and NH4+ constitute a
base/conjugate–acid pair. If
the ammonium ion donates
a proton to a base, it will
revert to the ammonia
molecule.
-
14
Example—Identify the Brønsted–Lowry acids and bases and their conjugates in this reaction.
C. HNO3(aq) + H2O(l) H3O+(aq) + NO3
-(aq)
-
15
Neutralization Reactions• H+ + OH- H2O net ionic eqn.
• Acid + base salt + water
• Double-displacement reactions.Salt = cation from base + anion from
acid. Sometimes the salt in insoluble.
Cation and anion charges stay constant.
H2SO4 + Ca(OH)2 → CaSO4 + 2 H2O
• Some neutralization reactions are gas evolving, where H2CO3 (carbonic acid) decomposes into CO2 and H2O.
H2SO4 + 2 NaHCO3 → Na2SO4 + 2 H2O + 2 CO2sulfuric acid sodium bicarbonate sodium sulfate water carbon dioxide gas
-
16
Example — Write the equation for the reaction of
aqueous perchloric acid with strontium hydroxide.
1. Write the formulas of the reactants.
HClO4(aq) + Sr(OH)2(aq)
2. Determine the ions present when each reactant
dissociates.
(H+ + ClO4−) + (Sr2+ + OH−)
3. Exchange the ions. H+1 combines with OH-1 to
make H2O(l). Like other double displacements.
(H+ + ClO4−) + (Sr2+ + OH−) (Sr2+ + ClO4
−) + H2O(l)
-
17
4. Write the formulas of the products.
Cross charges and reduce subscripts if possible.
HClO4(aq) + Sr(OH)2(aq) Sr(ClO4)2 + H2O(l)
5. Balance the equation. Each atom and group on
left vs. right side of the equation.
May be quickly balanced by matching the numbers of
H and OH to make H2O.
Coefficient of the salt is always 1.
2 HClO4(aq) + Sr(OH)2(aq) Sr(ClO4)2 + 2 H2O(l)
Write the equation for reaction of aqueous
perchloric acid with strontium hydroxide,
Continued.
-
18
6. Determine the solubility of the salt.
Sr(ClO4)2 is soluble (look up in solubility table).
7. Write an (s) after the insoluble products and an
(aq) after the soluble products.
2 HClO4(aq) + Sr(OH)2(aq) Sr(ClO4)2(aq) + 2 H2O(l)
Write the Equation for reaction of aqueous
perchloric acid with strontium
hydroxide.Continued
The reaction occurs since one of the products formed is water,
and water molecules mainly stay as molecules and don't ionize
very much.
-
19
9-2 Strong or Weak
• A strong acid is a strong electrolyte.Practically all the acid molecules ionize, →. completely
• A strong base is a strong electrolyte.Practically all the base molecules form OH– ions, either
through dissociation or reaction with water, →. completely
• A weak acid is a weak electrolyte.Only a small percentage of the molecules ionize, .
• A weak base is a weak electrolyte.Only a small percentage of the base molecules form OH–
ions, either through dissociation or reaction with water, .
-
20
Strong Acids• The stronger the acid, the
more willing it is to donate H.
Use water as the standard base.
• Strong acids donate practically all their H’s.
100% ionized in water.
Strong electrolyte.
• [H3O+] = [strong acid].
[ ] = molarity.
HCl H+ + Cl-
HCl + H2O H3O+ + Cl-
No HCl is left after you dissolve
the compound in water.
-
21
Strong Acids, Continued
Hydrochloric acid HCl
Hydrobromic acid HBr
Hydroiodic acid HI
Nitric acid HNO3
Perchloric acid HClO4
Sulfuric acid H2SO4
-
22
Strong Acids, Continued
Pure waterHCl solution
HCl is a strong electrolyte. Its
solution will conduct current.Water is not an electrolyte and
will not conduct current.
-
23
Weak Acids
• Weak acids donate a small
fraction of their Hs.
Most of the weak acid
molecules do not donate H
to water.
Often less than 1% ionized
in water.
• [H3O+]
-
24
Weak Acids, Continued
Hydrofluoric acid HF
Acetic acid HC2H3O2
Formic acid HCHO2
Sulfurous acid H2SO3
Carbonic acid H2CO3
Phosphoric acid H3PO4
Stronger attraction between H and F than between H+ and water
-
25
Weak Acids, Continued
Pure water HF solution
HF in water
is a weak
electrolyte
and only
conducts
electricity
poorly – note
dimness of
light bulb.
-
26
Degree of Ionization
• The extent to which an acid ionizes in water
depends in part on the strength of the bond
between the acid H+ and anion compared
to the strength of the bond between the
acid H+ and the O of water.
HA(aq) + H2O(l) A−(aq) + H3O
+(aq)
In other words, which is stronger? the H-A bond or the
H-O bond in the hydronium ion. If the former, then the
acid will only ionize slightly. Fluorine for example
forms a stronger bond with H than does Cl, thus HF is a
weak acid and HCl is a strong acid.
-
27
Strong Bases• The stronger the base, the more
willing it is to accept H.
Use water as the standard acid.
• Strong bases, practically all molecules are dissociated into OH– or accept Hs.
Strong electrolyte.
Multi-OH bases completely dissociated.
• [HO–] = [strong base] x (# OH).
• Molarity will be discussed shortly....
NaOH Na+ + OH-
All the dissolved NaOH has
dissociated into sodium and
hydroxide ions.
-
28
Strong Bases, Continued
Lithium hydroxide LiOH
Sodium hydroxide NaOH
Potassium hydroxide KOH
Calcium hydroxide Ca(OH)2
Strontium hydroxide Sr(OH)2
Barium hydroxide Ba(OH)2
-
29
Weak Bases
• In weak bases, only a small
fraction of molecules accept
Hs.
Weak electrolyte.
Most of the weak base molecules
do not take H from water.
Much less than 1% ionization in
water.
• [OH–]
-
30
Weak Bases, Continued
Ammonia NH3(aq) + H2O(l) NH4+(aq) + OH−(aq)
Pyridine C5H5N(aq) + H2O(l) C5H5NH+(aq) + OH−(aq)
Methyl amine CH3NH2(aq) + H2O(l) CH3NH3+(aq) + OH−(aq)
Ethyl amine C2H5NH2(aq) + H2O(l) C2H5NH3+(aq) + OH−(aq)
Bicarbonate HCO3−(aq) + H2O(l) H2CO3 (aq) + OH
−(aq)
Most of the base molecules or species stay in the unprotonated form.
-
31
Autoionization of Water• Water is actually an extremely weak electrolyte.
Therefore, there must be a few ions present.
• About 1 out of every 10 million water molecules form
ions through a process called autoionization.
H2O + H2O H3O+ + OH–
• All aqueous solutions contain both H3O+ and OH–.
The concentration of H3O+ and OH– are equal in DI water.
[H3O+] = [OH–] = 1 x 10-7M at 25 °C in pure water. These are
important concentrations to remember (related to pH that will
we study shortly).
-
32
Ion Product of Water• The product of the H3O
+ and OH–
concentrations is always the same number for solutions at 25 Celsius.
• The number is called the ion product of water and has the symbol Kw.
• [H3O+] x [OH–] = 1 x 10-14 = Kw.
• As [H3O+] increases, the [OH–] must
decrease so the product stays constant.
Inversely proportional.
-
33
Acidic and Basic Solutions
• Neutral solutions have equal [H3O+] and [OH–].
[H3O+] = [OH–] = 1 x 10-7
• Acidic solutions have a larger [H3O+] than [OH–].
[H3O+] > 1 x 10-7; [OH–] < 1 x 10-7
• Basic solutions have a larger [OH–] than [H3O+].
[H3O+] < 1 x 10-7; [OH–] > 1 x 10-7
We can measure the concentration of hydronium ions in a
solution by using a pH meter. These devices are used in
the general chemistry courses to study acid-base behavior
and reactions. pH paper also works to give us the rough
pH of a given aqueous solution.
-
34
Example—Determine the [H3O+] for a 0.00020 M
Ba(OH)2 solution and Determine Whether the
Solution Is Acidic, Basic, or Neutral.
Ba(OH)2 = Ba2+ + 2 OH– therefore:
[OH–] = 2 x 0.00020 = 0.00040 = 4.0 x 10−4 M
4
14
3
3
100.4
101
OHOH
OHOH
w
w
K
K
[H3O+] = 2.5 x 10-11 M.
Since [H3O+] < 1 x 10−7, the solution is basic.
-
35
9-3a The pH scale
• The acidity/basicity of a solution is often
expressed as pH.
• pH = ─log[H3O+], [H3O
+] = 10−pH
The exponent on 10, but with a positive sign.
pHwater = −log[10-7] = 7.
Need to know the [H3O+] concentration to find pH.
• 3 cases:
case 1: pH < 7 is acidic; case 2: pH > 7 is basic;
case 3: pH = 7 is neutral.
-
36
pH, Continued• The lower the pH, the more acidic the solution; the
higher the pH, the more basic the solution.1 pH unit corresponds to a factor of 10 fold
difference in acidity of that solution.
• Normal range is 0 to 14.pH 0 is [H+] = 1 M, pH 14 is [OH–] = 1 M.
pH can be negative (very acidic) or larger than 14
(very alkaline, at high concentration).
-
37
pH of Common SubstancesSubstance pH
1.0 M HCl 0.0
0.1 M HCl 1.0
Stomach acid 1.0 to 3.0
Lemons 2.2 to 2.4
Soft drinks 2.0 to 4.0
Plums 2.8 to 3.0
Apples 2.9 to 3.3
Cherries 3.2 to 4.0
Unpolluted rainwater 5.6
Human blood 7.3 to 7.4
Egg whites 7.6 to 8.0
Milk of magnesia (saturated Mg(OH)2) 10.5
Household ammonia 10.5 to 11.5
1.0 M NaOH 14
-
38
Example—Calculate the pH of a 0.0010 M
Ba(OH)2 Solution and Determine if It Is
Acidic, Basic, or Neutral.
[H3O+] =
1 x 10-14
2.0 x 10-3= 5.0 x 10-12M
pH > 7 therefore, basic.
Ba(OH)2 = Ba2+ + 2 OH− therefore,
[OH-] = 2 x 0.0010 = 0.0020 = 2.0 x 10-3 M.
pH = −log [H3O+] = −log (5.0 x 10-12)
pH = 11.3
-
39
Practice—Calculate the pH of the
Following Strong Acid or Base
Solutions.• 0.0020 M HCl
• 0.0050 M Ca(OH)2
• 0.25 M HNO3
-
40
[H3O+] = 1 x 10
−14
1 x 10−2= 1.0 x 10−12
Practice—Calculate the pH of the
Following Strong Acid or Base
Solutions, Continued.
• 0.0020 M HCl strong acid therefore, [H3O+] =
0.0020 M.
• 0.0050 M Ca(OH)2 strong base, [OH–] = 0.010 M.
• 0.25 M HNO3 a strong acid, therefore, [H3O+] =
0.25 M.
pH = −log (2.0 x 10-3) = 2.70 acidic
pH = −log (1.0 x 10−12) = 12.00 basic
pH = −log (2.5 x 10−1) = 0.60 acidic
-
41
9-4 Acid-base buffers
• Buffers are solutions that resist changing pH when small amounts of acid or base are added.
• They resist changing pH by neutralizingadded acid or base.
• Buffers are made by mixing together a weak acid and its conjugate base.
Or weak base and its conjugate acid. For example, a mixture of ammonia and ammonium chloride is a buffer.
-
42
How Buffers Work
• The weak acid present in the buffer mixture
can neutralize added base.
• The conjugate base present in the buffer
mixture can neutralize added acid.
• The net result is little to no change in the
solution pH. As long as you don’t over
come the capacity of the buffer. Then the
pH will change drastically. Can change by
several pH units after capacity is overcome.
-
43
Acetic Acid/Acetate Buffer
The conjugate base
neutralizes any added
acid…. makes weak acid
(acetic acid)
The weak acid neutralizes
the added base, forms H2O.