Transcript Unit 10

UNIT 10
Acids & Bases
Acid/Base Video
pH Scale (0 – 14)
The pH scale is used
to determine
if a solution is acidic
or basic.
Acids have a pH < 7
Bases have a pH > 7
pH = 7 is neutral
pH of Common Substances
pH = 1  strong acid pH = 9  weak base
pH = 6  weak acid pH = 14  strong base
Acids
have a
pH
less
than 7
PROPERTIES OF ACIDS
•
•
•
•
•
•
•
•
Sour tasting
Feel & look like water
React with metals
Can conduct electricity when dissolved in water (strong
or weak electrolytes)
Are red on blue litmus paper
React with bases (metallic hydroxides) to form water
and a salt.
May burn or sting if you have an open cut
The formula usually starts with H (but not H2O!)
INDICATORS
Blue litmus paper turns red in
contact with an acid.
(and red paper stays red)
Sulfuric Acid = H2SO4
Highest volume
production of any
chemical in the U.S.
(approximately 60 billion pounds/year)
Used in the
production of paper
Used in production of
fertilizers
Used in petroleum
refining; auto batteries
Nitric Acid = HNO3
Used in the production
of fertilizers
Used in the production
of explosives
Nitric acid is a volatile
acid – its reactive
components evaporate
easily
Stains proteins yellow
(including skin!)
Hydrochloric Acid = HCl
Used in the “pickling”
of steel
Used to purify
magnesium from sea
water
Part of gastric juice, it
aids in the digestion of
proteins
Sold commercially as
Muriatic acid
Phosphoric Acid = H3PO4
A flavoring agent in
sodas (adds “tart”)
Used in the
manufacture of
detergents
Used in the
manufacture of
fertilizers
Not a common
laboratory reagent
Acetic Acid = HC2H3O2
(also called Ethanoic Acid, CH3COOH)
Used in the
manufacture of plastics
Used in making
pharmaceuticals
Acetic acid is the acid
that is present in
household vinegar
Bases
have a
pH
greater
than 7
PROPERTIES OF BASES
•
•
•
•
Bitter tasting
Feel slimy/slippery
Usually do not react with metals
Can conduct electricity when dissolved in water
(strong or weak electrolytes)
• Are blue on red litmus paper
• React with acids to form water and a salt
• Do not sting
INDICATORS
Red litmus paper
turns blue in contact
with a base (and blue paper Phenolphthalein
stays blue).
turns purple in a
base.
Examples of Bases
(metallic hydroxides)
 Sodium hydroxide, NaOH
(lye for drain cleaner; soap)
 Potassium hydroxide, KOH
(alkaline batteries)
 Magnesium hydroxide, Mg(OH)2
(Milk of Magnesia)
 Calcium hydroxide, Ca(OH)2
(lime; masonry)
Bases Neutralize Acids
Milk of Magnesia contains
magnesium hydroxide,
Mg(OH)2, which neutralizes
stomach acid, HCl.
2 HCl + Mg(OH)2
MgCl2 + 2 H2O
Magnesium salts can cause
diarrhea (thus they are used
as a laxative) and may also
cause kidney stones.
Acids & Bases
DEFINITIONS
Arrhenius
• Acids – any substance that dissociates in water to
produce free hydrogen ions (H+) or hydronium
ions (H3O+)
HCl + H2O  H3
H
H
Cl
O
H
+
O
H
–
+
O
H
+
Cl
H
–
Cl
Arrhenius
• Bases – any substance that dissociates in
water to produce free hydroxide ions (OH-)
NH3 + H2O 
+
NH4
H
H
H
N
H
O
H
H
–
+
O
N
H
+
OH
H
H
H
Bronsted-Lowry
• Acids are proton (H+) donors
HCl
→
H+
+
Cl-
• Bases are proton (H+) acceptors
NH3
+
H+
→
NH4+
Why is H+ a proton?
*H is made of 1 proton and 1
electron
*When H loses an e-, all that is left
is a proton.
Lewis
• Acids are electron pair acceptors
• Bases are electron pair donors
Most specific definiton→ least specific
Arrhenius, Bronsted-Lowry, Lewis
Conjugate Acid/Base Pairs
(Bronsted-Lowry)
• A “conjugate base” is the remainder of the
original acid, after it donates it’s hydrogen ion
• A “conjugate acid” is the product formed when
the original base gains a hydrogen ion
HCl + H2O 
acid
–
Cl
+
+
H3O
base
conjugate base
conjugate acid
Examples
H2O + HNO3  H3O+ + NO3–
NH3 + H2O  NH4 +
+
OH
AMPHOTERIC
Substances that can be considered both an acid
or a base.
NH3 + H2O ↔ NH4+ + OHbase
acid
c.a.
c.b.
HCl + H2O ↔ H3O+ + Clacid
base
c.a.
c.b.
Let’s examine the behavior of
an acid, HA, in aqueous solution.
HA
What happens to the HA molecules in solution?
100% dissociation of HA
HA
H+
Strong Acid
AWould the
solution be
conductive?
Partial dissociation of HA
HA
H+
Weak Acid
AWould the
solution be
conductive?
HA  H+ + A-
HA
H+
A-
Weak Acid
At any one
time, only a
fraction of
the molecules
are
dissociated.
Strong Acid/Base
– 100% ionized in water
– strong electrolyte
HCl
HNO3
H2SO4
HBr
HI
HClO4
-
+
NaOH
KOH
Ca(OH)2
Ba(OH)2
Weak Acid/Base
• does not ionize completely
• weak electrolyte
HF
CH3COOH
H3PO4
H2CO3
HCN
-
NH3
+
Calculations
• What is pH?
The “power of hydrogen” or pouvoir hydrogen
• What is pOH?
The power of hydroxide
• What is [H+]?
The molarity (concentration) of acid ions in a solution
• What is [OH-]?
The molarity (concentration) of base ions in a solution
Calculations
4 different values to find:
1) pH (this indicates if it’s ACID/BASE/NEUTRAL)
2) pOH
3) H+ (hydrogen ions) or H3O+ (hydronium ions)
4) OH- (hydroxide ions)
H+ ↔ pH
↕
OH- ↔ pOH
pH = -log[H3O+] or [H+]
pOH =
-log[OH ]
pH + pOH = 14
[H3O+] or [H+] = 2nd log (-pH)
[OH-] = 2nd log (-pOH)
How to check your work:
[H3O+][OH-] = 1.0  10-14
pH + pOH = 14
Example
Find the pH, pOH, OH- if H+ = 3.25 x 10-4
• What is the pH of 0.050 M HNO3?
pH = -log[H3O+]
pH = -log[0.050]
pH = 1.3
Acidic or basic? Acidic
pH
pOH
[H+]
[OH―]
3.21
5.87
2.51 x 10-2
M
.00875 M
Acid/Base
/Neutral
Testing pH in the Lab
• Litmus paper
• Indicators
• pH meter
pH of Common Substances
Indicators
• dyes that will change color in the
presence of an acid or base.
• Color changes occur when the molecules
are ionized (acid) or un-ionized (base)
• Indicators change color during specific
pH ranges
Indicators
Acid Indicators: change colors w/in the pH range
of 0 - 6.99
Base Indicators – change colors within the pH range
of 7.1 – 14
Neutral Indicators – change colors of pH 6-8
Universal Indicators – change colors throughout pH
range
Some of the
many pH
Indicators
and their
pH range
Neutralization
• Neutralization Reaction - a reaction in
which an acid and a base react in an
aqueous solution to produce a salt and
water:
HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l)
H2SO4(aq) + 2KOH(aq)  K2SO4(aq) + 2 H2O(l)
Neutralization
In general: Acid + Base  Salt + Water
All neutralization reactions are double displacement reactions.
HCl + NaOH

NaCl + HOH
HCl + Mg(OH)2 
H2SO4 + NaHCO3 
Titration
• A laboratory method for determining
the concentration of an unknown acid or
base using a neutralization reaction.
• A standard solution,(a solution of known
concentration), is used.
Titration Terminology
• Equivalence point - The point at which
there are equal concentrations of acid
and base.
[H+] = [OH-]
• End point – the point at which the
chosen indicator changes color to mark
a change in pH (also called indication
point)
Equivalence Point
Solving Titration Problems
• Use the titration formula
MaVa = MbVb
What is the concentration of HCl if 30.0 mL of
0.10 M NaOH neutralizes 50.0mL HCl?
Titration
• Titration is the process of adding a known
amount of solution of known concentration to
determine the concentration of another
solution
• Remember? - a balanced equation is a
mole ratio
• The equivalence point is when the moles of
hydrogen ions is equal to the moles of
hydroxide ions (= neutralized!)
Titration
• The concentration of acid (or base) in
solution can be determined by
performing a neutralization reaction
–An indicator is used to show when
neutralization has occurred
–Often we use phenolphthaleinbecause it is colorless in neutral
and acid; turns pink in base
Steps - Neutralization reaction
1) A measured volume of acid of
unknown concentration is added to
a flask
2) Several drops of indicator added
3) A base of known concentration is
slowly added, until the indicator
changes color; measure the volume
Neutralization
• The solution of known concentration
is called the standard solution
– added by using a buret
• Continue adding until the
indicator changes color
– called the “end point” of the titration