Transcript Strengths of Acids and Bases - Welcome to Westford Academy

Strengths of Acids and Bases
Main Idea: In solution, strong acids and
bases ionize completely, but weak acids
and bases ionize only partially.
1
Strength of Acids
• Ions carry electric current through the
solution
– If all of the acid molecules completely dissociate
to form ions, then the conductivity will be high
– Acids that ionize completely are called strong
acids
– Strong acid reactions are represented using a
single arrow:
HCl (aq) + H2O (l)  H3O+ (aq) + Cl- (aq)
2
List of Strong Acids - MEMORIZE
•
•
•
•
•
•
HCl – hydrochloric acid
HBr – hydrobromic acid
HI – hydroiodic acid
HNO3 – nitric acid
HClO4 – perchloric acid
H2SO4 – sulfuric acid
3
Weak Acids
• Weak acids are weak electrolytes (conduct electricity,
but poorly)
• Weak acids do not ionize completely in solution
• Weak acid reactions are represented by reversible
arrows because although some ionization occurs, the
ions can also rejoin to make the compound:
CH3COOH (aq)  CH3COO- (aq) + H+ (aq)
• There are numerous weak acids, so there’s no need to
memorize any (although acetic acid, above, is a classic
example) – just memorize the strong and if the acid in
question is not on that list, then it’s weak!
4
Strength and the Bronsted-Lowry
Model
• Conjugate acids and bases are the opposite
strength of what it came from:
– Strong acid = weak conjugate base
– Weak acid = strong conjugate base
– Strong base = weak conjugate acid
– Weak base = strong conjugate acid
5
Strengths of Bases
• What you have learned about acids can be
applied to bases, except that OH- ions, rather
than H+ ions are involved.
• Strong base = 100% ionization (one-way)
• Weak base = <100% ionization (reversible
reaction)
6
List of Strong/Weak Bases MEMORIZE
• STRONG bases: All Group 1 and 2, SOLUBLE
hydroxides
– EX: LiOH, NaOH, KOH, etc.
– NOTE: Ca and Mg have low solubility in water as
hydroxides, so technically they aren’t strong
bases, BUT to make life easy, we’ll consider them
as part of the list
• WEAK bases: anything that’s not a hydroxide!
The classic example is ammonia, NH3
7
DID YOU KNOW? (A Deadly
Compound)
Hydrogen cyanide, HCN, is a poisonous gas found in the
exhaust of vehicles, in tobacco and wood smoke, and in
smoke from burning nitrogen-containing plastics.
Some insects such as millipedes and burnet moths
release hydrogen cyanide as a defense mechanism.
A solution of hydrogen cyanide is called hydrocyanic acid.
Fruits that have a pit, such as cherries or peaches, contain
cyanohydrins, which convert to hydrocyanic acid in the
digestive system if the pits are eaten. However, no
hydrocyanic acid is produced in the flesh of these
fruits, so the fruit can be safely eaten.
8
HOMEWORK
1) Write chemical equations for the ionization of each acid
(think about which arrows are most appropriate!):
a)
HClO2
b) HNO2
c) HIO
2) Write the first and second ionization equation for H2SeO3.
3) Write chemical equations for the ionization of each base
(think about the B-L definition of bases for what to react
each of these with):
a)
b)
c)
d)
Hexylamine (C6H13NH2)
Propylamine (C3H7NH2)
Carbonate ion (CO32-)
Hydrogen sulfite ion (HSO3-)
9
MORE HOMEWORK
4) Describe the contents of dilute aqueous
solutions of the strong acid HI and the weak
acid HCOOH.
5) Relate the strength of a weak acid to the
strength of its conjugate base.
6) Identify the conjugate acid-base pairs in each
equation and identify their strength:
a) HCOOH (aq) + H2O (l)  HCOO- (aq) + H3O+ (aq)
b) NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)
10