Name:_________________
Chemistry 12
Unit
4.6 Strong and Weak Acids and Bases
4A-6
Hebden P. 121-126
By the end of this lesson I will be able to
D4
relate electrical conductivity in a solution to the total [ions] in the solution
define and give several examples for the following terms:
– strong acid – strong base – weak acid – weak base
write equations to show what happens when strong and weak acids and bases are dissolved in water
Homework: Unit 4 Workbook pg 7-8
For extra practice: Hebden pg. 125 #21-25
Vocab
 Strong acid
 Strong base
 Weak acid
 Weak base
 Concentrated Acid/base
 Diluted acid/base
A) Definitions
Definition: Strong Acid


An acid that is 100% ionized in solution.
o E.g. HCl(g) + H2O(l) → H3O+ (aq) + Cl-(aq)
o
1.0M
→ 1.0M
1.0M
Note: There is NO EQUILIBRIUM. Reaction goes to COMPLETION.
Definition: Strong Base


A base that is 100% ionized in solution.
o E.g. NaOH(s) → Na+(aq) + OH-(aq)
o
1.0M
→ 1.0M
1.0M
Note: There is NO EQUILIBRIUM. Reaction goes to COMPLETION.
Definition: Weak Acid


An acid that is less than100% ionized in solution.
o E.g. HF(aq) + H2O(l) ↔ H3O+(aq) + F-(aq)
o
1.0M
→ 0.03M
0.03M
Note: Reaction goes to EQUILIBRIUM.
Definition: Weak Base




A base that is less than100% ionized in solution.
o E.g. NH3(aq) + H2O(l) ↔ NH4+(aq) + OH-(aq)
o
1.0M
→ 0.004M
0.004M
Note: Reaction goes to EQUILIBRIUM.
100% ionized → STRONG
99% ionized → WEAK (however all weak acids/bases are less than 50% ionzied)
4.6 Notes
12
Review
Strong vs Weak → refers to the nature of the acid or base and whether it will ionize to 100% or not.
Strong = 100% ionized (goes to completion)
Weak < 100 % ionized (equilibrium)
Concentration → refers to the amount dissolved in solution
Concentrated → lots of chemical species in solution (high Molarity)
Diluted → very few chemical species in solution (low Molarity)
E.g. Consider the following 4 solutions: 10.0 M HF, 0.001M HF, 10.0M HCl, 0.001M HCl
Strong Acids:________________________________
Weak Acids:________________________________
Concentrated Acids:________________________________
Diluted Acids:________________________________
B) The Strong Acids
We will be using the “RELATIVE STRENGTHS OF BRONSTED-LOWRY ACIDS AND BASES” Table
on page 6 of your Data Pages Booklet
There are 6 STRONG ACIDS found on the top of the table on the left:
They are found at the top:
1. HClO4
2. HI
3. HBr
4. HCl
5. HNO3
6. H2SO4
Note:



H2SO4 is only strong for 1st dissociation (H2SO4 → HSO4-) HSO4- is NOT a strong acid
For all of these reactions, there is NO reverse reaction
H3O+ is just the net result of putting a strong acid in water.:
4.6 Notes
13
C) The Weak Acids
The weak acids are found on the left side of the table, below the strong acids from HIO3 to H2O.
HIO3 ↔ H+ + IO3∙
∙
H2O ↔ H+ + OHNote:
 Weak acids always go to equilibrium and thus always use ‘↔’
 The last 2 species (OH- and NH3) NEVER act as acids
D) The Strong Bases
The bottom two bases on the right side of the table are strong bases.
o E.g. NaO contains the O2- ion
 Metal hydroxides that will dissociate in water 100% are strong bases.
o All the metals that are soluble with OH- from Unit 3.
 NaOH
 KOH
 Sr(OH)2
Metal hydroxides provide the OH- ion for the following:
H2O ↔ H+ + OH-
E) The Weak Bases
The weak bases are on the right side of the table, from H2O to PO43H3O+ ↔ H+ + H2O
∙
∙
HPO42- ↔ H+ + PO43Note:
 Weak bases always go to equilibrium and thus always use ‘↔’
 The top 6 species (HSO4- and ClO4-) NEVER act as bases (conjugates of strong acids)
Practice
 NaOH
 HCl
 H2C2O4
 HF
 F HS-
 NH4+
 HClO4
 Mg(OH)2
 C6H5O-
 Al(H2O)63+  HCOO H2BO3 HSO4-
1. Use the list of chemicals above to answer the following questions. (Some can be used twice)
a. Strong Acids: _____________________________________
b. Strong Bases: _____________________________________
c. Weak Acids: _____________________________________________________________________
d. Weak Bases: _____________________________________________________________________
4.6 Notes
14
F) Using the Table Relative Strengths of Acids
Some trends in the table.
1. Acids get stronger as they go ↑
2. Bases get stronger as they go ↓
3. A stronger acid has a weaker
conjugate base
HX ↔ H+ + X4. A weaker acid has a stronger
conjugate base.
HX
↔ H+ + X
5. Amphiprotic species are on BOTH
sides
 HPO42- and HCO3 As acids:
HCO3- ↔ H+ + CO32HPO42- ↔ H+ + PO43 As bases:
H2CO3 ↔ H+ + HCO3H2PO4- ↔ H+ + HPO42-
G) The Leveling Effect
Compare 4 different solutions of acids:
1M HClO4 produces:
1M H3O+ + 1M ClO4(NO undissociated HClO4)
+
1M HCl produces:
1M H3O + 1M Cl (NO undissociated HCl)
1M HF produces:
0.97M HF + 0.03M H3O+ + 0.03M F1M CH3COOH produces:
0.996M CH3COOH + 0.004M H3O+ + 0.004M CH3COOIn Summary:





Strong Acids (HClO4 and HCl) dissociate 100% to ions and contain no undissociated species
Both strong acids dissociate to the same amount (100%) therefore:
THERE IS NO DIFFERENCE IN STRENGTH BETWEEN STRONG ACIDS
Weak acids (HF and CH3COOH) still contain large concentrations of undissociated species
(e.g. 0.97M HF)
HF is a stronger acid than CH3COOH as it will dissociate to a larger extent.
Leveling Effect:
 All strong acids dissociate to 100% and therefore are effectively solutions of H3O+
 All strong bases dissociate to 100% and therefore are effectively solutions of OH There is NO difference in the strength of the strong acids or the strong bases.
4.6 Notes
15
H) Conductivity of Acids and Base.
Review of Conductivity:
Definition: Conductivity


the movement of ions (e.g. electrons) in the form of electricity to travel through a
substance
For solutions more ions = more conductivity
Observations:
 100% water has very low
conductivity
 solid ionic compounds
(like solid bases) or 100%
pure liquid acids have zero
conductivity
 ionic solutions conduct
electricity
Notes:
In solutions, electrons can
jump from one ion to another,
and thus transfer electrons and
electricity
In pure water, there are very
few ions, so electrons cannot
move easily.
The more ions there are, the higher the conductivity
In Summary:
 More ions in solution = higher conductivity
 Conductivity of acids and bases, depends on 2 things:
o concentration of the acid/base
o strength of the acid/base (or more specifically – how much it will dissociate).
Example:
Consider
0.01M HCl produces:
0.01M H3O+ + 0.01M Cl0.01M HF produces: 0.0097M HF + 0.0003M H3O+ + 0.0003M F1M HF produces:
0.97M HF + 0.03M H3O+ + 0.03M FThe order of conductivity will be:
___1M HF ____ → ___ 0.01M HCl___ → ___ 0.01MHF ___
(most conductive) ∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙ (least conductive)
4.6 Notes
16
Practice
2. Rank the following acids from weakest to strongest: H2S, H3PO4, HF, HBr, CH3COOH
________________________________________________________________________
3. Rank the following bases from weakest to strongest: IO3-, HS-, C2O42-, CO32-, SO42-, NH2________________________________________________________________________
4. Which of the acids from #2. with a 0.25M would be the most conductive? ____________
Why? _____________________________________________________________________
___________________________________________________________________________
I) Acid Base Reaction Equations in Water
Example 1 : H2S as an acid in water:
Step 1: Write down the acid reaction from table.
H2S
↔ H+ + HS-
Step 2: Write down flipped base reaction from table
H+ + OH-↔H2O
+
(H must cancel on both sides)
Step 3: Add the two reactions together
Overall Rxn:
H2S(aq) + H2O(l) ↔ H3O+ (aq) + HS-(aq)
Example 2 : HS- acts as an base in water:
Step 1: Write down the base reaction from table.
HS- + H+ ↔ H2S
Step 2: Write down flipped base reaction from table
H2O ↔ H+ + OH+
(H must cancel on both sides)
Step 3: Add the two reactions together
Overall Rxn:
HS- (aq) + H2O(l) ↔ H2S (aq) + OH- (aq)
Practice
5. Using the table of “Relative Strengths of Acids”, write the equations of the following reactions
that occur in water.
a. H2CO3 acts as an acid: _________________________________________
b. C2O42- acts as a base: __________________________________________
c. HCN acts as an acid: __________________________________________
d. H2PO4- acts as a base: __________________________________________
e. H2PO4- acts as an acid: __________________________________________
f. H2O2 acts as an acid: ____________________________________________
4.6 Notes
17