Transcript Acids and bases review (L2 PPT)

Acids and Bases Review
Definition of Acid and Base
Conjugate acid-base questions
Amphiprotic substances
Strong/weak/conc/dilute – which is which?
Strong and weak equilibrium
Strong and weak bases
Summary – important strong and weak acids and bases
Properties of strong and weak acids
Results and explanations
Titration experiment
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Definition of Acid and Base
The Bronsted-Lowry definition of acids and bases:
An ACID DONATES PROTONS to water to form HYDRONIUM IONS (H3O+)
A BASE ACCEPTS PROTONS from water to form HYDROXIDE IONS (OH-)
Write equations to show how the following can act as acids or bases:
1. H2SO4
+ H2O  H3O+ + HSO4-
2. CH3COOH
+ H2O  H3O+
3. CH3NH2
+ H2O  OH- +
4. HCO3-
+ H2O  H3O+ +
+ CH3COOCH3NH3+
CO32-
+ H2O  OH- + H2CO3
5. NaOH
→
Na+ (aq) + OH- (aq)
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Conjugate acid-base questions
1.Write the formula for the conjugate acid of:
a) F-
HF
b) NO3-
HNO3
c) H2PO4-
H3PO4
d) NH3
NH4+
e) H2O
H3O+
2.Write the formula for the conjugate base of:
a) HCl
Cl-
b) H2SO4
HSO4-
c) HSO4-
SO42-
d) NH4+
NH3
e) H2O
OH-
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Amphiprotic substances
Some substances can both accept and donate protons depending on what
they are reacting with – they are called amphiprotic substances
HCO3- + H2O  H3O+ +
CO32-
HCO3- + H2O  OH- + H2CO3
Water is another important amphiprotic substance
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Strong/weak/conc/dilute – which
is which?
Acid molecule
Hydronium ion
Dilute, strong
Dilute, weak
conc, weak
conc, strong
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Strong and weak equilibrium
Strong acids
-Dissociate completely
-Equilibrium lies to the right
-All acid molecules dissociate into hydronium ions
HCl + H20
Cl- + H3O+
Weak acids
-Dissociate to a small extent only
-Equilibrium lies to the left
-Very few acid molecules dissociate into hydronium ions
CH3COOH + H20
CH3COO- +H3O+
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Strong and weak bases
Bases are the same as acids:
Strong bases
Are usually IONIC bases
Ionic bases dissociate completely to form OH-
NaOH is an IONIC base – it dissociates completely – 1 molL-1 NaOH means
there is 1 molL-1 OH- ions
Weak bases
Are usually MOLECULAR bases
Molecular bases accept a small amount of protons to form a small amount of
OHNH3 is a MOLECULAR base - 1 molL-1 NH3 means there is essentially 1 molL-1
NH3 molecules and very few OH- ions.
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Summary – important strong
and weak acids and bases
Strong Acids
Weak Acids
(organic)
HCl
H2SO4
HNO3
Strong Bases
(soluble
hydroxides)
NaOH
KOH
CH3COOH
HCOOH
Weak Bases
NH3
CH3NH2
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Properties of strong and weak
acids
Carry out the following experiments on 1 molL-1 HCl, 1 molL-1
CH3COOH, 4 molL-1 HCl and 4 molL-1 CH3COOH.
Experiment 1: reaction rate
Add a small piece of Mg to an equal volume of each. Time the
reaction
Experiment 2: conductivity
Test the conductivity of each using a simple series circuit consisting
of a bulb and a power supply.
Experiment 3: pH
Test the pH of each solution
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Results and explanations
Conductivity depends on the number of charged particles in solution – the more
charged particles, the higher the conductivity.
pH is a measure of the concentration of hydronium ions in molL-1 – the higher
the [H3O+], the lower the pH
The reaction of an acid is quicker with a higher concentration of H3O+.
4 molL-1 HCl is a high concentration of a strong acid. Strong acid means that
the equilibrium lies to the right and the HCl molecules are fully dissociated.
This along with the high concentration means that there are many ions
(hydronium and chloride) in solution. Therefore the solution has a high
conductivity, low pH, and the fastest rate of reaction.
1 molL-1 CH3COOH is a dilute solution of a weak acid. A weak acid has an
equilibrium lying to the left. Few molecules have dissociated into ions,
therefore the solution has few charged particles to conduct electricity, a high pH
(low [H3O+]) and a slow rate of reaction (low [H3O+]).
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Titration experiment
1. Place 10 mL of 1 molL-1 NaOH into each of 2 boiling tubes
2. Titrate one against 1 molL-1 HCl and the other against CH3COOH.
Predict the volume of each acid that will be used before you begin. Use
phenolphthalein as an indicator.
Results: record the volume of each acid that was required to reach the end
point.
Explain these results!
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pH
Calculate the pH of the following
solutions
(Hint – pH = - log [H3O+])
a) 0.4 molL-1 H3O+
0.398
b) 0.35 molL-1 HCl
0.456
c) 0.30 molL-1 NaOH
13.5
d) 1 x 10-8 molL-1 HCl
6.96
e) 0.0030 molL-1 H2SO4
2.22
f) 0.40 molL-1 KOH
13.6
g) Calculate the [OH-] and
[H3O+] of a solution of pH
5.5
3.16 x 10-9 molL-1 [OH-] and
3.16 x 10-6 molL-1 [H3O+]
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Calculations
1. Assumptions: a strong acid is fully dissociated – therefore the
concentration of the acid = the concentration of hydronium ions
2. pH + pOH = 14
3. OR [H3O+] x [OH-] = 1 x 10-14
4. A diprotic acid has 2 protons to donate (NB this should not come up in Level
3 exams)
5. At concentrations of lower than 1 x 10-7, the [H3O+] from the dissociation of
water molecules becomes significant
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Questions
Use the appropriate equations to show that
a) NH4Cl and
b) CH3COONa
Are acidic or basic.
NH4+ + Cl-
1. NH4Cl + H2O
NH4+ + H2O

(dissociation of ionic ammonium chloride in water)
NH3 + H3O+
2. CH3COONa + H2O
(reaction of molecular ammonium ions with water producing an
acidic solution)
CH3COO- + Na+ (dissociation of ionic sodium ethanoate in
water)
CH3COO- + H2O 
CH3COOH + OH-
(reaction of molecular ethanoate ions with water
producing a basic solution)