Transcript The Strengths of Acids and Bases

The Strengths of Acids and Bases
References:
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Chemistry, Nelson, pages 503-551
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Heath Chemistry, pages 570-590
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McGraw-Hill Ryerson, Chapters
14,15
Acids and Bases outcomes
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Refer to your outcomes handout
#11 Compare strong and weak acids and
bases using the concept of equilibrium
#12 Understand that acid and base systems
are quantitatively described using pH, pOH,
[H3O+], [OH-], Kw, Ka, Kb and concentration
#13 perform calculations from any of the
above from empirical (lab) data.
What properties can be used to determine if a
solution is acid or base?
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Tasks
1. Refer to table 14.4, page 503 to refresh
your memory about properties!!
2. From the lab data, put the household
acids in order form least acidic to most
acidic.
3. Do Lab Exercise 14D, Strengths of
Acids,page 503.
Strong and Weak Acids
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Acidic solutions of different substances at the
same concentration do not possess acid
properties to the same degree.
Recall Rates of Reaction Lab
What did the demonstration show ?
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Refer to Figure 14.4, page 504 for a recap.
Reflection: Think back to the Chemical Reactions
lab. Which reacted faster with marble, 6M acetic
acid or 6M HCl?
Weak Acid characteristics: If characteristic properties
are less than those of a common strong acid like
HCl; weak electrolyte; reacts at a slower rate, pH
closer to 7
Most common acids fit this category. (Fig. 14.15)
Strong Acids
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According to the revised Arrhenius theory,
HCl(aq) + H2O(l) ---> H3O+(aq) + Cl- (aq)
The H+ ion is a tiny proton that does not remain in
solution, but immediately unites with a water
molecule to make H3O+.
As a strong acid, when HCl reacts with water, no HCl
molecules are left in solution. HCl ionizes
completely.
From the table of Relative strengths of Acids and
Bases on page 611, list the acids that ionize 100%
Weak Acids
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When acetic acid reacts with water it only
partially ionizes (1.3% see page 505 and
611) This reaction can be described as an
equilibrium system
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CH3COOH(aq) + H2O(l) ---> H3O+(aq) + CH3COO- (aq)
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At equlibrium, a considerable amount of
CH3COOH molecules remain. There is a small
proportion of hydronium and acetate ions.
Weak conductor ---> weak acid.
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The Table of Relative Strengths of
Acids and Bases
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Page 611. Also last page of booklet.
To determine the ranking:
The pH of 0.10 mol/L solutions are measured
and the [H3O+] is calculated, the % ionization
is determined and the solutions are ranked in
order of decreasing strengths
See page 505 for example calculations
Comparing Strengths of Acids
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To use the table on pg. 611, or any other like
it, remember:
1. A base is a proton acceptor, an acid is a
proton donor
2. A strong base readily accepts protons
3. A weak base does not accept protons
readily
An example.
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When sodium hydrogen carbonate and hydrofluoric
acid are combined in aqueous solution, what
reaction is likely to occur?
Sodium ions are spectators……
HCO3- + HF
HCO3- is a stronger base than HF and HF is the
stronger acid, so the proton transfer occurs from HF
to HCO3HCO3- + HF
H2CO3 + F-
Weak Acid Equilibrium and Ka
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Refer pages 506-507
CH3COOH ionizes in water to the extent of
1.3% in a 1.0 mol/L solution. The equilibrium
constant can also be used to express the
extent of ion formation.
Equilibrium Law can be used to calculate the
equilibrium constant, known as the acid
ionization constant Ka
Ka= [H3O+] [CH3COO-]
[CH3COOH]
The table on page 611 shows Ka
for
Acetic acid to be 1.8 x 10-5 mol/L
The Ka value can be used over a
range of concentrations of an acid to
predict the [H3O+] and the pH.
CH3COOH(aq) + H2O(l) ---> H3O+(aq) + CH3COO(aq)
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According to the balanced equation,
[CH3COO-] = [H3O+] 1:1 ratio of moles
x
Ka
=
x
= x2
[CH COOH]
3
Perform the calculation and report the value of [H3O+]
pH calculation
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Recall, pH = - log [H3O+]
pH = - log 4.2 X 10-3
pH = 2.37
Examples: What pH results when 0.25 mol
acetic acid dissolves in water to make one
litre of solution?
A 0.24 M solution of carbonic acid has a pH
of 3.49. Determine the Ka of the weak acid.
Strong Bases (pg. 507)
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According to Arrhenius,a base is a substance that
increases the concentration of the hydroxide ion in
solution.
Ionic hydroxides are all strong bases - they
dissociate100% into the metallic cation and the
hydroxide ion.
Which is more basic, sodium hydroxide or barium
hydroxide? Explain
Qualitative Analysis and reviewing
the concepts!!
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Do Lab Exercise 14 E on
page 507
Solution 1 is _______
Solution 2 is _______
Solution 3 is _______
Solution 4 is ________
Solution 5 is ________
Solution 6 is _________
Solution 7 is _________
Weak Bases(pg. 510)
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Arrhenius - bases are soluble ionic hydroxides
LeChatelier - hydroxide ions added to water cause a
shift in the water ionization equilibrium decreasing
the hydronium ion concentration and producing a pH
greater the 7.
Some molecular and ionic compounds other than
hydroxides, also dissolve in water to produce basic
solutions. These are not as basic as the ionic
hydroxides and are called weak bases.
Kb, base ionization constant
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Weak bases react non-quantitatively with
water to form an equilibrium that includes
aqueous hydroxide ions
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Page 510
Base + H2O(l)
OH-(aq) + conjugate acid
Ammonia is an example. Write the ionization
equation for ammonia.
NH3(aq) + H2O(l)
OH-(aq) + NH4(aq)
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Example calculations
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The format is very similar to the calculations
involving Ka
The pH of a 0.100 mol/L sodium carbonate
solution is 11.66. What is the Kb for the
carbonate ion?
The algorithm for solving this is as follows:
The solution!
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1. Write a balanced equation.
2. Given the pH, calculate the pOH.
3. Calculate the [OH-]
4. Solve for Kb
5. Read about calculation error on pg. 510
Ka and Kb are related
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Well, that’s not surprizing!
To review……….
pH + pOH = 14
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[H3O+][OH-] = 1.00 x 10-14 = Kw
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Ka x Kb = Kw
This applies to conjugate acid-base pairs.
See page 511 for examples.
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