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Topic 08 introduction
1. IB Chemistry Power Points
Topic 08
Acids and Bases
www.pedagogics.ca
Introduction to
Acids and Bases
2. In aqueous solutions, a proportion of the water
molecules dissociate;
The ions formed are H+ or positively charged
hydrogen ions and negatively charged hydroxide ions
(OH-)
Technically
+ -
2 H2O(l) H3O (aq) + OH (aq)
+ − -14
Equilibrium Constant Kw = [H ][OH ] = 1 x 10
3. Some chemical compounds contribute additional H+
to make the solution more acidic. Other compounds
remove H+ ions.
A compound that increases [H+] is called an acid
Examples: HCl, H2SO4, HNO3, CH3COOH
4. A compound that removes H+ ions from an aqueous
solution is called a base.
Often this is done by adding OH- ions for example
NaOH, KOH, Ca(OH)2. Soluble bases are called
alkalis.
5. Types of Neutralization Reactions
With hydroxides
acid + base water + a salt
HCl + NaOH H2O + NaCl (aq)
• With metal oxides
acid + base water + a salt
2 HCl + Cu2O H2O + CuCl2 (aq)
• With ammonia
acid + base a salt
HCl + NH3 NH4Cl (aq)
7. Three theories of acids
Arrhenius (most common)
Bronsted-Lowry
Lewis
8. Arrhenius (most common):
+
an acid dissociates to yield H
-
and a base dissociates to yield OH
Hydrochloric acid + -
H + Cl
Sodium hydroxide + -
Na + OH
9. Bronsted-Lowry
+
an acid is a proton (H ) donor
and a base is a proton acceptor
11. Lewis:
An acid is an electron pair acceptor
Lewis
Acid Lewis
Base
and a base is an electron pair donor
A dative covalent bond is formed
12. This is a common example that is not an obvious acid/base rxn
Boron trifluoride acts as a Lewis Acid.
The boron has only 6 electron in valence shell so the lone
pair of electrons forms a dative bond and fills up the
valence shell of the boron
13. Indicators
Acids and bases are substances with
specific physical and chemical
properties.
We can determine if substances are
acidic or basic by testing their reaction
with indicators.
14. Indicators are organic substances that
change color in the presence of an acid or
a base.
Some common indicators
in acid in base
Litmus red blue
Phenolphthalein colorless pink
Methyl orange red yellow
15.
16. Reactions of acids – examples to know
React with active metals (above copper in reactivity
series)
2 HCl + Ca CaCl2 + H2
Reaction with carbonates
H2SO4 + Na2CO3 Na2SO4 + CO2 + H2O
Reaction with bicarbonates
HNO3 + NaHCO3 NaNO3 + CO2 + H2O
17. Acid/base properties of Period 3 oxides (topic 3)
Metal oxides Na2O and MgO react with water to form
hydroxides (basic solutions)
Na2O + H2O 2 NaOH (aq)
Aluminum oxide is amphoteric (will react as a base
with an acid or vice versa)
Al2O3 + 6 HCl 2 AlCl3 + 3 H2O
Other period 3 oxides (non-metal S, P, Cl oxides) react
with water to form acidic solutions
SO3 + H2O H2SO4 (aq)
see page 15 in study guide
18. Acid/base properties of Period 3 chlorides (topic 13)
Chlorides across Period 3 become more acidic across
the period
NaCl (aq) is neutral
MgCl2 (aq) is weakly acidic
Chlorides of Al, Si, P, S and Cl2 react with
water to produce HCl (aq) solutions
see Study guide page 16
19. Strong Acids vs Weak Acids
The strength of an acid or base depends on how
easily it dissociates in water.
The dissociation of an acid or base is an equilibrium.
HA(aq) H+(aq) + A-(aq)
BOH(aq) B+(aq) + OH-(aq)
Strong acids or bases dissociate (ionize) easily –
the equilibrium favors the ionic products : kc >> 1
20. Strong vs Weak
How to tell
Strong acids and bases are mostly ionized and therefore
solutions are good electrolytes (high conductivity). The pH of
the solution can also be measured.
21. Strong vs Weak
When the strength of an acid or base is discussed, it is
very important NOT to confuse “strength” with
“concentration”
3
A 5M acid solution contains 5 mol of acid per dm but its
strength is determined by how much of that acid is
ionized.
Strong acids : HCl, H2SO4, HNO3 (mono vs diprotic)
Strong bases : NaOH, KOH, Ba(OH) 2
Weak acids: CH3COOH, H2CO3, carbonic acid CO2(aq)
Weak bases: NH3, ethylamine CH3CH2NH2
23. Weak Acid
example CH3COOH
+ -
CH3COOH (aq) H (aq) + CH3COO (aq)
[H+ ][CH3COO- ]
k= << 1
[CH3COOH]
• partially dissociated
• pH of 0.1 M soln = 2.9
• weak electrolyte
• reacts slowly
24. What is the pH scale?
pH is a measurement of hydrogen ion concentration
It tells you how acidic or basic (or alkaline) something is
Ranges from 0 (most acidic) to 14 (most basic)
pH log[ H ]
25. How does scale work?
The scale is logarithmic. As you go up
or down, the concentration is changed
by a power of ten
Example pH 3 is 100 times more
concentrated than pH 5
neutral
pH 10 is 100 times less concentrated
than pH 8