Chem 1011 – Intersession 2011
Class #24
03-Jun-11
1
Class 24: Le Chatelier’s Principle /
Introduction to Acids and Bases
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Sec 14.9 – Le Chatelier’s Principle: How a
System at Equilibrium Responds to Disturbances
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The effect of a concentrations change on
equilibrium
The effect of a volume or pressure change
The effect of a temperature change
Sec 15.2 – The Nature of Acids and Bases
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Sec 15.3 – Definitions of Acids and Bases
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The Arrhenius Definition
The Brønsted-Lowry Definition
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LeChatelier’s Principle
• Le Chatelier's Principle states:
“When an equilibrium system is subjected to a
change in temperature, pressure or
concentration of a reacting species, the system
responds by attaining a new equilibrium that
partially offsets the impact of the change.”
3
The Effect of Concentration
• Increasing the concentration of a reacting species will
shift the equilibrium in the opposite direction.
2 SO2(g) + O2(g) ⇌ 2 SO3(g)
• Increasing the concentration of either SO2(g) or O2(g) will
favor the forward reaction
• Increasing the concentration of SO3(g) will favor the
reverse reaction
▫ Reaction shifts to remove any excess species added
• Decreasing the concentration of either SO2(g) or O2(g) will
favor the reverse reaction
• Decreasing the concentration of SO3(g) will favor the
forward reaction
▫ Reaction shifts to replace any species that are removed
LeChatelier’s Principle / Introduction to Acids
and Bases
1
Chem 1011 – Intersession 2011
Class #24
03-Jun-11
4
The Effect of Concentration
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The Effect of Pressure / Volume
1. Add or remove a gaseous reactant (or product)
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This has the same effect as increasing or decreasing the
pressure (increasing or decreasing concentration).
2. Add an inert (non-reactive) gas to the reaction
mixture, at constant volume
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This has the effect of increasing the total pressure of the
system. It will not change the equilibrium position.
3. Change the pressure by changing the volume of the
system.
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When the volume of an equilibrium mixture of gases is
reduced a net reaction occurs in the direction producing
the fewer moles of gases
When the volume is increased a net reaction occurs in the
direction producing more moles of gases.
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The Effect of Pressure / Volume
LeChatelier’s Principle / Introduction to Acids
and Bases
2
Chem 1011 – Intersession 2011
Class #24
03-Jun-11
7
Problem
• An equilibrium mixture of N2(g), H2(g), and
NH3(g) is transferred from a 1.50 L flask to a 5.00
L flask. In which direction does a net reaction
occur to restore equilibrium.
N2(g) + 3 H2(g) ⇌ 2 NH3(g)
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The Effect of Temperature
• Raising the temperature of an equilibrium mixture shifts
the equilibrium condition in the direction of the
endothermic reaction.
▫ Raising the temperature has the same effect as “increasing
the amount of heat,” if heat were to be considered like a
chemical species in the reaction
▫ This will favor the direction that absorbs (uses) the added
heat
• Lowering the temperature causes a shift in the direction
of the exothermic reaction.
▫ Lowering the temperature has the same effect as
“decreasing the amount of heat”
▫ This will favor the direction that releases (produces) heat –
restoring the heat that was removed
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The Effect of Temperature
LeChatelier’s Principle / Introduction to Acids
and Bases
3
Chem 1011 – Intersession 2011
Class #24
03-Jun-11
10
Problem
• Consider the reaction:
2 SO2(g) + O2(g) ⇌ 2 SO3(g)
DH = -197.8 kJ
Will the amount of SO3(g) formed from given
amounts of SO2(g) and O2(g) be greater at higher
or lower temperatures?
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The Effect of a Catalyst
• Catalysts provide an alternative, more efficient
mechanism.
• Works for both forward and reverse reactions.
• Affects the rate of the forward and reverse
reactions by the same factor.
• Therefore, catalysts do not affect the
position of equilibrium
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Chapter 15: Acids and Bases
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Acids:
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have a sour taste
dissolve many metals
turn blue litmus paper red
neutralize bases
LeChatelier’s Principle / Introduction to Acids
and Bases
4
Chem 1011 – Intersession 2011
Class #24
03-Jun-11
13
Common Acids
14
Bases
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Bases:
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have a bitter taste
have a slippery feel
turn red litmus paper blue
neutralize acids
15
Common Bases
LeChatelier’s Principle / Introduction to Acids
and Bases
5
Chem 1011 – Intersession 2011
Class #24
03-Jun-11
16
Acid / Base Theories
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There are three main theories that define acids
and bases:
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The Arrhenius Definition
The Brønsted-Lowry Definition
Lewis Definition

Today, we will look at the Arrhenius and
Brønsted-Lowry Definitions. Lewis will have to
wait…
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Arrhenius Definitions
• Arrhenius Acid
▫ A substance that produces
H+ ions in aqueous solution
HCl ionizes in water,
producing H+ and Cl– ions.
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Arrhenius Definition
• Hydronium Ion
▫ H+ ions are highly reactive, and do not exist in water.
Rather, they react with water to make complex ions.
That is, H3O+; the hydronium ion
H+ + H2O  H3O+
LeChatelier’s Principle / Introduction to Acids
and Bases
6
Chem 1011 – Intersession 2011
Class #24
03-Jun-11
19
Arrhenius Definition
• Arrhenius Base
▫ A substance that produces
OH– ions in aqueous solution
NaOH dissociates in water,
producing Na+ and OH– ions.
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Arrhenius Acid-Base Reactions
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The H+ from the acid combines with the OH− from
the base to make a molecule of H2O.
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The cation from the base combines with the anion
from the acid to make a salt.
acid + base → salt + water
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
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The Problem with Arrhenius...
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does not explain why molecular substances, like NH3,
dissolve in water to form basic solutions – even though they
do not contain OH– ions
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does not explain how some ionic compounds, like Na2CO3
or Na2O, dissolve in water to form basic solutions – even
though they do not contain OH– ions
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does not explain why molecular substances, like CO2,
dissolve in water to form acidic solutions – even though
they do not contain H+ ions
•
does not explain acid–base reactions that take place outside
aqueous solutions
LeChatelier’s Principle / Introduction to Acids
and Bases
7
Chem 1011 – Intersession 2011
Class #24
03-Jun-11
22
Brønsted-Lowry Definition
• Brønsted-Lowry Acid
▫ A species (atom, molecule or ion) that is capable of
donating a proton to another species
• Example:
HCl(aq) + H2O(l) → H3O+(aq) + Cl–(aq)
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HCl is a Brønsted-Lowry acid because it donates a
proton (H+) to a water molecule
H3O+ is the hydronium ion, and it is always formed
when an acid, such as HCl, ionizes in water
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Brønsted-Lowry Definition
• Brønsted-Lowry Base
▫ A species (atom, molecule or ion) that is capable of
accepting a proton from another species
• Example:
H3O+(aq) + NH3(aq) → NH4+(aq) + H2O(l)
▫ NH3 is a Brønsted-Lowry base because it accepts a
proton (H+)
▫ Any material that has atoms with lone pairs can
potentially be a Brønsted–Lowry base
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Week 5!
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Sec 15.3 – Definitions of Acids and Bases
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The Brønsted-Lowry Definition (continued)
Sec 15.4 – Acid Strength and the Acid Ionization
Constant (Ka)
Strong Acids
Weak Acids
The Acid Ionization Constant (Ka)
Sec 15.5 – Auto-ionization of Water
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The pH Scale: A Way to Quantify Acidity or Basicity
pOH and other p Scales
LeChatelier’s Principle / Introduction to Acids
and Bases
8