# Equilibrium Concept Review

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```					Chemical Systems and
Equilibrium
Learning Goals:
• Explain the concept of dynamic equilibrium,
using examples of physical and chemical
equilibrium systems.
• Explain the concept of chemical equilibrium
and how it applies to the concentration of
reactants and products in a chemical reaction
at equilibrium.
• Create and complete an ICE table for an
equilibrium system.
• Draw graphs of c vs. t to illustrate a chemical
system approaching equilibrium.
• Use appropriate terminology such as dynamic
equilibrium, reversible reaction, closed system,
equilibrium concentrations, phase equilibrium,
solubility equilibrium, chemical equilibrium,
equilibrium position
Dynamic Equilibrium
• rate of forward reaction = rate of reverse reaction
• an observable macroscopic property remains constant
summer haze

How do the following
images represent
dynamic equilibrium?

Dynamic Equilibrium?
•two kids balanced on a teeter-totter
•player substitutions during a hockey game
•outdoor pool
•indoor pool
Consider the following…

Over time, what happens
to an open beaker of
water?
Over time, what happens
to the water level if we
cover the beaker?

We can study equilibrium
under closed systems!
Write an equation to
describe the equilibrium
What conditions are required for a dynamic
equilibrium to exist?
• reaction is reversible
• closed system

What are the key characteristics of a dynamic
equilibrium?

• forward rate = reverse rate
• observable macroscopic property remains constant
3 types of Equilibrium
1. Solubility Equilibrium
2. Phase Equilibrium
3. Chemical Equilibrium

NaCl(s)   Na+ (aq) + Cl- (aq)
Chemical Equilibrium

Write an equation to
describe this equilibrium
system.

Initially, N2O4(g) is in the flask. Draw a graph to
illustrate the changes in concentration of R and P as
the system approaches equilibrium.
Does slope = 0 mean the reaction has stopped?
Are the concentrations of R and P equal at equilibrium?
Draw graphs to illustrate how equilibrium can be
reached from either direction.

Use an ICE table to discover how concentration
changes as the system approaches equilibrium.
Create and complete an ICE table for the following
chemical system.

If 0.80 mol of HI are placed in a 0.50 L flask, predict how the
concentrations will change as the system approaches
equilibrium. Write algebraic expressions for the [ ] .
Sample Problem 1 (p. 425)

• The initial concentrations of H2(g) and F2(g) are
2.0 M. What are the equil. conc. of H2(g) and
HF(g) if the equil. conc. of F2(g) is 0.48 M?
Sample Problem 2 (p. 426)

• A chemist adds 4.0 mol of ammonia gas to a 2.0 L
sealed container and heats it. Fig 7 shows the
changes in the amount of NH3(g) observed over time.
Determine the equil. conc. of N2(g)and H2(g).
• Practice Problems – p. 427 #1-3
Equilibrium Position
• The relative concentrations of R and P at equilibrium
is called the equilibrium position.
• Think of running up the down escalator. Can you
reach a state of dynamic equilibrium at different
positions on the escalator?
• How does the equil. position relate to the solubility
of a solute?
• What does a brown haze over a city in the summer
indicate about the equil. position of this system?
N2O4(g)        2 NO2(g)
• What happens in the winter?
Equilibrium Position lies…   extent of   reaction is…   arrow
and favours …                reaction

very far to the right,       100%        complete       
products
to the right,                >50%        reversible
products
in the middle,               =50%        reversible
neither R or P
to the left,                 <50%        reversible
reactants
very far to the left,        <1%         no reaction     NR
reactants
Draw a bar graph to illustrate the relative
concentrations of R and P for each equilibrium position.
Dynamic Equilibrium

BEWARE! 3 Common Misconceptions:
1.Concentrations of reactants and products
are equal @ equilibrium – equilibrium
position?
2. Equilibrium can only be approached from
1 side
3. The forward and reverse reactions stop @
equilibrium… Macroscopically there are
no changes, but what about the molecular
level?
Homework:
• Define key terms
• p. 428 #1-6

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