Unit 3: Chemical Systems and
Equilibrium
Mr. Krstovic
SCH4U0
Focus Questions
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1) What is dynamic equilibrium?
2) What types of equilibria exist?
3) Which type of equilibria is most
complex?
4) What is percent reaction?
5) How do we solve equilibrium
problems?
Dynamic Equilibrium
Dynamic equilibrium is reached when the RATE
of the forward process equals the rate of
the reverse process!
Analogy:
Water jets in a swimming pool pump water in at
the same rate as the rate at which water is
removed. The level of water in the pool
remains the same (dynamic equilibrium is
reached)
How is dynamic equilibrium
symbolized?
We use a DOUBLE arrow to show that a
process is in equilibrium.
H2O(l)
H2(g) + Cl2(g)
H2O(g)
2HCl(g)
Types of equilibria
1)
2)
3)
Solubility equilibrium
Phase equilibrium
Chemical reaction equilibrium
Let’s look at each type closely…
Solubility Equilibrium
Solubility equilibrium is a dynamic
equilibrium between a solute and a
solvent in a saturated solution in a
closed system (no matter leaves).
Solubility Equilibrium
When a solute is first added, its
ions dissociate from the crystal.
As we keep adding more solute,
more ions come into solution.
However, at some point ions
start to collide with the solute’s
crystals and re-crystallization
occurs. At solubility
EQUILIBRIUM, solute ions
dissolve and crystallize at the
same RATE.
Phase Equilibrium
In a closed system, a phase
change may establish an
equilibrium. Thus, a
phase equilibrium is a
dynamic equilibrium
between two different
physical states of a pure
substance.
Let’s look at evaporation of
water
Explaining last slide…
As molecules in a liquid gain enough collision
energy, they leave the surface of the liquid
and enter gas phase. As the number of gas
molecules increases, more of the gas
molecules collide with the liquid surface and
lose enough energy to join the condensed
phase. In time, the rate of evaporation = the
rate of condensation.
H2O (l) H2O (g)
Chemical Reaction Equilibrium
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A dynamic equilibrium is reached
between reactants and products of
a chemical reaction in a closed
system.
Chem rxn equilb. Is more complex
than solubility or phase equilb.
because greater variety of
substances can be involved.
Notes from the previous
slide…
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Both 1-pentene and cyclopentane are gases
Rx: 1-pentene  cyclopentane
Initially we put 1-pentene in a closed container at
1.000 atm
Cyclopenate starts to be produced immediately.
However at certain time, the pressure-time curve
for both 1-pentene and cyclopentane will level off
Leveling indicates that the rate of the forward
reaction = rate of the reverse reaction (chemical
equilibrium is reached)
Percent Reaction at Chemical
Equilibrium
Percent reaction tells us the yield of the product
measured at equilibrium compared to the
maximum theoretical yield assuming the
reaction went to completion.
Percent Reaction:
actual product yield/ theoretical product yield x 100%
The real question….
The question is not whether reactions go in both
direction, BUT to what extent do they go one way or
the other.
 Three categories of reactions:
-Rxns that favor reactants (percent reaction is less than
1%)
-Rxns tha favor products (percent reaction is more than
99 %)
-Rxns that achieve noticeable equilibrium conditions
(percent reaction is between 1-99%). If %rxn is less
than 50% reactants are favored, if % rxn is more
than 50% products are favored.
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Calculating Concentrations at
Equilibrium