Chemical Equilibrium - Valhalla High School

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CHEMICAL EQUILIBRIUM
A dynamic process.
Chemical Equilibrium
When compounds react they eventually form a mixture of
products and un-reacted reactants in a dynamic equilibrium.


Dynamic Equilibrium – consists of a forward reaction, in which
substances react to give products; and a reverse reaction in which
products react to give the original reactants.
Chemical Equilibrium – is the state reached by a
reaction mixture when the rates of the forward
and reverse reactions have become equal.
Chemical Equilibrium
Equilibrium is a state where there are no observable changes.
Chemical Equilibrium is achieved when:
 The rates of the forward and reverse reactions are equal.
(Opposing Reactions)


These opposing reactions occur simultaneously and the rate at which
product is formed during the forward reaction is equal to the rate at
which reactants are re-formed during the reverse reaction.
The concentrations of the reactants and products remain
constant.

The quantities of reactants and products do NOT have to be equal but,
they must remain constant to have equilibrium.
Chemical Equilibrium

Chemical Equilibrium occurs in a closed system
(nothing can get in or out), therefore the reactions
are continuous.
o
Double arrow = equilibrium
Types of Equilibrium
Physical Equilibrium – equilibrium of physical
processes such as phases of matter or dissolving.
1.
a.
Phase Equilibrium - equilibrium between two phases in
a closed system.
o
Evaporation and condensation occur at equal rates
The rates of forward and reverse reactions are equal. As 0.50
moles of water are evaporated into the space above the water 0.50
moles of gaseous water are condensed back to liquid.
The concentration of the reactants and products remain constant.
Because the rates of the forward and reverse reaction are equal there
is no net change to the amount of reactants or products produced.
The may certainly be more of one than the other but those amounts do
not change.
1. Physical Equilibrium
a. Phase equilibrium
o
Melting and freezing occur at the same rate.
There may not be the same amounts of solid
and liquid present, but the rate of melting will
be equal to the rate of freezing
1. Physical Equilibrium
b. Solution equilibrium
b.
Solution Equilibrium (two types)
I.
gases in Liquids – equilibrium exists between the gas in the
liquid and the gas above the liquid.
The rates of forward and reverse reactions are equal. As 0.50
moles of CO2 gas comes out of solution into the space above the
soda, 0.50 moles of it are dissolved back into solution.
The concentration of the reactants and products remain
constant. Because the rates of the forward and reverse reaction
are equal there is no net change to the amount of reactants or
products produced. There is certainly more gas dissolved into the
liquid than in the space above it, but at equilibrium the amounts in
each area remain constant.
1. Physical Equilibrium
b. Solution equilibrium
I.
Gases in Liquids – equilbrium exists between the gas in the liquid
and the gas above the liquid.
Factors that affect the equilibrium of gases in liquids


Increasing Temperature – decreases the solubility of gas in the liquid (warm
soda has less bubble than cold soda).
Increasing Pressure – increases the solubility of gas in the liquid (bottling
soda).
1. Physical Equilibrium
b. Solution equilibrium
II.
solids in liquids – equilibrium exists between dissolving and
re-crystallizing of a solute in a liquid.

Solids and liquids exist in equilibrium in a saturated solution.
Factors that affect the equilibrium of solids in liquids

Increasing/Decreasing Temperature - increases/decreases the solubility of
solids in liquids (more sugar will dissolve in hot tea than iced tea).
Types of Equilibrium
2.
Chemical Equilibrium – Forward and Reverse
chemical reactions occur at the same time and
rate.
Upon Initially mixing the reactants
(H2 and I2) no products are
present. As time goes on and
more and more products are
formed and we start to see an
increase in the rate or the reverse
(backward) reaction.
Equilibrium is reached when
the rates of the forward and
reverse reactions are equal.
As a certain amount of HI is
produced in the forward
reaction that same amount of
H2 and I2 is produced in the
reverse reaction.
Reactions that go to Completion

Will not Reverse
 1)
Closed System - if products are removed as they are
made. The equilibrium is upset and the reaction will NOT
reverse.
 2)
Open System – reactions not taking place in a closed
system.
a)
b)
c)
Precipitate formed; Pb(NO3)2(l)+ 2KI(l) 2KNO3(l) + PbI2(s)
Gas formed; FeS(s) + 2HCl(aq)  FeCl2(aq) + H2S(g)
Un-ionized product is formed;
HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l)
Questions
1.
2.
3.
Which factors must be equal when a reversible chemical process reaches equilibrium.
a.
mass of products and mass of reactants.
b.
rate of forward reaction and rate of reverse reaction.
c.
concentration of product and concentration of reactants.
d.
activation energy of the forward reaction and activation energy of the reverse reaction.
A solute is added to water and a portion of the solute remains un-dissolved. When equilibrium between the
dissolved and un-dissolved solute is reached, the solution must be
a.
dilute
b.
saturated
c.
unsaturated
d.
supersaturated
Which description applies to a system in a sealed flask is half full of water?
a.
only evaporation occurs, but it eventually stops.
b.
only condensation occurs but it eventually stops.
c.
neither evaporation nor condensation occurs.
d.
both evaporation and condensation occur.
Questions
1.
2.
3.
Which factors must be equal when a reversible chemical process reaches equilibrium.
a.
mass of products and mass of reactants.
b.
rate of forward reaction and rate of reverse reaction.
c.
concentration of product and concentration of reactants.
d.
activation energy of the forward reaction and activation energy of the reverse reaction.
A solute is added to water and a portion of the solute remains un-dissolved. When equilibrium between the
dissolved and un-dissolved solute is reached, the solution must be
a.
dilute
b.
saturated
c.
unsaturated
d.
supersaturated
Which description applies to a system in a sealed flask is half full of water?
a.
only evaporation occurs, but it eventually stops.
b.
only condensation occurs but it eventually stops.
c.
neither evaporation nor condensation occurs.
d.
both evaporation and condensation occur.
Questions
4.
5.
6.
Solution equilibrium always exists in a solution that is.
a.
unsaturated.
b.
saturated.
c.
dilute.
d.
concentrated.
Given a reaction at equilibrium. The addition of a catalyst will
a.
shift the equilibrium to the right
b.
shift the equilibrium to the left
c.
increase the rate of forward and reverse reactions equally
d.
have no effect on the forward or reverse reactions
If a catalyst is added to a system at equilibrium and the temperature and pressure remain constant, there will be
no effect on the
a.
rate of the forward reaction.
b.
rate of the reverse reaction.
c.
activation energy of the reaction.
d.
heat of reaction (∆H).
Questions
4.
5.
6.
Solution equilibrium always exists in a solution that is.
a.
unsaturated.
b.
saturated.
c.
dilute.
d.
concentrated.
Given a reaction at equilibrium. The addition of a catalyst will
a.
shift the equilibrium to the right
b.
shift the equilibrium to the left
c.
increase the rate of forward and reverse reactions equally
d.
have no effect on the forward or reverse reactions
If a catalyst is added to a system at equilibrium and the temperature and pressure remain constant, there will be
no effect on the
a.
rate of the forward reaction.
b.
rate of the reverse reaction.
c.
activation energy of the reaction.
d.
heat of reaction (∆H).
Questions
7.
8.
9.
In a reversible reaction, chemical equilibrium is attained when the
a.
rate of the forward reaction is greater than the rare of the reverse reaction.
b.
rate of the reverse reaction is greater than the rate of the forward reaction.
c.
concentration of the reactants reaches zero.
d.
concentration of the products remains constant.
The addition of a catalyst to a system at equilibrium will increase the rate of
a.
the forward reaction only.
b.
the reverse reaction only.
c.
both the forward and reverse reactions.
d.
neither the forward nor the reverse reaction.
A system is said to be in a state of dynamic equilibrium when the
a.
concentration of the products is greater than the concentration of the reactants.
b.
concentration of the products is less than the concentration of the reactants.
c.
rate at which the product are formed is greater than the rate at which reactants are formed.
d.
rate at which the products are formed is the same as the rate at which reactants are formed.
Questions
7.
8.
9.
In a reversible reaction, chemical equilibrium is attained when the
a.
rate of the forward reaction is greater than the rare of the reverse reaction.
b.
rate of the reverse reaction is greater than the rate of the forward reaction.
c.
concentration of the reactants reaches zero.
d.
concentration of the products remains constant.
The addition of a catalyst to a system at equilibrium will increase the rate of
a.
the forward reaction only.
b.
the reverse reaction only.
c.
both the forward and reverse reactions.
d.
neither the forward nor the reverse reaction.
A system is said to be in a state of dynamic equilibrium when the
a.
concentration of the products is greater than the concentration of the reactants.
b.
concentration of the products is less than the concentration of the reactants.
c.
rate at which the product are formed is greater than the rate at which reactants are formed.
d.
rate at which the products are formed is the same as the rate at which reactants are formed.
Le Chatelier’s Principle



Any change in concentration, temperature, or
pressure on an equilibrium system is called a stress.
Le Chatelier’s Principle explains how a system at
equilibrium responds to a stress on the system.
The equilibrium will shift in a way that tehnds to
counteract the change.
Concentration Changes

An increase in concentration of any substance shifts the
reaction away from the location of the increased
substance.
 Increasing the concentration of a reactant will shift the reaction
to the right producing more products.
 Increasing the concentration of a product will shift the reaction
to the left producing more reactants.

An decrease in concentration of any substance shifts the
reaction toward the location of the decreased substance.
 Decreasing the concentration of a reactant will shift the reaction
to the left producing more reactants.
 Decreasing the concentration of a product will shift the reaction
to the right producing more product.
Concentration Changes - Example
Stress: More NH3
Stress: Less O2
Stress: Less NO
Stress: More H2O
Common Ion Effect


Adding a substance with an ion that is common to
the reaction.
Causes a shift in equilibrium because it favors the
formation of a precipitate.
What Happens if NaCl(s) is added?
What happens if KBr(s) is added?
Temperature Changes


An increase or decrease in temperature changes
the amount of energy into/out of the system.
We can consider heat as a reactant or product.
An increase in temperature shifts the reaction away
from heat.
 A decrease in temperature shifts the reaction towards
heat.

Temperature Changes - Example
Stress: More Heat (increased temperature)
Stress: Less Heat (decreased temperature)
Pressure Changes


Pressure does not affect the rate of reaction of
solids or liquids.
Only gases are affected!!
Pressure Changes

Pressure Increases: shift to the side with the least
moles of gas.


System shifts to the right forming more NH3(g)
Pressure Decreases: shift to the side with the most
moles of gas.

System shifts to the left forming more N2(g) and H2(g)
Pressure Changes


Both sides have the same number of moles of gas.
In this case, pressure changes have NO EFFECT on
the system.
Catalysts

Favor the forward and reverse reactions equally.

Does NOT shift equilibrium.
Equilibrium Diagrams

The graph below represents the following chemical equilibrium
before and after a stress has been imposed upon the system.
Stress: increase H2
Result: equilibrium shifts away from the increase
get decreased N2
get increased NH3
Practice Problem
o
What effect do the following changes have on the position of
equilibrium for this reversible reaction? How must the reaction
shift to relieve the “stress caused by change”?
o
o
o
o
addition of H2
decrease in pressure
adding heat
removal of HI as it is formed
Questions
1.
Consider the equation for the following reaction at
equilibrium;
The concentration of product could be increased by
a.
b.
c.
d.
adding a catalyst
adding more heat to the system
increasing the concentration of Y
decreasing the concentration of X
Questions
1.
Consider the equation for the following reaction at
equilibrium;
The concentration of product could be increased by
a.
b.
c.
d.
adding a catalyst
adding more heat to the system
increasing the concentration of Y
decreasing the concentration of X
Questions
2.
Consider the following equation
What stress would cause the equilibrium to shift to the left?
a.
b.
c.
d.
increasing the temperature
increasing the pressure
adding N2(g) to the system
adding H2(g) to the system
Questions
2.
Consider the following equation
What stress would cause the equilibrium to shift to the left?
a.
b.
c.
d.
increasing the temperature
increasing the pressure
adding N2(g) to the system
adding H2(g) to the system
Questions
3.
Consider the equation for the following reaction at
equilibrium.
When Na2SO4 is added to the system, how will equilibrium shift?
a.
The amount of CaSO4 will decrease, and the concentration of
Ca2+(aq) will decrease.
b.
The amount of CaSO4 will decrease, and the concentration of
Ca2+(aq) will increase.
c.
The amount of CaSO4 will increase, and the concentration of
Ca2+(aq) will decrease.
d.
The amount of CaSO4 will increase, and the concentration of
Ca2+(aq) will icrease.
Questions
3.
Consider the equation for the following reaction at
equilibrium.
When Na2SO4 is added to the system, how will equilibrium shift?
a.
The amount of CaSO4 will decrease, and the concentration of
Ca2+(aq) will decrease.
b.
The amount of CaSO4 will decrease, and the concentration of
Ca2+(aq) will increase.
c.
The amount of CaSO4 will increase, and the concentration of
Ca2+(aq) will decrease.
d.
The amount of CaSO4 will increase, and the concentration of
Ca2+(aq) will icrease.
Questions
4.
Consider the following equation
As the concentration of HCl(aq) decreases at constant
temperature, the rate of the forward reaction
a)
b)
c)
decreases
increases
remains the same
Questions
4.
Consider the following equation
As the concentration of HCl(aq) decreases at constant
temperature, the rate of the forward reaction
a)
b)
c)
decreases
increases
remains the same
Questions
5.
Consider the following equation
Which change will shift the equilibrium to the right?
a.
b.
c.
d.
decreasing the concentration of SO2
decreasing the pressure
increasing the concentration of O2
increasing the temperature
Questions
5.
Consider the following equation
Which change will shift the equilibrium to the right?
a.
b.
c.
d.
decreasing the concentration of SO2
decreasing the pressure
increasing the concentration of O2
increasing the temperature
Practice Problems
What effect do the following changes have on the position of
equilibrium for this reversible reaction? How must the
reaction shift to relieve stress?
1.
c.
addition of Cl2(g)
increase in pressure
removal of heat
d.
removal of PCl3 as it is formed
a.
b.
Practice Problem
How is the equilibrium position of this reaction affected by
the following changes?
2.
a.
b.
c.
d.
e.
f.
lowering temperature
increasing pressure
removing hydrogen from the equilibrium mixture
adding water to the equilibrium mixture
adding a catalyst
raising the temperature and decreasing the pressure
Practice Problems
The industrial production of ammonia is described by this
reversible reaction.
3.
What effect do the following changes have on the
equilibrium position?
a.
b.
c.
d.
e.
addition of heat
increase in pressure
addition of a catalyst
removal of heat
removal of NH3
Review of Le Chatelier’s Principle
Review of Le Chatelier’s Principle
Review of Le Chatelier’s Principle

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