Transcript Chemical Kinetics and Equilibrium

Chemical Kinetics and
Equilibrium
Reaction Rates
• How fast or slow the reaction
occurs
Collision Theory
• 2 conditions must be satisfied for a
chemical reaction to occur
Particles of reactants must collide with
one another; and
Colliding particles must have sufficient
energy
Activation Energy
• Minimum amount of energy needed for a reaction to
occur
F> Ea
Ea
F < Ea
Ea
Energy Diagram for Exothermic
reaction
increasing
energy
Average energy
of reactants
Energy difference bet.
reactants and products
Average energy
of products
Energy Diagram for
Endothermic reaction
increasing
energy
energy of the products
energy of
reactants
energy difference
Factors Affecting Rate of reaction
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Surface Area
Concentration of reactants
Temperature
Presence of Catalyst
Surface area of reactants
• Consider the block of wood or wood
shavings
The smaller the particle size of wood the
greater the surface area exposed to
oxygen thus the rate of reaction increases.
Concentration of Reactants
• An increase in concentration of the
reactants causes an increase in the
rate of chemical reaction
Temperature
• Higher temperature increases reaction
rates
• Ex: ripe fruits are placed in the refrigerator
to slow down the ripening process.
Presence of Catalyst
• Catalyst
Substance that speeds up chemical
reaction without itself undergoing a
chemical change
Inhibitor
--->substances that slow down reaction
Activation energy of uncatalized
reaction
•
catalyzed activation energy
Uncatalyzed
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activation energy
Note: Catalysts increases
reaction rates by
providing alternative
reaction pathways
with lower activation
energies
Lower activation energy
• Means that a lower amount of energy is
needed to surpass the energy barrier.
Chemical Equibrium
• It is always assumed that chemical
reaction go to completion
• However, only few chemical reactions
proceed to completion
• Reversible reaction
N2O4(g)
2NO
Reversible Reaction
• A chemical reaction in which the products can
regenerate the original reactants
N2O4(g)
colorless
2NO2
brown
N2O4(g) -------> 2NO2(g) ( forward Reaction)
2NO2 (g)-------> N2O4(g) (backward Reaction)
Chemical Equilibrium
• The rate of forward reaction and backward
reactions are equal
• Is dynamic
 The reactants constantly forms the products
while the products constantly forms the
reactants
Note:
In equilibrium, concentration of reactants and
products do not change
Ex: dynamic Equilibrium
• Carbonated Drinks
• CO2 + H20 ---> H2CO3
carbonic acid
When the bottle is opened the pressure is released, carbon
dioxide evolved from the decomposition of carbonic acid
H2CO3 ------> CO2 + H2O
• CO2 + H20
H2CO3
Equilibrium Constants(K)
Can be obtained in any equilibrium reaction
Equal to the Ratio of the equilibrium
concentration of the products to the equilibrium
concentration of the reactants
aA + bB ----->cC + dD
Where:
a,b,c & d -- are the coefficients
A,B,C & D– are the chemical species
Equilibrium Constant(K)
aA + bB ----->cC + dD
• K = [C]c [D]d
[A]a [B]b
=
[ products ]
[reactants]
--> Experimentally at equilibrium, Concentration of the
products raised to a certain power divided the
concentration of the reactants also raised to a certain
power is constant at( fixed temperature)
Is also called equilibrium expression
Note:
• K = [ products]
[reactants]
• is the concentration of products over that of
the reactants
• If the K value is small, it means that the
equilibrium concentration of the products is
small while the reactants are large. This
indicates that at equilibrium, the system
consist mostly of reactants
• If the K value is great , it means that the
equilibrium system consist mostly of the
products
Writing equilibrium expression
• If a pure liquid or solid is involved in a
reaction, its concentration is omitted in the
equilibrium expression because it has
constant concentration in mol/L at constant
Temperature
Ex:
• Write the equilibrium expression for the
reaction below
• H2(g) + F2(g)
2HF(g)
• K = [ HF]2
•
[H2] [F2]
Ex:2
• Write the equilibrium expression for the
reaction below
• N2(g) + 3H2(g)
2NH3(g)
• K = [NH3]2
[N2][H2]3
Ex:3
• Write the equilibrium expression for the
reaction below
•
CaCO3(s)
CaO(s) +CO2(g)
• K = [ CO2]
Ex:4
• Write the equilibrium expression for the
reaction below
• 2Hg(l) + O2(g)
2HgO(s)
• K= 1
•
[O2]
since solid and liquid is omitted
Exercises
• Write the expression of the following
Reactions
a)
b)
c)
2H2S(g) + 3O2(g) 2H2O(g) + 2SO2(g)
CO2(g) + H2(g) ---> CO(g) + H2O(l)
FeO(s) + H2(g) --->
Le Chatelier’s Principle
• States that if a change in conditions is
imposed on a system at equilibrium, the
equilibrium position will shift in the
direction that tends to reduce the effects
of that change
-->The effect of change in concentration
 The effect of change in Volume
The effect of change in temperature
Effects of Change in Concentration on reaction
Equilibrium
• Consider the equation below
N2 + 3H2
2NH3(g)
This equation shows equilibrium because of the two arrows of the
same length that is rate of forward reaction is equal to the rate of
backward reaction.
What will happen if you increase the concentration of N2?
increasing the concentration of the reactants means more
collision between them that increases the rate of forward
reaction as shown below
N2 + 3H2
2NH3(g)
Notice that the arrow pointing to the right has a greater length
which indicates forward reaction increases meaning more
production of NH3
What happens if you increase the concentration of
NH3 in the previous example?
N2 + 3H2
2NH3(g)
Increasing the concentration of NH3 will shift the reaction to
the left.
Note:
If a reactant or product is added to the system, the
system shifts away from the added component
If a reactant or product is removed, the system shifts
toward the removed component
Exercise
•
Suppose the reaction system
2SO2(g) + O2(g)
2SO3 (g)
Has reached equilibrium. Predict the effect of each of
the following changes on the position of the
equilibrium
a) SO2(g) is added to the system
Ans. Shift to the right( forward)
b) The SO3(g) present is liquefied and removed from the
system
Ans. Shift to the right(forward)
b)Some of the O2 gas is removed from the system
Ans. Shift to the left ( backward)
The effect of Change in Volume and pressure
Recall :
Boyle’s law : Pressure is inversely proportional to Volume of gases
The smaller the volume, the higher the pressure between molecules
Avogadro’s Law : V is directly proportional to the # of moles at constant T
•
Consider the equation below: suppose the gases below are mixed in a vessel
at equilibrium
2 NOCl(g)
2NO(g) + Cl2(g)
What will happen to the equilibrium position if we reduce the volume?
 Reducing volume means increasing the pressure between the molecules of
the gases, the system moves in the direction that lowers its pressure. In the
equation above, the direction will shift to the left because the products has
more molecules (3 molecules) than the reactant (2molecules).
 In other words, the equilibrium position will shift toward the side of the
reaction that involves the smaller number of gaseous molecules in the
balanced equation
2 NOCl(g)
2NO(g) + Cl2(g)
What happens when volume is increased in the
previous example?
2 NOCl(g)
2NO(g) + Cl2(g)
• When the volume is increased, it lowers
the pressure of the system, thus the
direction will shift to the right to increase
the total number of gaseous molecules
present
Predict the shift in equilibrium of the reaction below
when volume is reduced
a)
P4(s) + 6Cl2(g)
6 gaseous molecules
4PCl3(l)
0 gaseous molecules
The direction will shift toward the right since it has 0 gaseous molecules
b)
PCl3(g) + Cl2(g)
PCl5(g)
2 gaseous molecules
1 gaseous molecule
The direction will shift to the right since it has lesser number of gaseous
molecules
c) PCl3(g) + 3NH3(g)
P(NH2)3(g) + 3HCl(g)
4 molecules
4 molecules
Since the # of gaseous molecules in both sides are equal, a change in
volume will have no effect on the equilibrium position
Practice:
•
a)
For each of the following reaction predict the direction in which the
equilibrium will shift when volume in the container is increased
4NH3(g) + 5O2(g)
4NO(g) + 6H2O(g)
b)
FeO(s) + H2(g)
Fe(s) + H2O(g)
The effect of change in Temperature
The change in concentration and volume in the system
alter the equilibrium position but not the equilibrium
constant (K)
The effect of temperature on equilibrium is different because
the value of equilibrium constant (K) is change
 If the reaction is exothermic that means it releases heat
the direction will shift to the left since heat is is one of the
products
A) C(s) + O2(g) <-------> CO2(g) + heat
direction will shift away from heat( to the left)
 If the reaction is endothermic, energy is one of the
reactants so the direction will shift to the right
Exercise:
• Predict the shift would each of the
changes below have on the reaction
O2(g) + 2CO(g)
2CO2(g) + heat
a)Increasing [CO]
b)Increasing [CO2]
c) Increasing Temperature
d) Decreasing volume