Rates of Reaction
A chemical reaction occurs when:
• Collisions between molecules have sufficient energy to break
the bonds in the reactants.
• Molecules collide with the proper orientation.
• Bonds between atoms of the reactants are broken, and new
bonds form.
Rates of Reaction
Activation Energy
• the minimum energy needed for a reaction to take place is
called the Activation Energy
Reaction Rates
Different reactions take different amounts of time to
proceed
Reaction rates are measured by the time needed:
• to make a certain amount of product
• to use up a certain amount of reactant
change in concentrat ion
rate of reaction 
change in time
4
What affects reaction rates
Temperature
•
•
•
kinetic energy
number of collisions
energy of colliding molecules
Concentrations of Reactants
•
•
number of reacting molecules
number of collisions
5
What affects reaction rates
Reaction rate:
A and B are reactants, C is the product. x and y are determined
experimentally
k is the reaction rate constant:
(Arrhenius equation)
A is a contant, Ea is the activation energy, R is the gas constant, T is
temperature
Concentration of Products and Temperature affects reaction rate
What about Activation energy?
Catalysts
Catalysts
•
•
•
•
Lower activation
energy
Provide alternate
pathway
NOT used up in
reaction
Ex: Enzymes
Catalysts
Chemical Equilibrium
Many reactions can go forward AND backwards
N2 (g)  O2 (g)  2NO (g)
If the opposite reaction can occur,
2NO (g)  N2 (g)  O2 (g)
then this reaction is a reversible reaction
At chemical equilibrium, the rate of the forward
reaction equals the rate of the reverse reaction
9
Chemical Equilibrium
• Reactions continue at equal (but opposite rates)
• No further changes in concentrations of reactants or products occurs
Equilibrium Constants
coefficients
aA  bB  cC  dD
reactants
products
c
d
products

C D

Kc 
a
b
reactants
A  B
2SO2 (g)  O2 (g)
2SO3 (g)
11
Heterogeneous Equilibrium
• Homogeneous equilibrium: all reactants and
products in same state (e.g. gas)
• Heterogeneous equilibrium:
• two or more states are present
• concentrations of liquids and solids are constant
• Liquids and solid not included in equilibrium
expressions
Mg (s)  O2 (g)
MgO (s)
12
Calculating Equilibrium Constants
Write the equilibrium equation for the reaction:

NO2 
products

Kc 
1
reactants
N 2O4 
2
N2O4 (g)
2 NO2 (g)
Must be determined experimentally
At 100 °C, once this reaction reaches equilibrium, the
concentration of N2O4 is 0.45 M and NO2 is 0.31 M.
What is the value of Kc?
NO2   0.31
1
1
N 2O 4  0.45
2
Kc 
2
 0.21
13
Equilibrium Constants: Large Kc
products
Kc 
reactants
Large Kc value:
– Reaction favors products
14
Equilibrium Constants: Small Kc
products
Kc 
reactants
Small Kc value:
– Reaction favors reactants
15
Equilibrium Constants
Will each of the following favor reactants or products?
• Reaction with Kc = 2.9 x 10-12
• Reaction with Kc = 0.001 x 105
16
Calculating Concentrations at Equilibrium
1.
2.
3.
4.
Balance Equation
Write out Equilibrium Expression
Rearrange expression so unknown is alone on one side
Plug in concentrations and/or Kc
What is the concentration of methane gas (CH4), a product
(along with water vapor) in the equilibrium reaction
between carbon monoxide and hydrogen gas?
At equilibrium at 1000 °C, the concentrations of hydrogen,
carbon monoxide, and water are 0.30 M, 0.51 M, and 2.0 M,
respectively. (Kc at 1000 °C = 260)
17
Le Châtelier’s Principle
• At equilibrium, concentrations of substances
do not change.
• If a stress is put on the reaction at equilibrium,
the equilibrium will shift to relieve the stress.
What changes count as stress?
• Concentration
– Adding or removing reactant or product
• Volume (Pressure)
• Temperature
18
Effect of Concentration Changes
2NO2 (g)
2NO (g) + O2 (g)
What is the effect on the concentration of each substance?
• Add NO2?
• Add NO?
• Add O2?
• Remove NO2?
• Remove NO?
• Remove O2?
19
Effect of Volume Changes
• Each mole of gas exerts a certain pressure
• Decrease the volume…
– Increase the pressure…
– Shifts to side of reaction with FEWER moles of gas
2NO2 (g)
2NO (g) + O2 (g)
• What happens if we increase the volume?
• Shifts to side with MORE moles
• What happens if we increase the pressure?
• Shifts to side with FEWER moles
20
Effect of Volume Changes
With volume changes and concentration changes,
Equilibrium shifts to relieve new stress
…in order to return to equilibrium (same Kc)
21
Effect of Temperature Changes
• If T is increased, equilibrium shifts to remove the heat
• If T is decreases, equilibrium shifts to create heat
• Kc changes!
– Unlike for concentration and volume changes
Endothermic:
Heat + A (g)
B (g) + C (g)
Increase the temperature?
• Shifts to the products (to use up heat)
• Kc increases
22