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
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What affects reaction rates
Temperature
•
•
•
kinetic energy
number of collisions
energy of colliding molecules
Concentrations of Reactants
•
•
number of reacting molecules
number of collisions
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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
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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)
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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)
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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
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Equilibrium Constants: Large Kc
products
Kc 
reactants
Large Kc value:
– Reaction favors products
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Equilibrium Constants: Small Kc
products
Kc 
reactants
Small Kc value:
– Reaction favors reactants
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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
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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)
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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
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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?
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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
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Effect of Volume Changes
With volume changes and concentration changes,
Equilibrium shifts to relieve new stress
…in order to return to equilibrium (same Kc)
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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
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