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Kinetics
The rate of a chemical reaction is the change in concentration of a substance in a unit time
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Rate α[A]m[B]n
Where [] refers to the concentration of a species in the SI units of mol dm -3
M and n values are determined experimentally and proportional constant can be replaced
by k, rate constant
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The overall order of a reaction is the sum of the powers of the concentration terms in the
rate equation
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The rates of reaction are usually specified as initial rates where the
concentration of all species is exactly known
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By repeating
the experiment with differential initial concentrations of all reactants, the initial rate changes
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Changes in Temperature
Increasing the temperature always increases the rate of reaction
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At higher temp;
 Particles have more energy
 They move more quickly
 There are more collisions
 The collisions are more energetic
The increased energy of collisions is the main factor which increases rate of reaction, not the
frequency of the reactions
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If raising the temperature causes the rate to increase,
the rate constant must increase with increasing temperature
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The Rate
constant, k, is effected by the number of effective collisions per
time unit, which are dependent on the temperature of the
reaction
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The rate reaction increases exponentially
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The slowest step is called the rate determine step or rate limiting step
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A + 2B >>>>G
Rate=k[A][B]2
A+B>>>>C
B+C>>>>D
D+D>>>>G
C and D are intermediate species, 2nd step is rate determine step as 1 mole of A and 2 moles of B
have been involved
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Equilibria
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Incomplete reactions are reversible
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Le Chatelier’s Principle is when an equilibrium mixture undergoes change the equilibrium will
shift so that the system will counteract the change, minimising the effect of any imposed change on
conditions
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If product removed, more product
produced
Pressure
 Only considered if it involves gases
 Increase in pressure displaces equilibrium to side which has fewest gaseous moles, as it trys
to decrease the pressure change
 If equal number of gaseous moles on each side then the pressure will have no effect
Temperature
 Change in temperature alters the rates of both forward and backward reactions but by
different amounts

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If forward reaction is endothermic, heat energy is absorbed so an increase in temp willmove
the equilibrium towards the forward reaction as the system will try to reduce the change,
increasing amount of product
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Catalyst
 Increases the rate of both forward and backward reactions equally so get to equilibrium
faster
 No change in composition of equilibrium mixture
In industry processes must be designed to give the optimum yields
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KC is the equilibrium constant
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If Kc is large then the equilibrium position of the
reaction lies to the right, the concentration of products is
high and the concentration of reactants is low
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To find Kc;
Find for each species in reaction
Initial amount (mol)
Eqn amounts (mol)
Reaction ratio
Amount gained (mol)
Amount lost (mol)
Check ratio
Eqn amount (mol)
Concentation at eqm if needed
The value of Kc is not changed by a change in concentration, or pressure of any species, nor by
addition of catalyst
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Change in concentration
 At fixed temperatures Kc is constant
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Change in Total Pressure
 At fixed temperature Kc remains constant
Change in Temperature
 If forward reaction is exothermic, then backward reaction will be endothermic
 This does effect Kc because rate of forward and backward reactions effected by different
amounts
 Exothermic
-Heat energy is involved
-Equilibrium moves left toward endothermic reaction so reduces product
-Less product but equilibrium reached quicker as increased rate of reaction

 Endothermic
-Heat energy absorbed
-Increase in temp moves the quilibrium to right to reduce the change upon the system
-More product produced, increased rate of reaction
Kc and Kp are always effected by temperature unless ΔH≡=0
Catalyst Effect
 No effect on composition of the equilibrium mixture as it does not affect value of Kc
 Catalyst speeds up reaction rates of forward and backward reaction equally
 Composition unchanged, but equilibrium met faster so don’t affect the yield
 Provide alternate path for the reaction which has lower energy of activation



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