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Page 1

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Rates & Equilibrium
The Rate Equation
Definitions
Order of Reaction
Concentration-Time Graphs
Shapes of Graphs
Determining the Order Using Concentration-Time Graphs
Rate-Concentration Graphs
Shapes of Graphs
Determining the Order Using The Initial Rate Method
The Rate Constant
Calculation and Units
The Effect of Temperature on k
The Rate Determining Step…

Page 2

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The Rate Equation
The rate equation shows the relationship between the rate of a reaction and the initial
concentrations.




Rate is the chance in concentration per unit time.
[A] and [B] are the initial concentrations of the reactants. E.g. A could be
HCl and B NaOH.
a is the order…

Page 3

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ConcentrationTime Graphs
Shapes of Graphs

When plotting concentration time graphs, different orders of reaction cause the graphs to
look different:


Concentration decreases at
a constant rate.
Zero Order
Halflife decreases with
time.



In first order reactions the
halflife remains constant
First Order i.e. it is the same amount of
time.…

Page 4

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Start with known concentrations of A and B. Have one reactant (e.g. B) in excess
so its concentration stays constant.
Determine the concentration of A at different times in the reaction.
Plot the concentrationtime graph and match the shape with one of the above.

For example:
H+
Br2 + HCOOH…

Page 5

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Shapes of Graphs




The concentration of A
Zero Order does not affect the rate of
reaction.




The rate is proportional to
a change in the
First Order
concentration.




Rate is proportional to a
Second Order Drawn anther way: change in the concentration
squared.




Determining the Order Using The Initial Rate…

Page 6

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Measure the rate of reaction by timing how long it takes for a measurable change to
occur.
Repeat the experiment changing the initial concentration of A but keeping the initial
concentration of B constant.

Experiment [A]/moldm=3 [B]/moldm=3 Initial Rate/moldm3s1
1 0.5 3.0 0.18
2 1.0 3.0 0.36
3 2.0 3.0…

Page 7

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Calculation and Units

Using the values from the previous page we know:

Experiment [A]/moldm=3 [B]/moldm=3 Initial Rate/moldm3s1
1 0.5 3.0 0.18
2 1.0 3.0 0.36
3 2.0 3.0 0.72

Rate = k[A][B]2


Hence we get the equation: To calculate units we know:

0.18 = k(0.5)(3.0)2 moldm3s1 = k[moldm3][ moldm3]2

Rearranged…

Page 8

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This is best illustrated using a MaxwellBoltzmann distribution.



At




higher temperatures, more molecules will have energy greater than or equal to the activation
energy, which results in more successful collisions.

If the concentrations of reactants are kept constant as temperature increases, k must
increase as the rate increases. The increase…

Page 9

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reaction this is known as the ratedetermining step. The reactants that are involved in the
ratedetermining step are present in the rate equation.

Any step that occurs after the ratedetermining step will not affect the rate of the reaction
hence the reactants involved will not appear in the rate equation.…

Page 10

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When a system is in dynamic equilibrium it has three
main features:
The rate of the forward reaction equals the rate
of the backward reaction.
At equilibrium there is no net change in the
concentrations.
Equilibrium can only be reached in a closed
system.

The Equilibrium Constant Kc

Consider this…

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