Rates & Equilibrium A2 OCR Chemistry A Revision Notes

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Rates & Equilibrium
The Rate Equation
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
Reactions Involving Different Molar Quantities
The Equilibrium Constant Kc
The Units of Kc
The Effect of Changing Conditions on Kc
The Effect of Changing Concentration on Kc
The Effect of Changing Pressure on Kc
The Effect of A Catalyst on Kc
The Effect of Changing Temperature on Kc
Calculations Involving Kc
Kc From Equilibrium Constants
Kc From Initial Amounts
The Composition of a Reaction Mixture
Calculating the Amount of a Reactant Needed

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The Rate Equation
The rate equation shows the relationship between the rate of a reaction and the initial
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 of reaction with respect to A. This is determined by
b is the order of reaction with respect to B. This is determined by
experimentation.…read more

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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
In first order reactions the
halflife remains constant
First Order i.e. it is the same amount of
For second order
reactions, the halflife
Second Order
increases, i.e. it decreases
with every halflife taken.
This is a first order reaction as the measured halflife is constant.…read more

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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:
Br2 + HCOOH 2HBr + CO2
Time/s 0 30 60 90 120 180 240 360 480 600
0.01 0.009 0.0081 0.0073 0.0066 0.0053 0.0044 0.0028 0.0020 0.…read more

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Shapes of Graphs
The concentration of A
Zero Order does not affect the rate of
The rate is proportional to
a change in the
First Order
Rate is proportional to a
Second Order Drawn anther way: change in the concentration
Determining the Order Using The Initial Rate Method
Respect to A:
Start with known concentrations of A and B.…read more

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Measure the rate of reaction by timing how long it takes for a measurable change to
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 0.72
As can be seen, when the [A] doubles, the rate doubles, therefore a change in the rate is
proportional to the change in concentration. Thus in the rate equation A is first order.…read more

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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 this gives: Rearranged gives:
k = 0.18 k = moldm3s1
(9x0.5) [moldm3][ moldm3]2
Thus k = 0.…read more

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This is best illustrated using a MaxwellBoltzmann distribution.
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 of k with temperature is exponential.
The Rate Determining Step
Most reactions have what is called a reaction mechanism.…read more

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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.…read more

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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
Equilibrium can only be reached in a closed
The Equilibrium Constant Kc
Consider this reaction:
N2 + 3H2 2NH3
In this case Kc = [NH3]2
[N2], [H2] & [NH3] are the concentration of reactants and products at equilibrium.…read more



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