Pages in this set

Page 1

Preview of page 1
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…

Page 2

Preview of page 2
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…

Page 3

Preview of page 3
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

Page 4

Preview of page 4
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…

Page 5

Preview of page 5
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…

Page 6

Preview of page 6
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…

Page 7

Preview of page 7
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


Page 8

Preview of page 8
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…

Page 9

Preview of page 9
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

Preview of page 10
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…




Good notes



very useful

Similar Chemistry resources:

See all Chemistry resources »