1) Reaction rates can be measured by monitoring the change in concentration of reactants or products over time. For the reaction A + 2B → C, the concentration of A would decrease over time while concentrations of B and C would increase. 2) The rate of a reaction is calculated by taking the change in concentration divided by the change in time between two measurements. For example, the average rate between 100 and 200 seconds can be found by taking the change in concentration over that time interval. 3) Reaction rates typically decrease over time as there are fewer collisions between reactants as their concentrations decrease. Measuring rates at different time intervals shows the average rate decreasing as the reaction progresses.

2. What is a reaction rate?
How could we measure
the rate of reaction?
Measure the concentration
of A as it decreases
Measure the concentration
of B as it increases

3. What is a reaction rate?
The change in concentration of either
the reactants or products with time.
Official definition …
Reaction rates are measured in … mol dm-3 s-1

4. Measuring a reaction rate
Let’s look at a fictional reaction …
A + 2B → C
Concentration
Time
Questions …
… how would you label the
axes?
If we were to draw a graph of the
concentration against time …
… what direction would the
line for A be going?
… what direction would the
line for B be going?
… what direction would the
line for C be going?

5. Measuring a reaction rate
A + 2B → C
Concentration
Time
Questions …
Why does B fall faster than A?
For each molecule of A, two
molecules of B are used up
Why does C increase when A
and B are decreasing?
C is the product and its
concentration will increase
over time

6. Measuring a reaction rate
The rate – or speed – of a reaction
can be worked out using …
Change in concentration (or moles)
Time interval
Remember …
Rate will change throughout
the experiment as the
concentrations change
To find these values on a graph
we need to use tangents

7. Measuring a reaction rate
Concentration
Time
Question …
Measure the rate of
reaction 5 seconds into
the reaction
1. Find 5 seconds on your graph and mark it on the
line.
2. Draw a tangent line touching the curve at your
point.
3. Using sensible numbers, get the a and b values of
your line.
4. The gradient at 5 seconds is the rate of reaction.
5 10
b
a
a = 2 mol dm-3 b = 10s
2
4
gradient =
a
b
conc
time( )
= 2
10
Rate of reaction at 5
seconds is 0.2 mol dm-3 s-1

8. Measuring a reaction rate
We know how to work out the rate and any one time, but what
about the average rate?
Question …
What is the average rate between
100 and 200 seconds?
conc
time( )rate =
0.0820 – 0.0671
200 - 100
=
0.000149 mol dm-3 s-1

9. Measuring a reaction rate
Let’s look at the
average rate
between each time
interval
Question …
What do you
notice about the
reaction rate?
The average rate
decreases as time goes
on
As the reaction
continues there are
fewer and fewer
collisions

10. Measuring a reaction rate
The rate can only be determined by experiment.
You cannot determine the rate using the equation
alone.