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.
