1. Rate Graphs

2. Collision Theory Do you remember collision theory?
What is the unifying statement?

3. Collision Theory
Chemical reactions can only happen if reactant particles collide with enough energy. The more frequently particles collide, and the greater the proportion of collisions with enough energy, the greater the rate of reaction.

4. What things effect the rate of reaction?
Temperature
Surface Area
Concentration
Catalyst

5. Rate Graphs
We’re going to look at how you use graphing techniques to prove these relationships.
As well as a little maths
If rate is defined as change in products over time, then plotting a graph of change over time will help calculate the rate.
But we need to measure the products in some way

6. Things to Measure
The measurement itself depends on the nature of the reactant or product:
The mass of a substance - solid, liquid or gas
The volume of a gas
For example, if 24 cm3 of hydrogen gas is produced in two minutes, the mean rate of reaction = 24 ÷ 2 = 12 cm3 hydrogen / min.

7. Effect of Temperature on Rate
If the temperature is increased:
the reactant particles move more quickly
they have more energy
the particles collide more often
The rate of reaction increases

8. Measuring Rates: Using mass
Another method used to follow a reaction monitors change in mass. CO2 is generated and the mass loss is seen on the scale according to the reaction:
Hydrochloric
Acid
CaCO3 + 2HCl  CO2 + CaCl2 + H2O
Marble chips
(Calcium carbonate)
245.4

9. Measuring Rates
245.2

10. Measuring Rates
245.0

11. Measuring Rates
244.8

12. Measuring Rates
244.6

13. Measuring Rates
As the marble chips dissolve, the mass decreases. The mass value can be recorded every 30 seconds.
244.4

14. Measuring Rates
As the marble chips dissolve, the mass decreases. The mass value can be recorded every 30 seconds.
244.2

15. Measuring Rates
As the marble chips dissolve, the mass decreases. The mass value can be recorded every 30 seconds.
244.0

16. Measuring Rates
As the marble chips dissolve, the mass decreases. The mass value can be recorded every 30 seconds.
243.8

17. Measuring Rates
As the marble chips dissolve, the mass decreases. The mass value can be recorded every 30 seconds.
243.6

18. Measuring Rates
As the marble chips dissolve, the mass decreases. The mass value can be recorded every 30 seconds.
243.4

19. Using this technique you can draw a graph for the experiment.
E.g. Mass vs. time
Volume gas vs. time
Pressure vs. time

20. Difference in Products
If the gradient is…
Difference in Y
Difference in X
And Rate is
Difference in Products
Difference in Time
So long as the x-axis is time, then the gradient will be the rate at that point.

21. Draw axes using a large portion of the graph paper
Add labels for each axis as well as units.
Plot points
Draw a best fit line
Draw a tangent line at the required point. (t=60)
Calculate the gradient.
INCLUDE THE UNITS
Gradient = change in Y
change in X
Gradient = cm3
11 seconds
Gradient = cm3/sec
~5.5 units = 11 cm3
Amount of gas collected (cm3)
~10.5 units = 105 seconds
Time (seconds)

22. Initial Rate Vs. Temperature
If you calculate the gradient of a straight line from your first measurement to 0, you can find the initial rate of reaction.
Then you can directly graph rate of reaction.
This is also good for showing data transformation.

23. Sally Thinks…
Sally thinks that you could look at the total change in mass to work out how much the temperature changes the rate.
Is she right?
Why or why not?