1. RATES OF REACTION

2. Rates of reaction
Reactions can be very fast, like fireworks or explosives, but they can also be very slow – such as an apple turning brown.

3. Rates of reaction
Chemical reactions occur when particles of reactant collide with enough energy to react.

4. Speeding up reactions
Anything that increases the chance of effective collision increases the rate (speed) of reaction. Factors include:
Increased surface Area
Increased concentration
Increased temperature
Use of a catalyst

5. Extra surface for molecules to collide with.
Surface area
The reactions of solids can clearly only take place at the surface of the solid.
If we break a solid into smaller pieces we get more area and a faster reaction.
Molecules collide with the surface of the solid
Extra surface for molecules to collide with.

6. Surface area
If we grind up a solid to a powder we massively increase the surface area.
We therefore massively increase the rate of any reaction
Slow
Very fast

7. An indigestion tablet fizzes in water – but fizzes much faster if it is crushed.
Which glass has the crushed tablet? A B
Solid
Crushed

8. Collisions infrequent
Concentration
Reactions in solution involve dissolved paticles that must collide before reaction is possible.
The more crowded (concentrated) the solution, the faster the reaction.
Collisions infrequent
Collisions frequent

10. And so - pressure speeds up gas reactions
Reactions involving gases are affected by the pressure of the gases present.
If we cover one end of a bicycle pump and push in the plunger we increase the pressure.
Low pressure
High pressure
What we are doing is squeezing the gas molecules closer together or making them more concentrated.
And so - pressure speeds up gas reactions

11. Fewer collisions per second
Temperature
At higher temperatures molecules move faster. As a result there are more collisions per second and so a faster reaction occurs. Slow molecules are also less likely to lead to a reaction than fast ones.
Fewer collisions per second
More collisions per second

12. Food spoils because of chemical reactions that occur.
Why does food remain good for so much longer if it is kept in a freezer?
The reactions that cause the food to go off will be slower because there will be fewer and “softer” collisions between molecules at a reduced temperature.

13. Before microwave ovens were common many people used pressure cookers.
It was a pan that stopped the water boiling until it reached about 115oC.
How would this help cooking?
The molecules move faster and collide more often and with more energy.
Cooking times were greatly reduced.

14. Catalysts and rate of reaction
For chemical reactions to occur:
Existing bonds have to begin breaking so that new ones can be formed.
The molecules have to collide in such a way that the reacting parts of the molecules are brought together.
Catalysts can help with either or both of these processes.
A catalyst is a substance that speeds up a reaction without getting used up in the process.

15. Measuring reaction rates
Rate implies we are measuring how things change over a period of time.
To measure the rate of a reaction we have to track the manner in which the amount of product (or reactant) changes over time.
Rate of gas formation can be measured using a syringe.
For a reaction in which sulfur is precipitated we can time how long the solution takes to go cloudy.

16. Percentage completion of reaction
Slower and slower
Reactions do not proceed at a steady rate.
They start fast and get slower and slower.
This is not surprising because the reactant concentration (and the chance of collision) gets lower and lower as time progresses.
fast 25
slower 75
very slow
100
stopped
product
reactants
Percentage completion of reaction

17. Rates and Graphs
These show the increasing amount of product or the decreasing amount of reactant.
Amount of product
Time
Amount of reactant
Time
Steep gradient Fast reaction
Shallow gradient Slow reaction
Steep gradient Fast reaction
Shallow gradient Slow reaction

18. Rate graphs and reactant concentrations
Amount of product
Time
All product
Mix of reactant
And product
Reactant Concentration falls
Rate of Reaction falls
reactants
product
Gradient of graph decreases
All reactant

21. Some Reaction Rates Experiments
The following slides describe the four chemical reactions that are commonly used as examples.

22. Marble chips are calcium carbonate.
Acid and marble
Marble chips are calcium carbonate.
They react with acid to evolve a gas.
calcium carbonate
+ hydrochloric acid
calcium chloride
+ water
+ carbon dioxide
CaCO3(s) + 2HCl(aq)  CaCl2(aq) + H2O(l) + CO2(g)
Marble chips
Glass tube
Gas syringe
Hydrochloric acid
The gas given off can be collected in a syringe and readings taken every 30 seconds or so.

23. Measure the agreed mass of marble chips
Acid and marble
Measure the agreed mass of marble chips
Set up the syringe, flask and connector
Measure the acid / water.
Add the marble chips and quickly insert the bung and start stop clock.
Take syringe readings at 30 second intervals.
Time
Reading
0 s
0 cm3

24. Acid and marble

25. Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g)
Acid and metal
Reactive metals (eg. Magnesium) react with acid to evolve hydrogen gas.
magnesium
+ hydrochloric acid
 magnesium chloride
+ hydrogen
Mg(s) HCl(aq)  MgCl2(aq) H2(g)
As the gas given off leaves the flask the total mass of the flask and its contents decreases slightly.
Readings of the mass(g) can be taken. Typically at 1 minute intervals. Mg
HCl
11.72
11.73
11.71
11.71
11.74
11.80
11.77

26. Measure the agreed volume of acid / water into the conical flask.
Acid and metal
Measure the agreed volume of acid / water into the conical flask.
Have a loose plug of cotton wool to prevent “spitting” of droplets of liquid.
Have a piece of magnesium of known mass ready.
Add the magnesium, place the cotton wool in the neck and start taking mass readings immediately. Mg
HCl
Cotton wool
11.72
11.73
11.71
11.71
11.74
11.77
11.80
Time
Reading
0 s
0 cm3 60
120

27. Acid and metal

28. Decomposition of Hydrogen Peroxide
Hydrogen peroxide decomposes into water and oxygen.
Hydrogen peroxide
 water
+ oxygen
2H2O2(aq)  2H2O(l) O2(g)
Oxygen gas is given off and can be measured using a gas syringe or a balance.
The reaction is catalysed by a wide range of solids.
Remember the catalyst NEVER produces more product - just quicker

30. Acid and Sodium Thiosulphate
In this reaction sulphur is precipitated which makes the solution turn cloudy.
Sodium thiosulphate
+ hydrochloric acid
 sodium chloride
+ water
+ sulphur dioxide
+ sulphur
Na2S2O3(aq) HCl(aq)  2NaCl(aq) + H2O(l) + SO2(g) + S(s)
The effect of changing conditions such as temperature or concentrations can be studied by measuring how long it takes to produce enough sulphur to make the solution opaque (non see-through).

31. Measure the agreed volume of thiosulphate / water into the conical flask.
Prepare a piece of paper with a cross drawn on it.
Measure the required volume of acid in a measuring cylinder.
Add the acid to the flask, start the clock, swirl the flask.
Look down through the flask until the cross disappears. Note the time.
Look down here

32. Sketch a graph of the results.
Imran studied the effect of temperature upon the time it took for the flask to go cloudy.
Sketch a graph of the results.
Using the reactions at 25oC and 40oC, explain how the time taken lets you work out the relative rate of reaction.
Time(s) vs Temp(oC)
Time
Temp
Temp
(oC)
Time taken
(s) 25
100 30 60 35 40 45 15 50 10

33. Some Rates Questions

35. A pupil performed an investigation into the rate of reaction between a metal and an acid. The results below where obtained.
i) Plot a graph of gas volume (y-axis) against time (x-axis)
ii) When was the rate of reaction fastest?
iii) Use the graph to find the volume of gas produced after 35 seconds.
iv) Use the graph to tell after how long the reaction stopped.
v) On the graph sketch a line showing the experiment repeated
at a higher temperature.

36. Experimental Results Answer
300
250
200
150
Volume / cm3
100 50 10 20 30 40 50 60 70 80 90
Time / seconds
ii) The reaction was fastest at about 25 seconds as the gradient of the line is highest at this point.
iii) About 175 cm3
iv) About 55 seconds.
v) Higher temperature reaction is in red.

37. A flask was connected to a gas syringe by a glass delivery tube
A flask was connected to a gas syringe by a glass delivery tube. 30cm3 of water and 0.5g of manganese dioxide were added to the flask. Then 5cm3 of hydrogen peroxide was added and the stopper quickly fitted. Readings of the volume of gas produced were taken every 10 seconds.
i) Plot a graph of volume of gas (y-axis) against time (x-axis). Label this curve A.
ii) Without emptying the flask another 10cm3 of water and a further 5cm3 of hydrogen peroxide were added. Sketch the shape of the second experiment and label it B.

38. Answer A B Time (s) Notes: Curve B is an experiment with half the
concentration of hydrogen peroxide. This should
produce about half the rate as shown by a line with half the gradient of A. However, the same amount was added so 58cm3 of gas will still be produced.

39. i) The results above were obtained from an experiment where the loss in mass was recorded as lumps of zinc reacted with hydrochloric acid. Plot a graph of mass loss (y-axis) against time (x-axis).
On the graph sketch the lines you would expect if
a) the concentration of acid was reduced,
b) the temperature was increased.

40. Answer
(b)
(a)

41. Which of these would speed up the rate at which magnesium dissolves in acid?
Cool the acid.
Cut up the magnesium.
Add water.
Coat the magnesium in oil.

42. Why does breaking up solids increase the rate of reaction?
Makes more solid.
Creates more energy.
Increases surface area.
Increases the concentration.

43. Why does temperature increase the rate of reaction?
Acts as a catalyst.
Increases the concentration.
Increases number of molecules.
Makes collisions more frequent and harder.

44. Why does a catalyst increase the rate of reaction?
Provides a route with a lower activation energy.
Helps provide energy for the reaction.
Increases the speed of reactant molecules.
Reduces the number of molecular collisions.

45. Why do most reactions start fast and get slower and slower?
They run out of energy.
They run out of catalyst.
The concentration of reactant molecules gets less and less.
The surface area increases.