1. 14. Rates of Reaction

2. Learning Outcomes

3. Learning Outcomes

4. Experiments!
To monitor the rate of production of oxygen from hydrogen peroxide using manganese dioxide as a catalyst
To study the effect of concentration on the rate of reaction using sodium thiosulphate solution and hydrochloric acid
To study the effect of temperature on rate of reaction using sodium thiosulphate and hydrochloric acid

5. Rate of Reaction
The change in concentration per unit time of any one reactant or product.
Full marks given for: Rate of reaction = change in concentration change in time

6. Total time for the reaction to go to completion
Average Rate
Total volume produced
Total time for the reaction to go to completion
Unit = cm3/min

7. The rate of reaction at any one particular time during the reaction.
Instantaneous Rate
The rate of reaction at any one particular time during the reaction.
Steps:
Draw graph (volume v time)
Find tangent to curve at a particular time
Find slope of tangent (rise/ run or y2-y1/ x2-x1)
Slope = instantaneous rate

8. Monitoring the rate of production of oxygen

9. Graph Volume Oxygen v Time
160
V O L U M E G A S
cm3
140
120
100 80 60 40 20 10 20 30 40 50 60 70 80 90
TIME sec

10. 160
V O L U M E G A S
cm3
140
120
100 80 60 40 20 10 20 30 40 50 60 70 80 90
TIME sec

11. What volume of gas was collected after 30 s ?
120cm3
What volume of gas was collected after 30 s ?
160
V O L U M E G A S
cm3
140
120
100 80 60 40 20 10 20 30 40 50 60 70 80 90
TIME sec

12. When maximum volume of oxygen obtained
160
V O L U M E G A S
cm3
140
120
After what time was the reaction finished ?
100 80 60 40 20 10 20 30 40 50 60 70 80 90
TIME sec

13. When half maximum volume of oxygen obtained
160
V O L U M E G A S
cm3
140
120
When was the reaction half finished ?
100 80 60 40 20 10 20 30 40 50 60 70 80 90
TIME sec

14. What was the instanteous rate at 30 s ?
160
V O L U M E G A S
cm3
Y2=150
140
120
Rate= Y2-Y X2-X1
What was the instanteous rate at 30 s ?
100
Y1=95 80
Rate=
Rate=
Rate= cm3/sec 60 40 20 10 20 30 40 50 60 70 80 90
X1=10
X2=50
TIME

15. How would the graph change if the solution as more concentrated ?
160
V O L U M E G A S
cm3
140
120
How would the graph change if the solution as more concentrated ?
100 80 60 40 20 10 20 30 40 50 60 70 80 90
TIME sec

16. How would the graph change if the temperature was higher ?
160
V O L U M E G A S
cm3
140
120
How would the graph change if the temperature was higher ?
100 80 60 40 20
TIME

17. Rate of a Reaction
The change in concentration per unit time of any one reactant or product
Average Rate
Total volume of gas produced (cm3)
Total time to go to completion
Instantaneous Rate
The rate at a particular point in time during the reaction
(slope of tangent to curve)

18. For a reaction to take place:
The reacting particles must collide with each other
A collision only results in the formation of products if a certain minimum energy is exceeded

19. Factors Affecting Rate of Reaction

20. Factors Affecting Rates of Reactions
By experiment it is found that the rate of a reaction depends on five factors:
Nature of reactants
Particle size
Concentration
Temperature
Catalysts

21. 1. Nature of Reactants Covalent Compounds Ionic Compounds
- Slow reactions
- Fast reactions
- Requires energy to break covalent bonds
- Does not require energy to break bonds

22. 2. Particle Size
- Finer particles = greater the surface area
The greater the surface area, the quicker the reaction
E.g:
Reaction of marble (calcium carbonate) with dilute HCl
The smaller the particle size the larger the surface area.
If the surface area of the substance is larger, more of it is available to react
This increases the rate of reaction
*As CO2 is realeased, the mass of the flask drops

23. 2. Particle Size

24. 2. Particle Size 16 places of attack- increased surface ares

25. Coal dust explosion Pittsburgh
2. Particle Size
Dust EXPLOSIONS
If very fine dust in the air builds up, an explosive reaction can occur
Coal dust explosion Pittsburgh
Grain silo explosion

26. 2. Particle Size Conditions for dust explosion:
The explosion must take place in an enclosed space
The particles must be combustible
There must be a source of ignition (spark, flame, etc)
The particles must be dry
There must be oxygen present

27. 3. Concentration
The higher the concentration, the faster the rate of reaction

28. To investigate the effect of concentration on reaction rate
Add the HCl to the Na2S2O3 in the flask
Record the time taken for the X to disappear
Repeat with different concentrations of Na2S2O3
Inverse the time to get the rate

29. Effect of concentration on rate
Conclusion ?
R A T E
Rate is directly proportional to concentration 1
Time
CONCENTRATION

30. 4. Temperature
Hotter temperature = faster rate Cooler temperature = slower rate

31. To investigate the effect of temperature on reaction rate
Add the HCl to the Na2S2O3 in the flask
Record the temperature after mixing
Record the time taken for the X to disappear
Inverse the time to get the rate

32. Repeat at different temperatures

33. Effect of temperature on rate
Conclusion ?
R A T E
Rate increases with increasing temperature 1
Time
TEMPERATURE

34. What is the main factor increasing rate between A and B
Effect of temperature on rate
What is the main factor increasing rate between A and B
R A T E 1
Time
Rate increases due to increased collisions /sec B A
TEMPERATURE

35. What other factor caused the large rate increase between B and C
Effect of temperature on rate
What other factor caused the large rate increase between B and C C
R A T E
More colliding particles have E Act 1
Time B
TEMPERATURE

36. 5. Catalysts
Catalyst
A catalyst is something that alters the rate of a chemical reaction but is not consumed in the reaction
Negative catalyst: (inhibitor) can slow down the rate of a reaction
You need to know:
The general properties of catalysts
Types of catalysis
Mechanisms of catalysts
Catalytic converter

37. a) Properties of Catalysts
Remain chemically unchanged
Only needed in small amounts
In equilibrium reaction, a catalyst helps equilibrium to be achieved quicker

38. a) Properties of Catalysts
4. Specific – work for one reaction but not the other
E.g: Enzymes
Catalase – made in the liver, decomposes hydrogen peroxide
Enzyme
A substance that is produced by a living cell and acts as a biological catalyst

39. a) Properties of Catalysts
5. The action of a catalyst can be destroyed by catalyst poisons
E.g:
Lead: when lead was added to petrol, it was found that the lead destroyed the catalysts in the catalytic converter
Arsenic: Inhibits the actions of certain enzymes in the body
Catalyst poison
A substance that makes a catalyst inactive

40. Learning Check

41. b) Types of Catalysis Types of Catalysis Homogeneous Catalysis
Autocatalysis
Heterogeneous Catalysis

42. Homogeneous Catalysis
b) Types of Catalysis
Homogeneous Catalysis
Catalysis in which both the reactants and the catalyst are in the same phase,
i.e: there is no boundary between the reactants and the catalyst
***You must say phase***
(-3 marks for saying state)
Phase= a specific state of matter that has uniform chemical and physical properties
Example: The iodine snake (aqueous potassium iodide and hydrogen peroxide)

43. Heterogeneous Catalysis
b) Types of Catalysis
Heterogeneous Catalysis
Catalysis in which the reactants and the catalyst are in different phases
i.e: there is a boundary between the reactants and the catalyst
Example: oxidation of methanol by platinum catalyst Pt
2CH3OH + O2  2HCHO + 2H2O

45. Starts slow but speeds up as the product (catalyst) starts to form
b) Types of Catalysis
Autocatalysis
Catalysis in which one of the products acts as a catalyst for the reaction
Starts slow but speeds up as the product (catalyst) starts to form

46. c) Mechanisms of Catalysis
The Intermediate Formation Theory
The Surface Adsorption Theory

47. The Intermediate Formation Theory
c) Mechanisms of Catalysis
The Intermediate Formation Theory
One or more of the reactants combine with the catalyst to form an intermediate compound

48. The Intermediate Formation Theory
c) Mechanisms of Catalysis
The Intermediate Formation Theory
C= Catalyst
Consider: W + X → Y + Z
One of the reactants combines with the catalyst to form an intermediate compound- this does not last very long:
W + C → [WC]
Intermediate compound then reacts with the other reactant (X) to give the final product:
[WC] + X → Y + Z + C
Overall reaction: W + X → Y + Z C

49. Example of the Intermediate Formation Theory
c) Mechanisms of Catalysis
Example of the Intermediate Formation Theory
Iodine Snake
The decomposition of hydrogen peroxide (H2O2) catalysed by the presence of I- ions.
2H2O2 → 2H2O + O2
1. One of the hydrogen peroxide molecules reacts with an I- ion to form the IO- intermediate
H2O2 + I- → H2O + IO- I-

50. Example of the Intermediate Formation Theory
c) Mechanisms of Catalysis
Example of the Intermediate Formation Theory
2. The intermediate then reacts with another hydrogen peroxide molecule to form the products and regenerate the catalyst
H2O2 + IO- → H2O + O2 + I-
Adding the equations in steps 1 & 2 gives us the overall equation: I-
2H2O2 → 2H2O + O2

51. The Intermediate Formation Theory
c) Mechanisms of Catalysis
The Intermediate Formation Theory
One or more of the reactants combine with the catalyst to form an intermediate compound

52. c) Mechanisms of Catalysis
Exam Q!!
The oxidation of potassium sodium tartrate by hydrogen peroxide catalysed by cobalt (II) ions provides evidence for the intermediate formation theory of catalysis.
State the observation you would make when carrying out the experiment.
Explain how these observation provide evidence for the intermediate formation theory

53. c) Mechanisms of Catalysis
Exam Q!!
Observations:
The solution is pink at the start of the experiment
After adding 100-volume H2O2, fizzing is observed
The solution changes to a green colour
After a short while, the fizzing stops and the green colour changes back to pink
Explanations:
Pink → green indicates the formation of a new substance
Green pink indicates the substance was an intermediate
Fizzing when green indicates the intermediate is reacting
Pink colour at the end indicates the cobalt (II) ions were not used up and have been regenerated

54. c) Mechanisms of Catalysis
Exam Q!!
Observations:
The solution is pink at the start of the experiment
After adding 100-volume H2O2, fizzing is observed
The solution changes to a green colour
After a short while, the fizzing stops and the green colour changes back to pink
Explanations:
Pink → green indicates the formation of a new substance
Green pink indicates the substance was an intermediate
Fizzing when green indicates the intermediate is reacting
Pink colour at the end indicates the cobalt (II) ions were not used up and have been regenerated

55. The Surface Adsorption Theory-
c) Mechanisms of Catalysis
The Surface Adsorption Theory-
Heterogeneous Catalysis Mechanism
3 steps:
Adsorption: Molecules settle on the surface of the catalyst and are held there by temporary bonds
Reaction on the surface: Chemical reaction occurs. High concentration of molecules on the surface makes it more likely that they will collide with each other (react)
Desorption: Products leave the surface of the catalyst
Absorption= when a substance moves in to another
Adsorption= accumulation of substances only at the surface of another substance

56. The Surface Adsorption Theory-
c) Mechanisms of Catalysis
The Surface Adsorption Theory-
Heterogeneous Catalysis Mechanism
More finely divided the catalyst = the better its performance.
Greater surface area on which adsorption can take place.
When a catalyst is ‘poisoned’ a lot of the surface of the catalyst is blocked off due to permanent bond formation between the metal and the ‘poison’.

57. d) Catalytic Converters
A device in the exhaust of a motor vehicle which contains catalysts to convert pollutants in the exhaust gases to less harmful substances
ENGINE GASES
NO2 CO
CXHY
(HYDROCARBONS)
EXHAUST
GASES N2
CO2
H2O

58. d) Catalytic Converters
A catalytic converter consists of a thin coating of the metals platinum, palladium and rhodium (the catalysts) on a ceramic or metal ‘honeycomb’ inside a stainless steel base
*Common exam q!*
Name the catalysts used in a catalytic converter

59. d) Catalytic Converters
The main reaction that occurs:
Pt/Pd/Rh
2CO + 2NO → 2CO2 + N2
*This is heterogeneous catalysis
*Common exam q!*
Name the type of catalysis that occurs in catalytic converters

60. ENVIRONMENTAL BENEFIT OF REMOVAL
d) Catalytic Converters
REACTANT
PRODUCT
ENVIRONMENTAL BENEFIT OF REMOVAL
Carbon monoxide
Carbon dioxide
Carbon monoxide is a poisonous gas.
Nitrogen monoxide
Nitrogen
Nitrogen monoxide is poisonous and contributes to acid rain.
Hydrocarbons
Carbon dioxide and water
Hydrocarbons cause smog and contribute to the greenhouse effect.

61. Collision Theory and Activation Energy

62. Collision Theory
Attempts to explain why these five factors influence the rate of reaction
For a reaction to occur, the reacting particles must collide with each other.
A collision only results in the formation of products if a certain minimum energy is exceeded in the collision. Such a collision is called an effective collision
An effective collision is a collision which results in the formation of products

63. Collision Theory
Attempts to explain why these five factors influence the rate of reaction
When an effective collision occurs between colliding particles, bonds are broken and new bonds are formed,
i.e: the effective collision brings about a chemical reaction. This results in the formation of products
Rate of reaction: depends on not only the number of collision, but how many of these are effective

64. Collision Theory
Attempts to explain why these five factors influence the rate of reaction
3. For a collision to be effective, it must have a certain amount of energy – Activation energy
Activation Energy
The minimum energy that colliding particles must have for a reaction to occur,
i.e: the minimum energy required for effective collisions between particles to occur

65. Reaction Profile Diagram
A graph which shows the change in energy of a chemical reaction with time as the reaction progresses

66. Activation Energy and Reaction Profile Diagrams
Large Activation Energy
Only a small number of molecules have enough energy to change into products.
Therefore the rate is slow

67. Activation Energy and Reaction Profile Diagrams
Small Activation Energy
Greater number of molecules have enough energy to change into products.
Therefore the rate is fast.

68. Exothermic Reactions Exothermic:
A chemical reaction that gives out heat.
Products have less energy than the reactants

69. Endothermic Reactions
A chemical reaction that takes in heat.
Products have more energy than the reactants

70. Using the Collision Theory to Explain the 5 Factors:
*NB for LC qs
1. Nature of Reactants:
Reactions involving covalent bonds involve breaking bonds
much higher activation energy
slower rate of reaction
Reactions involving ionic bonds
lower activation energy
faster rate of reaction.
2. Particle Size:
Smaller the particle size:
greater the surface area
number of collisions increase
number of effective collisions also increase
increased rate of reaction.

71. Using the Collision Theory to Explain the 5 Factors:
*NB for LC qs
3. Concentration
Concentration of the reactants is increased
number of collisions increased (particles closer together)
number of effective collisions also increased
increased rate of reaction.

72. Using the Collision Theory to Explain the 5 Factors:
*NB for LC qs
4. Temperature
At higher temperatures
Particles have more energy
A greater number of collisions reach the activation energy
Increased rate of reaction.

73. Using the Collision Theory to Explain the 5 Factors:
*NB for LC qs
5. Catalysts
A catalyst works by providing an alternative reaction pathway with a lower activation energy.
more reactant molecules now possess the energy required for effective collisions to occur
increased rate of reaction.