1. Chemical Kinetics T2 T2 Edexcel new Specification T1 T1 Application of
Core principles
Of chemistry
Chemical Kinetics T1 T2 T2 T1
Edexcel new
Specification
Hemantha Welihena

2. Content of specification
Recall the factors that influence the rate of chemical reaction, including concentration, temperature, pressure, surface area and catalysts B
Explain the changes in rate based on a qualitative understanding of collision theory C
Use, in a qualitative way, the Maxwell-Boltzmann model of the distribution of molecular energies to relate changes of concentration and temperature to the alteration in the rate of a reaction D
Demonstrate an understanding of the concept of activation energy and its qualitative relationship to the effect of temperature changes on the rate of reaction E
Demonstrate an understanding of the role of catalysts in providing alternative reaction routes of lower activation energy and draw the reaction profile of a catalysed reaction including the energy level of the intermediate formed with the catalyst F
Carry out simple experiments to demonstrate the factors that influence the rate of chemical reactions, eg the decomposition of
hydrogen peroxide.

3. Chemical reaction
Reaction rate
Reaction between hydrogen and oxygen
Reaction between potassium and water
Precipitation of insoluble metal hydroxides
Displacement reaction of copper and silver nitrate solution
Fermentation of sucrose
Rusting of iron
Weathering of marbles by acid rain
Explosively fast
Vigorous, takes a fraction of a second
Instantaneous
Takes a several minutes
Takes a few weeks
Takes a few months
Slow, take centuries

4. Factors Affecting Reaction Rates
concentration
particle size
pressure
catalyst
temperature
light

5. 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. (unit volume) (unit volume)
Collisions infrequent
Collisions frequent

6. Effect of concentration on rate of reaction
The higher the concentration of a dissolved reactant, the faster the rate of a reaction.
Why does increased concentration increase the rate of reaction?
At a higher concentration, there are more particles in the same amount of space. This means that the particles are more likely to collide and therefore more likely to react.
lower concentration
higher concentration

7. Concentration and particle collisions

8. The effect of concentration on rate1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9

9. The effect of concentration on rate
2.0 M HCl
(b) 1.0 M HCl
(c) 0.5 M HCl
Reaction rate:
(a) > (b) > (c)

10. Effect of pressure on rate of reaction
Why does increasing the pressure of gaseous reactants increase the rate of reaction?
As the pressure increases, the space in which the gas particles are moving becomes smaller.
The gas particles become closer together, increasing the frequency of collisions. This means that the particles are more likely to react.
lower pressure
higher pressure

11. Effect of surface area on rate of reaction
Any reaction involving a solid can only take place at the surface of the solid.
If the solid is split into several pieces, the surface area increases. What effect will this have on rate of reaction?
low surface area
high surface area
This means that there is an increased area for the reactant particles to collide with.
The smaller the pieces, the larger the surface area. This means more collisions and a greater chance of reaction.

12. Surface area and particle collisions

13. Reaction between a carbonate and acid
Marble chips are made of calcium carbonate. They react with hydrochloric acid to produce carbon dioxide.
CaCO3 (s)
2HCl
(aq)
CaCl2 (aq) +
H2O (l)
CO2 (g)
The effect of increasing surface area on the rate of reaction can be measured by comparing how quickly the mass of the reactants decreases using marble chips of different sizes.

14. The effect of surface area on rate

15. Effect of temperature on rate of reaction
Na2S2O3(aq) + 2HCl(a) NaCl(aq) + SO2(g) + H2O(l) + S(s)
t = time taken for the disappearance of the cross
(a)
(b)
Temperature
rate  1/t
t2 < t1 (rate: b > a)

16. Effect of temperature on rate of reaction
 temperature   kinetic energy of reactant particles  reactant particles move faster   collision frequency   reaction rate

17. Effect of catalyst on rate of reaction
e.g. H2O2(aq)  2H2O(l) + O2(g)
A laboratory set-up for studying the effect of catalyst on the decomposition
of hydrogen peroxide solution

18. Effect of catalyst on rate of reaction
A catalyst speeds up a reaction by providing an alternative reaction pathway with a lower activation energy
More reactant particles will possess energy equal to or greater than the new activation energy, and thus the reaction rate increases

19. Molecular distribution Maxwell and Boltzmann
Number of molecules With energy, E
Most probable
energy
Molecules with
intermediate energy
Barrier of the activation energy
High energy
Molecules
Low Energy
Molecules
Kinetic Energy

20. What are catalysts?
Catalysts are substances that change the rate of a reaction without being used up in the reaction.
Catalysts never produce more product – they just produce the same amount more quickly.
reaction (time)
energy (kJ)
Ea without catalyst
Different catalysts work in different ways, but most lower the reaction’s activation energy (Ea).
Ea with catalyst

21. The effect of temperature on reaction rate in
Molecular distribution of Maxwell and Boltzmann
Activation energy with a catalyst
Activation energy without a catalyst

22. Activation Energy
In order to be effective, collisions between particles must exceed a certain energy threshold.
When particles collide effectively, they reach an activated state. The energy difference between the reactants and the activated state is the activation energy (Ea) for the reaction.
The lower the activation energy, the faster the reaction.
Smaller Ea
higher rate

23. Energy-level diagram for a reaction.
Collisions must occur with sufficient energy to reach an activated state.
This particular reaction is reversible and is exothermic in the forward direction.

24. Reaction energy diagram for a catalyzed (green)
Alternative reaction route and uncatalyzed (red)
process.
activated state

25. Reaction energy diagram for the two-step NO2-F2 reaction

26. The effect of temperature on reaction rate in
Molecular distribution of Maxwell and Boltzmann
barrier of the activation energy

27. The catalyzed decomposition of H2O2.
A homogeneous catalyst is in the same phase as the reaction mixture.
Oxygen gas forms quickly as Br-(aq) catalyzes the H2O2 decomposition; the intermediate Br2 turns the solution orange.
A small amount of NaBr is added to a solution of H2O2.

28. Reaction energy diagrams and possible transition states for two
reactions.
Endothermic reaction Exothermic reaction

29. Everyday catalysts
Many catalysts are transition metals or their compounds. For example:
Nickel is a catalyst in the production of margarine (hydrogenation of vegetable oils).
Iron is a catalyst in the production of ammonia from nitrogen and hydrogen (the Haber process).
Platinum is a catalyst in the catalytic converters of car exhausts. It catalyzes the conversion of carbon monoxide and nitrogen oxide into the less polluting carbon dioxide and nitrogen.

30. Rates of Reaction
Chemical reactions occur when particles of reactant collide with enough energy to react.

31. Collision theory

32. A collision between the reacting particles.
Collision theory
A collision between the reacting particles.
The reacting particles must have sufficient energy to break their existing bonds, so that new bonds can form
They must be collide in the correct orientation.
Correct Orientation
Incorrect Orientation O O C N C N O O

33. Collision Theory
The occurrence of a chemical reaction can be explained by the collision theory
The theory states that reactants must collide for a reaction to occur.
The collision must occur with enough energy to break the bonds in the reactants so that new bonds can be formed in the products
This minimum amount of energy required for the reaction to occur is known as the activation energy

34. Successful Collision- Effective Collision
The minimum energy needed in a collision before a reaction take place.
It can also be defined as the minimum energy colliding particles must have in order to have a “successful” collision.
A collision in which the molecules have sufficient energy for a reaction to take place is called a SUCCESSFUL COLLISION.

35. 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

36. Temperature and Collision Energy
An increase in temperature causes an increase in the kinetic energy of the particles. This leads to more frequent collisions and reaction rate increases.
At a higher temperature, the fraction of collisions with sufficient energy equal to or greater than Ea increases. Reaction rate therefore increases.

38. The effect of temperature on reaction rate in
Molecular distribution of Maxwell and Boltzmann
barrier of the activation energy

39. The diagram below shows the Maxwell-Boltzmann distribution of molecular energies for a gaseous system at two temperatures.
300 K
Number of molecules
with energy E
310 K
Energy E
(a) The energy plotted on the horizontal axis is mainly
A activation. B kinetic. C rotation. D vibration.

40. A the activation energy increases. B the activation energy decreases.
(b) The rate of a chemical reaction increases with temperature mainly because
A the activation energy increases.
B the activation energy decreases.
C more collisions occur with energy greater than the activation energy.
D the molecules collide more frequently.
(c) The total area under each curve
A decreases with increasing temperature.
B increases with increasing temperature.
C increases or decreases with increasing temperature, depending on the size of the molecules.
D does not change with temperature

41. 10 The Maxwell-Boltzmann distribution of molecular energies is useful for explaining why increasing temperature affects the rate of a chemical reaction.
(a) Which of the following statements describes how the shape of the Maxwell-Boltzmann distribution curve changes as temperature increases?
A The peak decreases in height and moves to the left.
B The peak increases in height and moves to the left.
C The peak decreases in height and moves to the right.
D The peak increases in height and moves to the right.
(b) The main reason that reaction rates increase with temperature is that
A all the molecules move faster.
B all the molecules collide more frequently.
C more molecules collide with the correct orientation.
D a larger proportion of molecules have high energies.

42. The reaction of heated magnesium with steam is faster than the reaction of magnesium with cold water. This is mainly because
A in cold water, the water molecules do not collide as frequently with magnesium.
B the coating of oxide on magnesium decomposes when it is heated.
C the fraction of particles with energy greater than the activation energy is higher in the reaction with steam.
D the reaction with steam goes by an alternative route with lower activation energy.