1. Edexcel Topic – Rates of reaction and energy changes
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PiXL KnowIT!
GCSE Chemistry
Edexcel Topic – Rates of reaction and energy changes   
© Copyright The PiXL Club Ltd, 2017
This resource is strictly for the use of member schools for as long as they remain members of The PiXL Club. It may not be copied, sold nor transferred to a third party or used by the school after membership ceases. Until such time it may be freely used within the member school.
All opinions and contributions are those of the authors. The contents of this resource are not connected with nor endorsed by any other company, organisation or institution.
The PiXL Club Ltd, Company number

2. Overview Rates of reaction and energy changes
Rate of reaction
Practical methods
Calculating rates of reaction
Collision theory
Effects of changes in temperature, concentration, surface area and pressure on rates of reaction
Catalysts
Heat energy changes in chemical reactions
Types of change that cause a change in heat energy
Exothermic reactions
Endothermic reactions
Overall heat energy change for a reaction
Calculate energy change (HT)
Activation energy
Energy profiles

3. LearnIT! KnowIT! Rate of reaction Part 1 Practical methods
Calculating rates
of reactions

4. Rate of reactions part 1 – Calculating rates of reactions
The rate of a chemical reaction can be found by measuring the quantity of a reactant used or the quantity of product formed over time.
mean rate of reaction = quantity of reactant used
time taken
mean rate of reaction = quantity of product formed
The quantity of reactant or product can be measured by the mass in grams or by a volume in cm3.
The units of rate of reaction may be given as g/s or cm3/s.

5. Rate of reactions part 1 – Calculating rates of reactions
Worked example 1
25cm3 of carbon dioxide was given off in the first 2 seconds of a reaction. Calculate the mean rate of reaction and give the units.
Mean rate of reaction = quantity of product formed
time taken
Mean rate of reaction = cm3
2 s
Mean rate of reaction = cm3/s
Worked example 2 (Higher Tier)
The above reaction was carried out again. The new results showed that 2 dm3 of carbon dioxide was released in 200 seconds. Calculate the mean rate of reaction in mol/dm3
(1 mole of any gas occupies 24 dm3 at STP)
Moles of carbon dioxide = 2 dm3 = 0.83 moles
24 dm3
Mean rate of reaction = moles = mol/s
200 s
Slope A will have a greater rate of reaction as it is steeper.

6. QuestionIT! Rate of reaction Part 1 Practical methods
Calculating rates
of reactions

7. Calculating rates of reactions – QuestionIT
State two ways of finding the rate of reaction.
State two units of rate of reaction. (HT: state 3)
State two ways of measuring the quantity of reactant or product.

8. Calculating rates of reactions – QuestionIT
A student carries out an experiment reacting hydrochloric acid (HCl) with calcium carbonate (CaCO3) to give calcium chloride (CaCl2) carbon dioxide and water. Write the balanced symbol equation for this reaction.
The student collects 50 cm3 of carbon dioxide gas in 10 seconds. What is the rate of reaction? Include the units.

9. Calculating rates of reactions – QuestionIT
(HT only) The student repeats the experiment again, this time they find the mass of the carbon dioxide collected. They collect 11 g of carbon dioxide in 10 seconds. Calculate the rate of reaction in mol/s.
(HT only) What mass of carbon dioxide are they collecting per second if the rate of reaction is mol/s?

10. AnswerIT! Rate of reaction Part 1 Practical methods Calculating rates
of reactions

11. Calculating rates of reactions – QuestionIT
State two ways of finding the rate of reaction.
Measuring the quantity of reactant used or product formed.
State two units of rate of reaction. (HT: state 3)
g/s; cm3/s; (mol/s)
State two ways of measuring the quantity of reactant or product.
Mass in grams or volume cm3

12. Calculating rates of reactions – AnswerIT
A student carries out an experiment reacting hydrochloric acid (HCl) with calcium carbonate (CaCO3) to give calcium chloride (CaCl2) carbon dioxide and water. Write the balanced symbol equation for this reaction.
CaCO3 (s) HCl (aq)  CaCl2 (aq) + CO2 (g) + H2O (l)
The student collects 50 cm3 of carbon dioxide gas in 10 seconds. What is the rate of reaction? Include the units.
rate of reaction = volume of gas collected = 50
time taken 10
5 cm3/s

13. Factors which affect the rates of reactions – AnswerIT
(HT only) The student repeats the experiment again, this time they find the mass of the carbon dioxide collected. They collect 11 g of carbon dioxide in 10 seconds. Calculate the rate of reaction in mol/s.
11g/44g = 0.25 moles of carbon dioxide
so 0.25 moles/10 seconds
= mol/s
(HT only) What mass of carbon dioxide are they collecting per second if the rate of reaction is mol/s
0.075 moles of CO2 is 44 x so 3.3 g/s

14. LearnIT! KnowIT! Rate of reaction Part 2 Collision theory
Effects of changes in temperature, concentration, surface area and pressure on rates of reaction.

15. Rates of reactions part 2 – Factors which affect rates of reactions
Factors which affect the rates of chemical reactions include:
The concentrations of reactants in solution
The pressure of reacting gases
The surface area of solid reactants
The temperature
The presence of a catalyst
Collision theory explains how these factors affect rates of reactions. According to this theory, chemical reactions can occur only when reacting particles collide with each other and with sufficient energy. The minimum amount of energy that particles must have to react is called the activation energy.
The explanations on the next slide are very important and you will need to use them accurately in the exams to gain credit.

16. Rates of reactions part 2 – Factors which affect rates of reactions
Increasing the concentration of reactants in solution increases the frequency of collisions, and so increases the rate of reaction.
Increasing the pressure of reacting gases increases the frequency of collisions, and so increases the rate of reaction.
Increasing the surface area of solid reactants increases the frequency of collisions, and so increases the rate of reaction.
Increasing the temperature increases the frequency of collisions and makes the collisions more energetic, and so increases the rate of reaction.

17. Rates of reactions part 2 – Factors which affect rates of reactions
Increasing the surface area, temperature or using a catalyst will increase the rate of reaction so the gradient of the line increases from B to A. Finishing at the same final volume of gas.
Increasing the concentration provides more reacting particles therefore more product. So the gradient of the line increases and the final volume of gas increases.

18. QuestionIT! Rate of reaction Part 2 Collision theory
Effects of changes in temperature, concentration, surface area and pressure on rates of reaction.

19. Factors which affect rates of reactions – QuestionIT
What is meant by the term ‘collision theory’?
What happens to the gradient of a line if the rate of reaction is increased?
According to collision theory, chemical reactions can only occur when…

20. Factors which affect rates of reactions – QuestionIT
4. Other than concentration, give three factors that affect the rate of reaction.
Draw a labelled graph to show how changing any one of these factors may affect the rate of reaction. Include the line before and after the change.

21. Factors which affect the rates of reactions – QuestionIT
6. The graph below shows how the reaction is affected when the concentration of hydrochloric acid is doubled when reacting with excess magnesium. Explain why the amount of hydrogen gas doubles and why the rate of reaction doubles. Use collision theory in your response.

22. AnswerIT! Rate of reaction Part 2 Collision theory
Effects of changes in temperature, concentration, surface area and pressure on rates of reaction.

23. Factors which affect rates of reactions – QuestionIT
What is meant by the term ‘collision theory’?
Explains how reactions occur when particles collide, and how rates of reaction are increased when the frequency and/ or energy of collisions is increased.
What happens to the gradient of a line if the rate of reaction is increased?
Becomes steeper.
According to collision theory, chemical reactions can only occur when…
reacting particles collide with each other with sufficient energy.

24. Factors which affect rates of reactions – AnswerIT
4. Other than concentration, give three factors that affect the rate of reaction.
Any from: temperature, surface area, pressure and a catalyst
5. Draw a labelled graph to show how changing any one of these factors may affect the rate of reaction. Include the line before and after the change.

25. Factors which affect the rates of reactions – AnswerIT
6. The graph below shows how the reaction is affected when the concentration of hydrochloric acid is doubled when reacting with excess magnesium. Explain why the amount of hydrogen gas doubles and why the rate of reaction doubles. Use collision theory in your response.
If concentration of acid is doubled then there are twice the number of collisions with magnesium atoms.
There will be twice the number of successful collisions so rate of reaction doubles.
As there are twice as many acid particles (and the magnesium is in excess) there will be twice the volume of (hydrogen) gas released

26. LearnIT!
KnowIT!
Rate of reaction
Part 3
Catalysts

27. Rates of reactions part 3 – Factors which affect rates of reactions - catalysts
Catalysts speed up the rate of chemical reactions without altering the products of the reaction, being itself unchanged chemically and in mass at the end of the reaction.
This means that the catalyst is still there, unchanged, at the end of the reaction.
Enzymes are biological catalysts. Enzymes are used in the production of alcoholic drinks by fermentation.
Catalysts increase the rate of reaction by providing a different pathway for the reaction that has a lower activation energy.
A reaction profile for a catalysed reaction can be drawn as shown on the right.
You should be able to explain catalytic action in terms of activation energy. For example, “from the reaction profile I can see that the catalyst lowers the activation energy”.

28. QuestionIT!
Rate of reaction
Part 3
Catalysts

29. Factors which affect rates of reactions - catalysts– QuestionIT
What is a catalyst?
What is a biological catalyst?
Name a process which makes use of biological catalysts.
How does a catalyst alter the rate of reaction?

30. Factors which affect rates of reactions - catalysts– QuestionIT
A student carried out three reactions to investigate how quickly oxygen gas was given off by decomposing hydrogen peroxide.
2H2O2  O H2O
Each time she changed the chemical she was adding to see if it was a catalyst.
Here are her results:
Which chemical was a catalyst? Explain your answer.
Chemical
Time taken to collect 50 cm3 in seconds
Without chemical 33 A B
No oxygen given off C 15

31. Factors which affect rates of reactions - catalysts– QuestionIT
Draw the reaction profile for 2H2O2  O H2O with and without a catalyst and label the activation energies.

32. AnswerIT!
Rate of reaction
Part 3
Catalysts

33. Factors which affect rates of reactions - catalysts– QuestionIT
What is a catalyst?
Speeds up the rate of reaction; does not alter products of reaction; unchanged chemically and in mass at end of reaction.
What is a biological catalyst?
Enzyme.
Name a process which makes use of biological catalysts.
Fermentation/ making alcoholic drinks.
How does a catalyst alter the rate of reaction?
Reduces activation energy of reaction.

34. Factors which affect rates of reactions - catalysts– AnswerIT
5. A student carried out three reactions to investigate how quickly oxygen gas was given off by decomposing hydrogen peroxide.
2H2O2  O H2O
Each time she changed the chemical she was adding to see if it was a catalyst. Here are her results.
Which chemical was a catalyst? Explain your answer.
C, the time taken for the reaction was shorter.
Chemical
Time taken to collect 50 cm3 in seconds
Without chemical 33 A B
No oxygen given off C 15

35. Factors which affect rates of reactions - catalysts– AnswerIT
6. Draw the reaction profile for 2H2O2  O H2O with and without a catalyst and label the activation energies.

36. LearnIT! KnowIT! Energy Changes - part 1 Exothermic reactions
Endothermic reactions

37. Exothermic and endothermic reactions part 1 – Exothermic reactions
Changes in heat energy accompany the following changes:
Salts dissolving in water
Neutralisation reactions
Displacement reactions
Precipitation reactions.
When these reactions take place in solution, temperature changes can be measured to monitor these heat changes.

38. Exothermic and endothermic reactions part 1 – Exothermic reactions
Energy is conserved in chemical reactions. The amount of energy in the Universe at the end of a chemical reaction is the same as before the reaction takes place.
H2 (g) + Cl2 (g)  2HCl (g)
In the above reaction, energy is given out and it gets hotter.
An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases – “it gets hotter”.
The two HCl molecules made will not hold as much energy as the H2 and Cl2 molecules at the start, so the spare energy is released as heat.

39. Exothermic and endothermic reactions part 1 – Endothermic reactions
We have already learnt that energy is conserved in chemical reactions.
2CH3COOH(aq) + Na2CO3(s)  2CH3COONa(aq) + CO2(g) + H2O (l)
In the above reaction, energy is taken in- it gets colder.
An endothermic reaction is one that takes energy from the surroundings so the temperature of the surroundings decreases – “it gets colder”.
The sodium ethanoate, carbon dioxide and water molecules made will hold more energy than the ethanoic acid and sodium carbonate molecules at the start, so the energy needed is
taken in as heat.
Other examples of endothermic reactions are:
Thermal decomposition
Sports injury packs

40. QuestionIT! Energy Changes Part 1 Exothermic reactions
Endothermic reactions

41. Energy Changes part 1 – QuestionIT
Why is the amount of energy in the universe at the end of a chemical reaction the same as before the reaction takes place?
What is an exothermic reaction?
Give an example of an exothermic reaction.
What is an endothermic reaction?
Give an example of an endothermic reaction.
Give an everyday use of endothermic reactions.

42. Energy Changes part 1 – QuestionIT
How would you know if an exothermic reaction had occurred?
How would you know if an endothermic reaction had occurred?
Below is a table of results for four reactions, the temperatures before and after the reactions are also given.
a/ Which reaction is endothermic? Explain how you know this.
b/ Which reaction is exothermic? Explain how you know this.
Reaction
Temperature at start OC
Temperature at end OC A 22 28 B 20 C 21 12 D 25

43. AnswerIT! Energy Changes Part 1 Exothermic reactions
Endothermic reactions

44. Energy Changes part 1 – QuestionIT
Why is the amount of energy in the universe at the end of a chemical reaction the same as before the reaction takes place?
Energy is conserved in chemical reactions.
What is an exothermic reaction?
Heat energy is given out.
Give an example of an exothermic reaction.
Combustion/ oxidation/ neutralisation.
What is an endothermic reaction?
Heat energy is taken in.
Give an example of an endothermic reaction.
Thermal decomposition
Give an everyday use of endothermic reactions.
Sports injury packs.

45. Energy Changes part 1 – AnswerIT
How would you know if an exothermic reaction had occurred?
The reaction would give out heat/get warmer/ temperature increase.
How would you know if an endothermic reaction had occurred?
The reaction would take in heat/get colder/ temperature decrease.

46. Energy Changes part 1 – AnswerIT
Below is a table of results for four reactions, the temperatures before and after the reactions are also given.
a/ Which reaction is endothermic? Explain how you know this.
Reaction C, it gets colder/temperature falls.
b/ Which reaction is exothermic? Explain how you know this.
Reaction A, it gets warmer/temperature increases.
Reaction
Temperature at start OC
Temperature at end OC A 22 28 B 20 C 21 12 D 25

47. LearnIT! KnowIT! Energy Changes part 2
Overall heat energy change for a reaction
Calculate energy change (HT)

48. The energy change of reactions
Breaking bonds is endothermic
Making bonds is exothermic
The overall heat energy change for a reaction is:
Exothermic if more heat is released in forming bonds in the products than is required to break bonds in the reactants.
Endothermic if less heat is released in forming the bonds in the products than is required in breaking bonds in the reactants.

49. The energy change of reactions (HT only)
For the reaction of methane with oxygen we can write out the balanced symbol equation:
CH O2  CO H2O
We can draw out the bonds between the atoms e.g.
Each line represents a bond, two lines represent a double bond e.g. in the oxygen molecule.

50. The energy change of reactions (HT only)
During a chemical reaction:
Energy must be supplied to break bonds in the reactants
Energy is released to form bonds in the products.
The energy needed to break bonds and the energy released when bonds are formed can be calculated from bond energies.
This means that 411kJ/mol of energy needs to be put in to break the carbon-hydrogen bond.
It also means that 459 kJ/mol is given out when the oxygen hydrogen bond is made in water.
Bond
Bond Energy kJ/mol
C-H
411
O=O
494
C=O
799
O-H
459

51. The energy change of reactions (HT only)
The difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed is the overall energy change of the reaction.
Worked example
For the reactants
For the products
There are four C-H bonds so
4 x 411kJ/mol = 1,644kJ/mol
There are two C=O bonds so
2 x 799kJ/mol = 1,598kJ/mol
There are two O=O bonds so
2 x 494kJ/mol = 988kJ/mol
There are four O-H bonds so
4 x 459kJ/mol = 1,836kJ/mol
The sum of these is
the energy supplied to break the bonds in the reactants
1, = 2,632kJ/mol
the energy released when bonds in the products are formed
1, ,836 = 3,434kJ/mol

52. The energy change of reactions (HT only)
Overall energy change = energy needed to – energy released as break the bonds bonds are made
= ,632kJ/mol ,434kJ/mol
Overall energy change = kJ/mol
This is an exothermic reaction, so the sum of the difference between the calculations is negative. For an endothermic reaction it would be positive.
Students should be able to calculate the energy transferred in chemical reactions using bond energies supplied.

53. QuestionIT! Energy Changes - part 2
Overall heat energy change for a reaction
Calculate energy change (HT)

54. Energy Changes part 2 QuestionIT
During a chemical reaction energy is needed to…
In a chemical reaction energy is released when…
What are bond energies used to calculate?
How do you calculate the overall energy change of a reaction?
Is bond making exothermic or endothermic? Why?
Is bond breaking exothermic or endothermic? Why?

55. The energy change of reactions (HT only) QuestionIT
The bond energy between a hydrogen and a nitrogen atom is 386 kJ/mol, the bond energy between two hydrogen atoms is 432 kJ/mol and the bond energy between two nitrogen atoms is 942kJ/mol. Using these bond energies, calculate the overall energy change for the following reaction.
2NH3  N H2
Is the reaction exothermic or endothermic? Explain your answer.

56. AnswerIT! Energy Changes - part 2
Overall heat energy change for a reaction
Calculate energy change (HT)

57. Energy Changes part 2 QuestionIT
During a chemical reaction energy is needed to…
break bonds in the reactants.
In a chemical reaction energy is released when…
bonds in the products are formed.
What are bond energies used to calculate?
The energy change in a reaction.
How do you calculate the overall energy change of a reaction?
Difference between: sum of energy needed to break bonds in the reactants and the sum of the energy released when the products are formed.
Is bond making exothermic or endothermic? Why?
Exothermic; energy is given out.
Is bond breaking exothermic or endothermic? Why?
Endothermic; energy is taken in.

58. The energy change of reactions (HT only) AnswerIT
The bond energy between a hydrogen and a nitrogen atom is 386 kJ/mol, the bond energy between two hydrogen atoms is 432 kJ/mol and the bond energy between two nitrogen atoms is 942kJ/mol. Using these bond energies calculate the overall energy change for the following reaction.
2NH3  N H2
For the reactants
There are three nitrogen to hydrogen bonds in each molecule and there are two molecules so
386kJ/mol x 6 = 2,316kJ/mol
For the products
There are three hydrogen to hydrogen bonds (432kJ/mol) and one nitrogen to nitrogen triple bond so
3 x 432kJ/mol + 942kJ/mol
= 2,238kJ/mol
Overall energy change = energy needed to – energy released as break the bonds bonds are made
2,316kJ/mol ,238kJ/mol
Overall energy change = + 78kJ/mol
Is the reaction exothermic or endothermic? Explain your answer.
The reaction is endothermic as the overall energy change is positive.

59. LearnIT! KnowIT! Energy Changes Part 3 Activation energy
Reaction profiles

60. Energy Changes part 2 – Reaction profiles
Chemical reactions can only occur when reacting particles collide with each other with sufficient energy.
The minimum amount of energy that particles must have to react is called the activation energy
You have given a reaction its activation energy when you have used a lit spill to light a Bunsen burner. Without the activation energy from the lit spill, the methane gas and oxygen in the air will not combust and release the heat energy.
When we look at this reaction we see the following.
CH O2  CO H2O

61. Energy Changes part 2 – Reaction profiles
CH O2  CO H2O
We know this reaction is exothermic; this means energy is released. So the CH4 and 2O2 , the reactants, must have more energy than the products, CO2 and 2H2O
We can show this as a reaction profile. On it we need to include the formulae or names of the products and reactants. We also need to show the relative energies of the reactants and products e.g.

62. Energy Changes part 2 – Reaction profiles
More information needs to be included in the reaction profile.
This will show the activation energy of the reaction. It is shown by a curved line rising above the reactants energy.

63. Energy Changes part 2 – Reaction profiles
We can now see the overall change in energy within the reaction.
The products have less energy than the reactants. This will have been lost as heat as the reaction is exothermic.

64. Energy Changes part 2 – Reaction profiles
We saw earlier that the following reaction was endothermic:
2CH3COOH(aq) + Na2CO3(s)  2CH3COONa(aq) + CO2(g) + H2O (l)
What would the reaction profile look like for this reaction?
We can see that the products have more energy than the reactants. This will have been taken in as heat energy; it feels colder.

65. QuestionIT!
Energy Changes
Part 3
Activation energy
Reaction profiles

66. Energy Changes part 2 QuestionIT
What needs to happen for a reaction to take place?
What is meant by the term activation energy?
What can a reaction profile be used to show?
What is an exothermic reaction?
What is an endothermic reaction?

67. Energy Changes part 2 QuestionIT
On the reaction profile below, what is shown by the letters? A B C D

68. AnswerIT!
Energy Changes
Part 3
Activation energy
Reaction profiles

69. Energy Changes part 2 QuestionIT
What needs to happen for a reaction to take place?
Reacting particles collide with sufficient energy.
What is meant by the term activation energy?
Minimum amount of energy particles need in order to react.
What can a reaction profile be used to show?
Relative energies of reactants and products, activation energy and energy change of a reaction.
What is an exothermic reaction?
Heat energy given out.
What is an endothermic reaction?
Heat energy taken in.

70. Energy Changes part 2 AnswerIT
On the reaction profile below what is shown by the letters?
A Reactants
B Products
C Activation energy
D Overall energy change/that the reaction is exothermic