1. Monitoring and Controlling Chemical Reactions
09/11/2018
09/11/2018
Monitoring and Controlling Chemical Reactions
OCR Gateway 2016 Chemistry topic 5
W Richards
The Weald School

2. C5.1 Monitoring Chemical Reactions
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C5.1 Monitoring Chemical Reactions

3. Recap of the “Mole” (HT only)
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Definition:
A mole of a substance is the relative formula mass of that substance in grams,
For example, 12g of carbon would be 1 mole of carbon...
...and 44g of carbon dioxide (CO2) would be 1 mole etc...
Q. How many moles are the following?
23g of sodium
48g of magnesium
36g of carbon
28g of iron
1 mol
2 mol
3 mol
0.5 mol

4. Concentration (HT only)
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Concentration means “how much of a chemical there is in a fixed volume” and can be measured in g/dm3 or mol/dm3.
A solution of low concentration (“dilute”)
A solution of high concentration (“strong”)

5. Titration
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1) Fill a burette with sodium hydroxide solution of known concentration
2) Accurately measure out 25cm3 of acid and place it in the conical flask
3) Add phenolphthalein indicator to the flask
4) Slowly add the alkali until the mixture in the flask turns pink (down to the next drop!)
5) Repeat until you get similar results

6. Titration Equations (HT only)
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Q. 0.05dm3 of HCl neutralises 0.1dm3 of NaOH of concentration 0.5mol/dm3. What is the concentration of the acid?
The key steps:
1) Look at the equation to compare the numbers of moles:
HCl + NaOH NaCl + H2O
Notice that 1 mole of HCl neutralises 1 mole of NaOH
2) Use this equation:
No. of moles = concentration x volume
So, the number of moles of NaOH is (0.5 x 0.1) = 0.05mol
According to the equation, this will neutralise 0.05mol of HCl
Therefore we have (0.05mol/0.05dm3) = 1mol/dm3 HCl

7. Titration Equations
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0.2dm3 of HCl neutralises 0.1dm3 of NaOH of concentration 0.5mol/dm3. What is the concentration of the acid?
H2SO4 of concentration 0.4mol/dm3 neutralises 0.1dm3 of NaOH of concentration 0.2mol/dm3. How much acid was used?
HCl + NaOH NaCl + H2O
0.25mol/dm3
H2SO4 + 2NaOH Na2SO4 + 2H2O
0.025mol/dm3

8. Molar Volume (HT only)
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An important fact: 1 mole of a gas at room temperature (20OC) and pressure (1 atm) occupies a volume of 24dm3.
What is the volume of 2 moles of oxygen?
What is the volume of 0.25 moles of carbon dioxide?
How many moles would be in 8dm3 of nitrogen?
How much volume would 80g of argon occupy?
A balloon contains 12dm3 of carbon dioxide. What is the mass of this much CO2?
48dm3
6dm3
1/3 mol
48dm3
22g

9. Questions on Concentration (HT only)
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To calculate the concentration of a substance you could use one of these formulae:
Conc. = Mass of substance (g)
Volume of solvent (dm3)
Conc. = Amount of solute (mol)
Calculate, with units, the concentration of the following:
A solution of 10g salt in 1dm3 of water
2mol of hydrochloric acid in 500cm3 of water
10kg of salt in 200dm3 of water
0.5mol of sodium hydroxide in 100cm3 of water

10. Converting concentrations (HT only)
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To convert g/dm3 into mol/dm3 you can use the following formula:
Concentration = Concentration (g/dm3)
(mol/dm3) Molar mass
Convert the following:
0.5mol/dm3 of sodium hydroxide into g/dm3.
2mol/dm3 of HCl into g/dm3.
20g/dm3 of NaCl into mol/dm3.
500g/dm3 of CaCl2 into mol/dm3.

11. Calculating the mass of a product (chem only)
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E.g. what mass of magnesium oxide is produced when 60g of magnesium is burned in air?
IGNORE the oxygen in step 2 – the question doesn’t ask for it
Step 1: READ the equation:
2Mg + O MgO
Step 2: WORK OUT the relative formula masses (Mr):
2Mg = 2 x 24 = MgO = 2 x (24+16) = 80
Step 3: LEARN and APPLY the following 3 points:
48g of Mg makes 80g of MgO
1g of Mg makes 80/48 = 1.66g of MgO
60g of Mg makes 1.66 x 60 = 100g of MgO

12. Calculating the mass of a product using moles (chem only)
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Calculating the mass of a product using moles (chem only)
Let’s try this question again but using moles:
Step 1: READ the equation:
2Mg + O MgO
“2 moles of magnesium + 1 mole of oxygen forms 2 moles of magnesium oxide”
Step 2: WORK OUT the relative formula masses (Mr) of MgO:
2MgO = 2 x (24+16) = 80
Step 3: Apply these steps:
60g of Mg is equal to 1.25 moles (60/48)
Therefore we will make 1.25 moles of magnesium oxide
Therefore we make 100g of MgO (1.25 moles)

13. Mr: 2Al2O3 = 2x((2x27)+(3x16)) = 204 4Al = 4x27 = 108
When water is electrolysed it breaks down into hydrogen and oxygen:
2H2O H2 + O2
What mass of hydrogen is produced by the electrolysis of 6g of water?
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Work out Mr: 2H2O = 2 x ((2x1)+16) = H2 = 2x2 = 4
36g of water produces 4g of hydrogen
So 1g of water produces 4/36 = 0.11g of hydrogen
6g of water will produce (4/36) x 6 = 0.66g of hydrogen
2) What mass of calcium oxide is produced when 10g of calcium burns?
2Ca + O CaO
Mr: 2Ca = 2x40 = CaO = 2 x (40+16) = 112
80g produces 112g so 10g produces (112/80) x 10 = 14g of CaO
3) What mass of aluminium is produced from 100g of aluminium oxide?
2Al2O Al + 3O2
Mr: 2Al2O3 = 2x((2x27)+(3x16)) = Al = 4x27 = 108
204g produces 108g so 100g produces (108/204) x 100 = 52.9g of Al2O3

14. So mass of product = (4/36) x 6g = 0.66g of hydrogen
Another method
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Try using this equation:
Mass of product IN GRAMMES
Mass of reactant IN GRAMMES
Mr of product
Mr of reactant
Q. When water is electrolysed it breaks down into hydrogen and oxygen:
2H2O H2 + O2
What mass of hydrogen is produced by the electrolysis of 6g of water?
Mass of product IN GRAMMES 4 6g 36
So mass of product = (4/36) x 6g = 0.66g of hydrogen

15. Percentage Yield Percentage yield = Actual yield Predicted yield
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Percentage yield =
Actual yield
Predicted yield
X 100%
Some example questions:
The predicted yield of an experiment to make salt was 10g. If 7g was made what is the percentage yield?
Dave is trying to make water. If he predicts to make 15g but only makes 2g what is the percentage yield?
Sarah performs an experiment and has a percentage yield of 33%. If she made 50g what was she predicted to make?
70%
13%
150g

16. Percentage Yield (harder)
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Theoretical yield = the amount of product that should be made as calculated from the masses of atoms
Actual yield = what was actually produced in a reaction
Percentage yield = actual yield (in g)
theoretical yield
Example question:
65g of zinc reacts with 73g of hydrochloric acid and produces 102g of zinc chloride. What is the percentage yield?
Zn + 2HCl ZnCl2 + H2
The theoretical yield is 136g (using Cl = 35.5) so the % yield is 75%

17. Total masses of products
Atom Economy
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Percentage atom economy =
Relative formula mass of useful product
Total masses of products
Calculate the atom economies of the following:
Converting ethanol into ethene (ethene is the useful bit):
C2H5OH C2H4 + H20
Making zinc chloride from zinc and hydrochloric acid:
Zn + 2HCl ZnCl2 + H2
61%
99%

18. Choosing a Reaction Pathway (HT only)
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Hi. We’re trying to make some chemicals and need to decide which reaction pathway to use. What factors do we need to consider when deciding which reaction pathway to use?
What is the atom economy of each pathway?
What is the % yield of each pathway?
What rate does each reaction happen at?
What is the equilibrium position of each pathway?
How useful are the by-products of each reaction?

19. C5.2 Controlling Reactions
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20. Measuring the Rate of Reaction
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Two common methods:
Table of results:
Time/s
Mass or reactants/g OR volume of gas formed/cm3

21. Measuring the Rate of Reaction
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Another common method using the formation of a precipitate:
Table of results:
Concentration of acid or sodium thiosulfate
Time taken for cross to disappear

22. Amount of gas formed/cm3
Example Results
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Time/s
Amount of gas formed/cm3 10 20 30 40 50 60 70 80 90
100
110
120 37 44 56 63 65

23. Rate of reaction graph
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Amount of product formed/ amount of reactant used up
Slower rate of reaction here due to reactants being used up
Fast rate of reaction here
Slower reaction
Time/s
Rate of reaction = amount of product formed/reactant used up
(in g/s or cm3/s) time

24. Using Tangents to calculate rate of reaction (HT )
09/11/2018
Q. Can we work out the rate of reaction at a specific time using the graph?
Amount of product formed
Diff. in y
For example, what is the rate of reaction here?
Diff. in x
Step 1 – draw a tangent at that point
Step 2 – calculate the gradient of the tangent using:
gradient = difference in y / difference in x
Time/s

25. Collision Theory
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Chemical reactions will only happen if particles ________ with each other with enough ________.

26. Rates of Reaction
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Oh no! Here comes another one and it’s got more energy…
Here comes another one. Look at how slow it’s going…
No effect! It didn’t have enough energy!
Hi. I’m Mike Marble. I’m about to have some acid poured onto me. Let’s see what happens…
It missed!
Here comes an acid particle…

27. Rates of Reaction and Collision Theory
09/11/2018
Chemical reactions occur when different atoms or molecules _____ with enough energy (the “________ Energy”):
Basically, the more collisions we get the _______ the reaction goes. The rate at which the reaction happens depends on four things:
If a ______ has been used
The _______ the reactants are under
Their concentration
Their surface area
The ______ of the reactants – this is the biggest factor as it affects the number of collisions AND the energy in each collision
Words – activation, quicker, pressure, temperature, collide, catalyst

28. Energy level diagrams Energy level
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Energy level
Activation energy – the energy needed to start the reaction
Energy given out by reaction
Reaction progress

29. New pathway with a catalyst
How Catalysts Work
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Catalysts work by providing a different pathway for the reaction – one that needs a lower activation energy:
Activation energy – the energy needed to start the reaction
Energy level
New pathway with a catalyst
Energy given out by reaction
Reaction progress

30. Words – different, enzyme, speed up, activation, used up, cheaper
Catalyst Summary
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Catalysts are used to ____ __ a reaction to increase the rate at which a product is made or to make a process ________. They are not normally ___ __ in a reaction and they are reaction-specific (i.e. different reactions need _________ catalysts). Catalysts work by lowering the _______ energy needed for a reaction to happen.
An example of a catalyst is a biological catalyst called an _______. These catalyts are used to make alcoholic drinks.
Words – different, enzyme, speed up, activation, used up, cheaper

31. C5.3 Equilibria
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32. Exothermic and Endothermic Reactions recap
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Photosynthesis
Can you put these reactions into groups?
Burning
Self-heating cans
Hand warmer packs
Cooling packs

33. Exothermic and Endothermic Reactions
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Exothermic reactions
Ones that GIVE OUT energy to the surroundings
Endothermic reactions
Ones that TAKE IN energy from the surroundings

34. Reversible Reactions
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Some chemical reactions are reversible. In other words, they can go in either direction – the reactants will make the products and the products will then make the reactants again: A + B C D
heat
e.g. Ammonium chloride
Ammonia + hydrogen chloride
NH4Cl
NH3 + HCl
cool
The direction of the reaction can be affected by changing the conditions

35. Reversible Reactions and energy changes
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If a reaction is EXOTHERMIC in one direction what must it be in the opposite direction?
endothermic
Q. Which direction would you push the reaction in if you heated it up? A + B C D
exothermic
If a reaction is endothermic, it wants to take in energy. Therefore, by heating it, you help it.
For example, consider copper sulphate:
Hydrated copper sulphate (blue)
Anhydrous copper sulphate (white)
+ Heat
+ Water
endothermic
exothermic
CuSO4 + H2O
CuSO4.5H2O

36. Reversible Reactions and Equilibrium
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When a reversible reaction occurs in a CLOSED SYSTEM (i.e. no reactants are added or taken away) an EQUILIBRIUM is achieved – in other words, the reaction goes at the same rate in both directions (a “dynamic equilibrium”): A + B C D
Higher Tier only - What happens if we change the conditions of an equilibrium? What does it do to a reaction?
Henry Le Chatelier,
We can use Le Chatelier’s Principle to work out how changes in concentration, temperature and pressure will affect an equilibrium. My principle is:
“If the system is at equilibrium and a change is made, the system responds to counteract the change.” Here’s the idea:

37. Le Chatelier’s Principle 1 – Concentration (HT only)
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Consider a reaction where reactants A and B make products C and D: A + B C D
If the concentration of reactant A was increased, the system is no longer at equilibrium. More products will be formed to restore the equilibrium (and vice versa).

38. Le Chatelier’s Principle 2 – Temperature (HT only)
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Increasing the temperature of a system basically means giving the system energy, i.e. you will help an endothermic reaction. Unfortunately, this gets expensive!
Endothermic reactions
Increased temperature:
Decreased temperature:
Exothermic reactions
Increased temperature:
Decreased temperature: A + B C D A + B C D
More products
Less products A + B C D A + B C D
Less products
More products

39. Le Chatelier’s Principle 3 – Pressure (HT only)
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Consider a reaction where nitrogen and hydrogen are used to make ammonia:
Q. How many molecules are on this side of the equation?
How many are on this side?
Nitrogen + hydrogen Ammonia
N2 + 3H NH3
A. 4
A. 2
An increase in pressure will push the reaction towards the side with the lower number of molecules (i.e. you’ll make more ammonia). A decrease in pressure will make more hydrogen and nitrogen.

40. Summary
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Consider a reaction where reactants A and B make products C and D: A + B C D
So, we want to make lots of product BUT:
High temperatures and high pressures are expensive…
…and they may cause equipment to break
We could also use a catalyst to increase the rate of a reaction (but not the yield)
We need to choose the right conditions to get a good yield but not make it too expensive!