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Flashcards in Chemical Equilibrium Deck (63)
1

What is a reversible reaction?

One in which the products can react again to reform the reactants A + B C + D

2

What is the name of this reaction: A + B = C + D

Forwards reaction

3

What is the name of this reaction: C + D = A + B

Backwards reaction

4

What is a closed system?

One where no reactants or products can escape

5

When is a dynamic equilibrium set up?

When a reversible reaction takes place in a closed system and eventually the rates of the forwards and backwards reactions will be the same

6

What does dynamic mean?

The forwards and backwards reactions are continuing and the rates of reaction are constant

7

What does equilibrium mean?

When the total amount of the species present remains the same

8

What is Le Chatelier's principle?

If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change

9

What happens during the reversible reaction of the dehydration of copper sulfate?

If you heat the blue copper sulfate crystals, it forms white anhydrous copper sulfate is formed and water is given off Anhydrous copper sulfate is used to test for the presence of water - if you add water to the white solid, it turns blue and heat is given off The original change has been exactly reversed. Even the heat has been given out again.

10

What is the equation of the dehydration of copper sulfate?

CuSO4.5H20 = CuSO4 + 5H2O

11

Is the dehydration of copper sulfate exothermic or endothermic?

Endothermic

12

What happens when you heat ammonium chloride?

If you heat ammonium chloride, the white crystals disappear form the bottom of the tube and reappear further up

Heating ammonium chloride splits it into the colourless gases ammonia and hydrogen chloride (sublimation)

These gases recombine further up the tube where it is cooler

The reaction reverses when the conditions are changed from hot to cool

13

What is the equation of the heating of ammonium chloride?

NH4Cl = NH3 + HCl

14

Is the heating of ammonium chloride exothermic or endothermic?

Endothermic

15

What is the effect of changing the concentration of the substances present (by adding more A (or B)) to increase the product?

This will favour the forwards reaction as the position of equilibrium will shift to counteract the change. The reaction will remove the extra A by producing more C and D.

16

What is the effect of changing the concentration of the substances present (by continuously removing C (or D)) to increase the product?

This will favour the forwards reaction as the position of equilibrium will shift to counteract the change. The reaction will react more A and B to replace the C that is being removed from the equilibrium.

17

How would you favour the exothermic reaction by changing the temperature?

Reduce the temperature. Exothermic reactions release heat into their surroundings, increasing the temperature of their surroundings. Therefore, if you reduce the temperature of the surroundings, the position of equilibrium will shift to counteract the change, by favouring the exothermic reaction, as this is what will increase the temperature.

18

How would you favour the endothermic reaction by changing the temperature?

Increase the temperature. Endothermic reactions absorb heat from their surroundings, decreasing the temperature of their surroundings. Therefore, if you increase the temperature of the surroundings, the position of equilibrium will shift to counteract the change, by favouring the endothermic reaction as this is what will decrease the temperature.

19

How would you favour the side with less moles by changing the pressure?

Reduce the pressure. The position of equilibrium will shift to counteract the change, by favouring the side with less moles, to produce more molecules, increasing the pressure.

20

How would you favour the side with more moles by changing the pressure?

Increase the pressure. The position of equilibrium will shift to counteract the change, by favouring the side with more moles, to produce less molecules, decreasing the pressure.

21

What is the equation for the Haber Process?

N2(g) + 3H2(g) = 2NH3(g)

DeltaH = -92kJ/mol

22

Is the Haber Process exothermic or endothermic?

Exothermic

23

Describe the Haber Process

N2 from the air + H2 from methane → Compressor (200atm) → Reaction Chamber with Iron Catalyst, ammonia gas formed in the reaction (450degrees) → Heat Exchanger - where heat is recycled → Condenser - ammonia gas turns into liquid ammonia and is then stored and the unreacted H2 and N2 gases go back to the reaction chamber and are recycled as they do not condense.

24

Why is the nitrogen and ammonia recycled?

Only about 15% of the nitrogen and ammonia combine each time so must be recycled.

25

What are the uses of ammonia?

Making fertilisers Making nitric acid

26

What is the reason for the iron catalyst in the Haber Process?

It speeds up both the forwards and backwards reaction Does not affect the yield of ammonia

27

What are the conditions needed for the Haber Process

Iron catalyst

200 atm

450 degrees

1 volume nitrogen to 3 volumes hydrogen

28

What is the reason for the pressure (200 atm) in the Haber Process?

There are 4 molecules on the LHS and 2 on the RHS. If you have a high pressure, the position of equilibrium will shift to favour the forwards reaction, as this will produce more ammonia, reducing the pressure, and increasing the yield of ammonia A high pressure also increases the rate of reaction But a higher pressure than 200atm is not used as it is expensive and dangerous

29

What is the reason for the temperature (450degrees) in the Haber Process?

The forwards reaction is exothermic, so releases heat into their surroundings, increasing the temperature of the surroundings. By having a low temperature, the position of equilibrium will shift to favour the forwards reaction, as it will increase the heat of the surroundings But a low temperature would greatly reduce the rate of the reaction, so a compromise temperature of 450 degrees is used which gives an acceptable rate of reaction

30

What is the reason for the constant removal of ammonia in the Haber Process?

It is removed by cooling and condensing it. This will decrease the concentration of gaseous ammonia, favouring the forwards reaction, as the position of equilibrium will shift to counteract the change to make more ammonia.

31

What is the reason for the proportions of nitrogen and hydrogen?

because an excess would clutter the reaction vessel with molecules which wouldn't have anything to react with.

32

What does the Haber Process make? Out of what?

Ammonia, out of nitrogen from the air and hydrogen produced from natural gas.

33

What does the Contact Process make?

Sulfuric acid

34

What is Stage 1 of the Contact Process?

Making sulfuric dioxide: Burn sulfur in air - S(s) + O2(g) = SO2(g) OR heat sulfide ores strongly in air - 4FeS2(s) + 11O2(g) = 2FeO3 + 8SO2(g)

35

What is Stage 2 of the Contact Process?

Making sulfur trioxide: Oxidise sulfur dioxide - 2SO2(g) + O2(g) 2SO3(g)

36

Conditions for stage 2 of the Contact process?

450°C

1-2 atm

V2O5 catalyst (vanadium oxide)

37

Why is an excess of air used in stage 2 of the Contact Process?

Because it is important to make sure as much sulfur dioxide as possible is converted into sulfur trioxide. Having sulfur dioxide left over at the end of the reaction is wasteful and could possible cause dangerous pollution.

38

Why is the temperature 450°C in step 2 of the haber process?

Even though the forwards reaction is exothermic so according to Le Chatelier's principle a low temperature should be used, as with the Haber process with a low temperature the reaction would be very slow so 450°C is a compromise

39

Why can't Stage 3 of the Contact Process can't be done by just adding water to sulfur trioxide?

Sulfuric acid can't be made in this way because the reaction is so uncontrollable that it creates a fog of sulfuric acid

40

What is Stage 3 of the Contact Process?

Making sulfuric acid: Dissolve the sulfur trioxide in concentrated sulfuric acid - SO3(g) + H2SO4(l) = H2S2O7(l) - Oleum THEN dilute the Oleum with water - H2S2O7(l) + H2O(l) = 2H2SO4l)

41

Which is the equilibrium reaction in the Contact Process? Is the forwards reaction exothermic or endothermic?

Stage 2 (Making sulfur trioxide) 2SO2(g) + O2(g) 2SO3 (g) DeltaH = -196kJ/mol Exothermic

42

What are the conditions needed for the equilibrium in the Contact Process?

V2O5 catalyst 450degrees 1-2 atm Excess oxygen

43

What is the reason for the V2O5 catalyst in the Contact Process?

Increases the rate of the forwards and backwards reaction Does not affect the position of equilibrium or the yield of sulfuric acid

44

What is the reason for the temperature (450) in the Contact Process?

The forwards reaction is exothermic, so releases heat into their surroundings, increasing the temperature of the surroundings. By having a low temperature, the position of equilibrium will shift to favour the forwards reaction, as it will increase the heat of the surroundings But a low temperature would greatly reduce the rate of the reaction, so a compromise temperature of 450 degrees is used which gives an acceptable rate of reaction

45

What is the reason for the quite high pressure of 1-2atm in the Contact Process?

There are 3 molecules on the LHS and 2 on the RHS. Therefore a higher pressure would favour the forwards reaction and increase the yield as well as increasing the rate of reaction However, it is cheaper and safer to use a lower pressure (as you would need much stronger and more expensive pipes) and you still get a 99.5% conversion so a higher pressure is not worth the expense.

46

What is the reason for having excess oxygen in the Contact Process?

It is important to make sure as much sulfur dioxide as possible is converted to sulfur trioxide as sulfur dioxide causes dangerous pollution

47

What are the uses of sulfuric acid?

Sulfate containing fertilisers

Making paints

Making detergents

Laboratory acids

48

What are 'closed' conditions?

No substances are added to the reaction mixture and no substances escape from it.

49

What is a dynamic equilibrium?

The rate of both reactions is equal, the total amounts of the various things present is equal and the reactions are still continuing.

50

Can you have a dynamic equilibrium if the reaction is open?

No because if you remove the products at the start of the reaction they will not get a chance to change back.

51

What is the position of equilibrium?

It is a reference to the proportions of the various things in the equilibrium mixture

52

What is Le Chatelier's principle?

If a dynamic equilibrium is disturbed by changing the conditions, the reaction move to counteract the change.

53

What might you do to influence the reaction of a dynamic equilibrium?

• Increasing or decreasing the concentrations of substances present • Changing the pressure • Changing the temperature • Adding a catalyst (this has no effect of the position of the equilibrium)

54

What happens if you add more of a substance from a dynamic equilibrium?

The system responds by removing it again and producing more of the product. You would do this if you wanted more of the product.

55

What happens if you remove a substance from a dynamic equilibrium?

The reaction will respond by replacing it again by reaction more of the other side of the reaction.

56

When can you change the pressure of a dynamic equilibrium reaction?

With gas reactions and where the total number of molecules on each side of the equation is different.

57

What causes pressure?

Molecules hitting the walls of their container. If you have fewer molecules in the same volume at the same temperature, you will have a lower pressure.

58

According to Le Chatelier's principle, what should happen when you increase the pressure on one side of a dynamic equilibrium? How does this happen?

If you increase the pressure the reaction will respond by reducing it again. It can reduce the pressure by producing fewer molecules, i.e. the side of the reaction that has less moles. So increasing the pressure will always favour the side which produces the smallest number of molecules.

59

What happens if there are the same number of molecules on each side of the reaction and you increase the pressure?

The position of the equilibrium does not change.

60

If the forwards reaction is exothermic and the heat energy of ΔH = -100kJ mol-1, what is the heat energy of the backwards reaction?

The backwards reaction would be endothermic by the exact same amount so ΔH = 100kJ mol-1

61

What will happen if you decrease the temperature of a dynamic equilibrium if the forward reaction is exothermic?

The reaction will respond in such a way to raise the temperature and so more of the forwards reaction would happen as more heat would be given out because of the exothermic change. In the reaction A + 2B ⇌ C + D, more C + D would be produced.

62

What will happen if you increase the temperature of a dynamic equilibrium if the forward reaction is exothermic?

The reaction will respond in such a way to lower the temperature and so more of the backwards reaction would happen as more heat would be absorbed because of the endothermic change. In the reaction A + 2B ⇌ C + D, more A + 2B would be produced.

63

What does adding a catalyst to a dynamic equilibrium do?

Speeds up both reactions so the position of equilibrium doesn't actually change.