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Chemistry > SC8 > Flashcards

SC8 Flashcards

1
Q

What are electrolytes?

A

Ionic compounds that are:

  • In the molten state (heated so they become liquids) or
  • Dissolved in water
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2
Q

What happens in conditions when the electrolytes are molten?

A

The ions in electrolytes are free to move within the liquid or solution.

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3
Q

What is electrolysis?

A

A process in which electrical energy from a direct current (dc) supply, decomposes electrolytes. The free moving ions in electrolytes are attracted to the oppositely charged electrodes, which connect to the dc supply.

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4
Q

What are cations?

A

Positively charged ions. They move towards the negatively charged electrode, which is called the cathode.

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5
Q

What are anions?

A

Negatively charged ions. They move towards the positively charged electrode, which is called the anode.

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6
Q

What are the products of electrolysis?

A

When ions reach an electrode, they gain or lose electrons. As a result, they form atoms or molecules of elements:

  • Cations gain electrons from the negatively charged cathode.
  • Anions lose electrons at the positively charged anode.
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7
Q

Predict the products of electrolysis of molten aluminium oxide.

A

Aluminium will form at the negative electrode (anode) and oxygen will form at the positive electrode (cathode).

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8
Q

What can pure water do?

A

Conduct electricity because a small proportion of its molecules dissociate into ions. The two ions formed in water are, hydrogen ions, H+, and hydroxide ions, OH-.

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9
Q

What happens during the electrolysis of water?

A

H+ ions are attracted to the cathode, gain electrons and form hydrogen gas.
OH- ions are attracted to the anode, lose electrons and form oxygen gas.

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10
Q

What is the overall balanced equation for this process?

A

2H2O(l) → 2H2(g) + O2(g)

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11
Q

What does an electrolyte formed by dissolving an ionic compound contain?

A

2 pairs of negative and positive ions:

  • Positive hydrogen ions from the water, and positive ions from the compound.
  • Negative hydroxide ions from the water, and negative ions from the compound.
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12
Q

What do the ions do?

A

Compete at each electrode to gain or lose electrons.

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13
Q

What does whether hydrogen or a metal is produced at the cathode depend on?

A

The position of the metal in the metal reactivity series:

  • The metal is produced at the cathode if it is less reactive than hydrogen.
  • Hydrogen is produced at the cathode if the metal is more reactive than hydrogen.
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14
Q

What can be produced at the anode?

A

Oxygen or a non-metal:

  • for the most common compounds oxygen is produced (from the hydroxide ions)
  • If halide ions (chloride, bromide or iodide ions) are present, then the negatively charged halide ions lose electrons to form the corresponding non-metal halogen (chlorine, bromine or iodine).
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15
Q

Describe the practical that investigates electrolysis using inert (unreactive) electrodes.

A

Aim:
To investigate the electrolysis of copper sulfate solution using inert electrodes.
1. Pour some copper sulfate solution into a beaker.
2. Place 2 graphite rods into the copper sulfate solution. Attach 1 electrode to the negative terminal of a dc supply, and the other electrode to the positive terminal.
3. Completely fill 2 small test tubes with copper sulfate solution and position a test tube over each electrode as shown in the diagram.
4. Turn on the power supply and observe what happens at each electrode.
5. Test any gas produced with a glowing splint and a burning splint.
6. Record your observations and the results of your tests.

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16
Q

What would happen at the negative electrode?

A

A brown/pink solid forms

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17
Q

What would be observed at the positive electrode?

A

Bubbles of a colourless gas form.

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18
Q

What would the results of the gas tests be?

A

Nothing would happen at the negative electrode. The gas relights a glowing splint at the positive electrode.

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19
Q

Describe the practical to investigate electrolysis of copper sulfate solution using non-inert electrodes.

A

Copper electrodes take part in the reactions and are described as non-inert:
- At the positive electrode, copper atoms lose electrons and form copper ions, Cu2+.
- At the negative electrode, copper ions gain electrons and form copper atoms.
This process is used industrially to purify copper.
1. Pour some copper sulfate solution into a beaker.
2. Measure and record the mass of a piece of copper foil. Attach it to the negative terminal of a dc supply, and dip the copper foil into the copper sulfate solution.
3. Repeat step 2 with another piece of copper foil, but this time attach it to the positive terminal.
4. Make sure the electrodes do not touch each other, then turn on the power supply. Adjust the power supply to achieve a constant current as directed by your teacher.
5. After 20 minutes, turn off the dc supply.
6. Carefully remove one of the electrodes. Gently wash it with distilled water, then dip it into propanone. Lift the electrode out and allow all the liquid to evaporate. Do not wipe the electrodes clean. Measure and record the mass of the electrode.
7. Repeat step 6 with the other electrode. Make sure you know which is which.
8. Repeat the experiment with fresh electrodes and different currents.

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20
Q

What are the dangers and precautions when using copper sulfate solution?

A

Harm- causes skin and serious eye irritation.

Precaution - Wear gloves and eye protection.

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21
Q

What are the dangers and precautions of propanone?

A

Harm- Highly flammable liquid and vapour- may cause drowsiness or dizziness.
Precaution- Keep away from naked flames- use it in a fume cupboard

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22
Q

What happens as the current is increases?

A

The change in the mass of the electrodes is greater.

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23
Q

How is copper purified?

A

By electrolysis. Electricity is passed through solutions containing copper compounds, such as copper sulfate. The anode (positive electrode) is made from impure copper and the cathode (negative electrode) is made from pure copper.

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24
Q

Describe how pure copper forms on the cathode.

A
  1. A beaker with pure and impure copper rods is dipped into copper sulfate solutions.
  2. The pure copper rod is connected to the negative terminal of a battery and the impure rod is connected to the positive terminal.
  3. The pure copper rod has increased in size while the impure rod has deteriorated, leaving a pool of anode sludge at the bottom of the beaker
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25
Q

What happens during purification of copper during electrolysis?

A

The anode loses mass as copper dissolves, and the cathode gains mass as copper is deposited.

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26
Q

Describe what happens during the purification of copper by electrolysis?

A
  • Some copper ions are attached to the rod on the right and some are floating in the space between the rod.
  • A battery is connected between the rods and copper ions are pulled towards the other rod.
  • There are now copper ions attached to the other rod, with the other ions floating in the middle.
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27
Q

What is a half equation?

A

An equation, involving ions and electrons, that describes the process happening at an electrode.

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28
Q

What is a half equation used to represent?

A

The reaction that happens at an electrode during electrolysis. It shows what happens when ions gain or lose electrons. In half equations:

  • Electrons are shown as e-
  • The number of atoms of each element must be the same on both sides.
  • The total charge on each side must be the same (usually zero).
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29
Q

What do positively charged ions do?

A

Gain electrons at the cathode.
These are half equations for some reactions at the cathode:
Na+ + e- - Na

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30
Q

Write a balanced half equation for the formation of calcium ion, Ca2+

A

Ca2+ + 2e- → Ca

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31
Q

What do negatively charged ions do?

A

Lose electrons at the anode.
For example,
2Cl- → Cl2 + 2e-

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32
Q

Write a balanced half equation for the formation of bromine, from bromide ions, Br-.

A

2Br- → Br2 + 2e-

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33
Q

What is oxidation?

A

The loss of electrons.

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34
Q

What is reduction?

A

The gain of electrons.

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35
Q

Where does reduction happen?

A

At the cathode.

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36
Q

Where does oxidation happen?

A

At the anode.

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37
Q

What is the reactivity series of metals?

A

A chart showing metals in order of decreasing reactivity. In general, the more reactive metal is:

  • the more vigorous its reactions are
  • the more easily it loses electrons in reaction to form positive ions (cations).
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38
Q

What is formed when a metal reacts with water?

A

Metal hydroxide and hydrogen.

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39
Q

What is formed when a metal reacts with steam?

A

Metal oxide + hydrogen.

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40
Q

What is formed when a metal reacts with a dilute acid?

A

Salt + hydrogen.
The more reactive the metal, the more rapid the reaction is. A metal below hydrogen in the reactivity series will not react with dilute acids.

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41
Q

What happens in the reactions of metals with water, steam and acids?

A

The metals lose electrons and form cations. The metal is oxidised and the water is reduced.

42
Q

What are a metal’s relative tendency to form cations and its resistance to oxidation both related to?

A

Its position in the reactivity series. In general:

  • the higher up a metal, the greater the tendency to form cations.
  • the lower down a metal, the greater its resistance to oxidation.
43
Q

What can a more reactive metal displace?

A

A less reactive metal from its compounds.

44
Q

What are spectator ions?

A

An ion that is exactly the same on both sides of an ionic equation.

45
Q

What is the half equation for magnesium and copper sulfate?

A

Mg(s) → Mg2+(aq) + 2e- (oxidation)

Cu2+(aq) + 2e- → Cu(s) (reduction)

46
Q

What happens to the magnesium atoms?

A

They lose electrons- they are oxidised.

47
Q

What happens to the copper ions?

A

They gain electrons- they are reduced.

48
Q

Reduction and oxidation happen at the same time, so what are these reactions called?

A

Redox reactions.

49
Q

What are electrolysis reactions also?

A

Redox reactions.

50
Q

How do you write a half equation?

A

Remove the spectator ions so that it become an ionic equation. Then write the half equations for both elements.

51
Q

What are unreactive metals such as gold found in the Earth’s crust as?

A

The uncombined elements. However, most metal are found combined with other elements to form compounds.

52
Q

What is an ore?

A

A rock that contains enough of a metal or a metal compound to make extracting the metal worthwhile:

  • low-grade ores contain a small percentage of the metal or its compound.
  • high-grade ores contain a larger percentage.
53
Q

What does the extraction method used depend on?

A

The metal’s position in the reactivity series.

54
Q

How is a metal extracted if that metal is less reactive than carbon?

A

It can be extracted from its compounds by heating with carbon.

55
Q

What does a redox reaction involve?

A

The loss and gain of oxygen.

56
Q

What is oxidation?

A

The gain of oxygen by a substance.

57
Q

What is reduction?

A

The loss of oxygen by a substance.

58
Q

As iron is below carbon in the reactivity, how can it be extracted?

A

It can be displaced from its compounds by heating with carbon. This is because carbon is more reactive than iron.

59
Q

Write a balanced equation for the reaction between tin(IV) oxide and carbon, forming molten tin and carbon dioxide.

A

SnO2(s) + C(s) → Sn(l) + CO2(g)

60
Q

How must aluminium be extracted?

A

By electrolysis because it is more reactive than carbon.

61
Q

How is aluminium ore treated?

A

To produce pure aluminium oxide. The electrolytes used in electrolysis are ionic compounds:

  • in the molten state, or
  • dissolved in water.
62
Q

What is aluminium oxide in water?

A

Insoluble, so it must be molten to acts as an electrolyte. However, the melting point of aluminium oxide is high. A lot of energy must be transferred to break its strong ionic bonds, and this is expensive. To reduce costs, powdered aluminium oxide is dissolved in molten cryolite. This ionic compound melts at a lower temperature than aluminium oxide, reducing costs.

63
Q

What happens during electrolysis at the cathode?

A

Aluminium ions gain electrons and form aluminium atoms.

64
Q

What happens during electrolysis at the anode?

A

Oxide ions lose electrons and form oxygen gas.

65
Q

What does the oxygen react with?

A

The carbon anodes, forming carbon dioxide. So the anodes gradually wear away. They must be replaced frequently, adding to the cost of producing aluminium.

66
Q

What does recycling a metal involve?

A

Collecting used metal items and producing a new metal from them.

67
Q

What are the step usually needed to recycle a metal?

A
  • Collecting and transporting the used items to a recycling centre.
  • Breaking up and sorting the different metals.
  • Removing impurities from the metals.
68
Q

What are the advantages of recycling metals?

A
  • More economic - less energy is needed to produce a metal.
  • Less damage to the environment - fewer quarries and mines, less noise and less heavy traffic.
  • Saves valuable raw materials - reserves of metal ores will last longer.
69
Q

What are the disadvantages of recycling?

A
  • The collection and transport of used items needs organisation, workers, vehicle and fuel.
  • It can be difficult to sort different materials from one another.
70
Q

What is a life-cycle assessment or LCA?

A

A ‘cradle to grave’ analysis of the impact of a manufactured product on the environment. The main stages are:

  1. obtaining the raw materials needed.
  2. manufacturing the product.
  3. using the product.
  4. disposing of the product at the end of its useful life.
71
Q

At all stages, what is an LCA likely to include information about?

A

The use of energy, transport of materials, and the release of waste substances into the environment.

72
Q

Describe the raw materials stage.

A

All the raw materials we need come from the Earth’s crust, atmosphere or oceans, or are due to living organisms. Obtaining these materials has an impact on the environment, including:

  • Using up limited resources such as ores and crude oil.
  • Damaging habitats through quarrying, mining, or felling trees.
73
Q

Describe the manufacture stage.

A

The manufacture of products has an impact on the environment, including:

  • Using up land for factories.
  • The use of machines and people.
74
Q

Describe the use stage.

A

The impact of a product on the environment during its use depends on the type of product. For example, a wooden chair has very little impact, unless it needs cleaning or repair. On the other hand, a car will have a significant impact.

75
Q

Describe the disposal stage.

A

The disposal of old products has an impact on the environment including:

  • Using up land for landfill sites.
  • Whether any or all of the product can be recycled or reused.
76
Q

What can some alternative methods be used to do?

A

Extract metals using living organisms. These have advantages and disadvantages compared to the usual extraction methods.

77
Q

How do plants absorb mineral ions?

A

Through their roots.

78
Q

What makes use of this to extract metals?

A

Phytoextraction.

79
Q

What is phytoextraction?

A

Using plants to absorb metal compounds from the ground through their roots. The plants are then burned to produce an ash containing a high concentration of the metal compounds.

80
Q

Describe the process of phytoextraction.

A
  1. Plants are grown on a low-grade ore that contains lower amounts of metal.
  2. The plants absorb metal ions through their roots and concentrate these ions in their cells.
  3. The plants are harvested and burnt.
  4. The ash left behind contains a higher concentration of the metal than the original ore.
  5. The ash is processed to obtain the metal.
81
Q

What is a disadvantage of phytoextraction?

A

It is slow.

82
Q

What are the advantages of phytoextraction?

A
  • Reduces the need to obtain new ore by mining.

- Conserves limited supplies of more valuable ores with higher metal content.

83
Q

Suggest reasons why phytoextraction decreases the damage done to the environment.

A

Mining for metal ores involves quarries, which are large holes in the ground. These create noise, dust and traffic. They also destroy natural habitats. Phytoextraction reduces the need for mining, so reduces this damage.

84
Q

What is bioleaching.

A

Certain bacteria can break down low-grade ores to produce an acidic solution containing metal ions. The solution is called a leachate and the process is called bioleaching.

85
Q

What metal can be extracted by using bioleaching?

A

Copper. It does not need high temperatures, but it produces toxic substances, including sulfuric acid, which damage the environment.

86
Q

What metal is more reactive than copper?

A

Iron. It can displace copper from the leachate.

Since iron is cheaper than copper, the use of scarp iron is a cost-effective way to produce copper from the leachate.

87
Q

In principle, what are all chemical reactions?

A

Reversible reactions. This means that the products can be changed back into the original reactants.

88
Q

What are some examples of reactions that go to completion?

A
  • Complete combustion of a fuel.
  • Many precipitation reactions.
  • Effervescence reactions in which a gas escapes.
89
Q

What happens when ammonium chloride (a white solid) is heated?

A

It breaks down, forming ammonia and hydrogen chloride. When these two gases are cool enough, they react together to form ammonium chloride again. This reversible reaction can be modelled as:
Ammonia chloride ⇌ ammonia + hydrogen.
The symbol ⇌ is used in equations that model reversible reactions:
- the forward reaction is the one that goes to the right.
- the backward reaction is the one that goes to the left.

90
Q

What is blue copper sulfate described as?

A

Hydrated. The copper ions in its crystal lattice structure are surrounded by water molecules. This water is driven off when blue hydrated copper sulfate is heated, leaving white anhydrous copper sulfate. This reaction is reversible.
Hydrated copper sulfate ⇌ anhydrous copper sulfate + water.

91
Q

What can be achieved if a reversible reaction happens in a close container?

A

Dynamic equilibrium.

92
Q

What happens at equilibrium?

A
  • The forward and backward reactions are still happening.
  • The rates of the forward and backward reactions are the same.
  • The concentrations of the reactants and products remain constant (they do not change).
    Equilibrium can only be achieved if none of the reactants or products can escape.
93
Q

Ammonia is an important industrial product used to make what?

A

Fertilisers, explosive and dyes.

94
Q

What is used to manufacture ammonia?

A

The Haber process.

95
Q

What does the Haber process involve?

A

A reversible reaction between nitrogen and hydrogen. The reaction can reach a dynamic equilibrium.

96
Q

Describe what happens in the Haber process,

A
  1. Nitrogen (extracted from the air) and hydrogen (obtained from natural gas) are pumped through pipes.
  2. a compressor increases the gas pressure to 200 atmospheres.
  3. the pressurised gases are heated to 450°C and passed through a reaction chamber containing an iron catalyst to speed up the reaction.
  4. the reaction mixture is cooled so that ammonia liquefies an can be removed.
  5. unreacted nitrogen and hydrogen are recycled.
    The conditions are chosen to ensure the process of making ammonia is profitable as possible.
97
Q

What is the equilibrium position of a reversible reaction a measure of?

A

The concentrations of the reactants and products at equilibrium. Using the Haber process as an example:
N2(g) + 3H2(g) ⇌ 2NH3(g)
The equilibrium position is:
- to the left if the concentration of N2 and H2 are greater than the concentration of NH3.
- to the right if the concentration of NH3 is greater than the concentrations of N2 and H2.

98
Q

How can the equilibrium position be changed?

A

By altering the reaction conditions, such as by:

  • changing the pressure.
  • changing the concentration.
  • changing the temperature.
99
Q

What happens if the pressure is changed?

A

In a reaction involving gases, if the pressure is increased, the equilibrium position moves in the direction of the fewest molecules of gas.
If the pressure is increased, the equilibrium position moves to the right.

100
Q

What happens if you change the concentration?

A

In a reaction involving solutions, if the concentration of a solute is increased, the equilibrium position moves in the direction away from the solute. For example, bismuth chloride reacts with water in a reversible reaction:
BiCl3(aq) + H2O(l) ⇌ BiOCl(s) + 2HCl(aq)
The concentration of hydrochloric acid can be increased by adding more hydrochloric acid. When this happens, the equilibrium position moves to the left, away from the HCL(aq) in the equation.

101
Q

What happens if you change the temperature?

A

In a reversible reaction, if the reaction is exothermic in one direction, it is endothermic in the other direction. If the temperature is increased, the equilibrium position moves in the direction of the endothermic process. For example, sulfur dioxide reacts with oxygen in a reversible reaction:
2SO2(g) + O2(g) ⇌ 2SO2(g) (forward reaction is exothermic)

If the forward reaction is exothermic, the backward reaction must be endothermic. Therefore, if the temperature is increased, the equilibrium position moves to the left.

Chemistry (3 decks)

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