Additional Science-Chemistry Chapter 6

GCSE AQA Additional Science-Chemistry Chapter 6

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  • Created by: Mel
  • Created on: 24-05-12 10:29

Electrolysis

The word electrolysis means 'splitting up using electricity'. In electrolysis, we use an electric current to breakdown (decompose) a substance made of ions.When electricity is passed through a molten ionic compound or a solution containing ions (electrolyte) where ions are free to move, electrolysis takes place.

The electrical circuit has two conducting rods called 'electrodes' that make contact with the electrolyte. The ions in the electrolyte move to the electrodes where they are discharged to produce elements.

We normally make the electrodes out of an unreactive (inert) substance like graphite or platinum so they do not react with either the electrolyte or the products made during electrolysis.

  • Positively charged ions are attracted to the negative electrode where they form metals. Hydrogen may be formed at the negative electrode if the ions are dissolved in water.
  • Negatively charged ions are attracted to the positive electrode where they lose their charge to form non-metallic elements.
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Electrolysis (cont.)

Positive electrode - Anode

Negative electrode - Cathode

What happens in electrolysis. Positively charged ions move to the negatively charged electrode; negatively charged ions move to the positive electrode (http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_74.gif)

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Changes at the Electrodes

  • When positively charged ions reach the negative electrode (cathode) they gain electrons (called reduction as the positive ions have been reduced) to become neutral atoms. Ions with a single positive charge gain one electron and those with a 2+ charge gain 2 electrons.
  • At the positive electrode (anode), negative ions lose electrons to become neutral atoms (oxidation). Some non-metal atoms combine to form molecule.

Oxidation Is Loss of electrons, Reduction Is Gain - OIL RIG

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Changes at the Electrodes (cont.)

Bromine forms Br2.

We can represent the changes at the electrodes by half equations. The equations for lead bromide are:

At the negative electrode: Pb2+ + 2e- --> Pb

At the positive electrode: 2Br- --> Br2 + 2e-

Water contains hydrogen ions and hydroxide ions. When solutions of ions in water are electrolysed, hydrogen may be produced at the positive electrode. This happens if the other positive ions are of metals more reactive than hydrogen.

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Electrolysing Brine

Brine is a solution of sodium chloride in water. When it is electrolysed, hydrogen gas is produced at the negative electrode form hydrogen ions in the water. Chlorine gas is produced at the positive electrode from the chloride ions. This leaves sodium ions and hydroxide ions (from water) in the solution.

At the positive electrode: 2Cl- --> Cl2 + 2e-

At the negative electrode: 2H+(aq) + 2e- --> H2 (g)

React chlorine with sodium hydroxide produced in the electrolysis of brine makes bleach. Chlorine is a poisonous green gas which causes great damage to our bodies if it is inhaled in even tiny quantities.

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Uses of Sodium Hydroxide, Chlorine and Hydrogen

  • Sodium Hydroxide is a strong alkali and has many uses including making soap, making paper, making bleach, neutralizing acids and controlling pH in many industrial processes.
  • Chlorine is used to kill bacteria in drinking water and swimming pools, and to make bleach, disinfectants and plastics (PVC).
  • Hydrogen is used to make margarine and hydrochloric acid.
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Electrolysis

At the anode

  • If halogen is present = halogen gas produced
  • If there is no halogen = oxygen gas produced

At the cathode

  • If metal is more reactive than hydrogen = hydrogen gas produced
  • If metal is less reactive than hydrogen = the metal produced
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Purifying Copper

When we remove copper from its ore it is possible to get copper that is about 99% pure. The impurities include precious metals like gold, silver and platinum which affect the conductivity of the copper and must be removed before it is used for electrical wires.

Copper can be purified by electrolysis using copper electrodes in a solution of a copper salt. The impure copper is used as the positive electrode and the negative electrode is a thin sheet of pure copper. The electrolysis takes place in a solution containing copper ions (usually copper sulfate solution).

Copper atoms on the positive electrode are oxidized, losing electrons to form copper ions that go into the solution. Copper ions from the solution at the negative electrode are reduced, forming copper metal.

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Purifying Copper Diagram

(http://www.chemguide.co.uk/inorganic/extraction/curefine.gif)

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Purifying Copper (cont.)

At the positive electrode: Cu (s) --> Cu2+(aq) + 2e-

At the negative electrode: Cu2+(aq) + 2e- --> Cu (s)

As the copper from the positive electrode dissolves, the impurities are released and collect as sludge at the bottom of the cell. The impurities are then extracted from the sludge.

Once we have purified the copper in the electrolytic cell, it is removed, melted and then formed into bars or ingots.

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