Electrolysis

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Electrolysis

Electrolysis is the decomposition of a compound using electricity:

Electrolysis

The decomposition of molten lead bromide occurs using the apparatus above. A current is passed through graphite rods called electrodes.

The negative terminal is attached to one rod, which becomes the negative electrode, the cathode.

The positive terminal is attached to the other rod. This becomes a positive electrode, the anode.

Note: The compound must be molten to allow the charged ions to flow. You cannot carry out electrolysis on solid lead bromide.

How does lead bromide decompose?

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The diagram above shows how the oppositely charged ions are attracted to oppositely charged electrodes.

Cations (positive ions - metal ions and hydrogen) travel to the negative electrode, the cathode.

Anions (negative ions - non-metal ions) travel to the positive electrode, the anode.

Cations are positive so the go to the negative electrode, the cathode.

Anions are negative so go to the positive electrode, the anode.

Summary of electrolysis:

  1. All ionic compounds when molten can be decomposed when electricity is passed through using electrolysis.
  2. The metal and hydrogen always forms at the cathode.
  3. Non-metal always forms at the anode.
  4. Cations travel to the cathode.
  5. Anions travel to the anode.
  6. The electrodes are made from inert material such as graphite, so that they do not involve themselves with the reaction.
  7. The molten substance been electrolysed is called the electrolyte.

Examples:

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At the cathode:

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At the anode:

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At the cathode:

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At the anode:

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When a salt is dissolved in water, its ions become mobile.

Hence, the solution can be electrolysed. However, the products from the salt solution will be different to the molten solution because of the presence of the water, which itself produces ions.

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During electrolysis, these ions compete with the metal and non-metal ions from the dissolved salts, to receive or give up electrons.

So who wins?

At the cathode:

The more reactive a metal is the more it prefers being ions.

Therefore, if a reactive metal such as zinc or magnesium is present it will remain as the ions. The H+ ions will accept the electrons and hydrogen gas will be given off at the cathode.

If a less reactive metal, such as copper or silver is present it would rather accept the electrons than H+.

Hence, the metal forms at the cathode.

At the anode:

If halide ions are present, Cl-, Br-, I-, they will give up there electrons to become molecules of Cl2, Br2 and I2 respectively.

If no halogen is present, OH- will give up electrons more readily than other non-metal ions, and oxygen forms.

Examples:

Potassium bromide solution (aq):

At the cathode:

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At the anode:

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Copper (II) nitrate solution (aq):

At the cathode:

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At the anode:

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