Electrolysis

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  • Created by: chris
  • Created on: 22-03-15 17:01

What is it?

  • PROCESS THAT USES ELECTRICITY TO BREAK DOWN IONIC COMPOUNDS INTO ELEMENTS
  • WHEN ELECTRICITY IS PASSED THROUGH A MOLTEN IONIC COMPOUND OR A SOLUTION CONTAINING IONS, ELECTROLYSIS TAKES PLACE
  • THE SUBSTANCE BROKEN DOWN IS CALLED THE ELECTROLYTE
  • THE ELECTRICAL CIRCUIT HAS 2 ELECTRODES THAT MAKE CONTACT WITH THE ELECTROLYTE
  • THESE ARE OFTEN MADE OF INERT SUBSTANCES THAT DON'T REACT WITH THE PRODUCTS
  • THE IONS IN THE ELECTROLYTE MOVE TO THE ELECTRODES WHERE THEY ARE DISCHARGED TO PRODUCE ELEMENTS
  • POSITIVELY CHARGED IONS ARE ATTRACTED TO THE NEGATIVE ELECTRODE WHERE THEY FORM METALS OR HYDROGEN, DEPENDING ON THE IONS IN THE ELECTROLYTE
  • NEGATIVELY CHARGED IONS ARE ATTRACTED TO THE POSITIVE ELECTRODE WHERE THEY LOSE THEIR CHARGE TO FORM NON-METALLIC ELEMENTS
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Changes at the Electrodes

OILRIG

Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons)

  • When positively charged ions reach the negatively charged electrode they gain electrons to become neutral atoms
  • Gaining electrons is called reduction, so the positive ions have been reduced. Ions with a single positive charge gain 1 electron and those with a 2+ charge gain 2 electrons
  • At the positive electrode, negative ions lose electrons to become neutral atoms. This is oxidation. Some non-metal atoms combine to form molecules. (For example bromine froms Br2
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Gas Production

  • Water contains Hydrogen ions and Hydroxide ions.
  • When solutions of ions in water are electrolysed, Hydrogen may be produced at the negative elctrode. This happens if the other positive ions in the solution are those of a metal more reactive than Hydrogen. (Consult reactivity series!)
  • Oxygen is usually produced at the positive electrode from aqueous solutions. However, if the solution contains a reasonably high concentratio of a halide ion, then a halogen will be produced.
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Electrolysis of Aluminium

  • Aluminium is more reactive than Carbon and so much be extracted from its ore by electrolysis.
  • Its ore contains aluminium oxide which must be purified and then melted so it can be electrolysed
  • The aluminium oxide is mixed with another ionic compound called cryolite so the mixture melts at 850 degrees. This makes the process more economically viable as the electrolysis can take place at a lower temperature, thus saving energy.
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Electrolysis of Aluminium

The reaction in the cell is :


Aluminium Oxide -> Aluminium + Oxygen


2Al2O3 (l) -> 4Al (l) + 3O2 (g)


The cryolite remains in the cell and fresh aluminium oxide is added as aluminium and oxygen are produced


  • At the negative electrode Aluminium ions are REDUCED to Aluminium atoms by gaining electrons. The molten Aluminium metal is collected from the bottom of the cell.

  • At the positive electrode oxide ions are OXIDISED to oxygen atoms by losing electrons and the Oxygen atoms for Oxygen molecules.


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Electrolysis of Aluminium

Half Equations -

Negative Electrode: Al 3+ (l) + 3e- -> Al (l)

Positive Electrode: 2O2- (l) -> 2O2 + 4e-

  • The positive electrodes used in the cell are made of carbon. At the high temperature of the cell the oxygen reacts with the carbon electrodes to produce CO2. This means that carbon electrodes gradually burn away and have to be replaced regularly
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Electrolysis of Brine

  • Brine is a solution of sodium chloride in water
  • The solution contains sodium ions (Na+ (aq)), chloride ions (Cl- (aq)), hydrogen ions (H+ (aq)) and hydroxide ions (OH- (aq))

When brine is electrolysed:

  • Hydrogen is produced at the negative electrode from the H+ ions
  • Chlorine is produced at the postive electrode from the Cl- ions.
  • This leaves a solution of sodium ions and hydroxide ions (NaOH (aq))
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Electrolysis of Brine

Half Equations -

Positive Electrode : 2Cl- -> Cl2 + 2e-

Negative Electrode : 2H+ + 2e- -> H2

  • Sodium Hydroxide is a strong alkali and has many uses including making soap, making paper, making bleach, neutralising acids and controlling pH.
  • Chlorine is used to kill bacteria in drinking water and in swimming pools, and to male bleach, disinfectants and plastics
  • Hydrogen is used to make margerine and hydrochloric acid
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Electroplating

  • Electroplating is a process that uses electrolysis to put a thin coating of metal onto an object.

It can be done for a number of reasons that include :

  • to make the object look more attractive
  • to protect a metal object from corroding
  • to increase the hardness of the surface
  • to reduce costs by using a thin layer of metal instead of the pure metal

For electroplating the object to be plated is used as the negative electrode and the positive electrode is made from the plating metal. The electrolyte is a solution containing ions of the plating metal,

At the positive electrode, atoms of the plating metal lose electrons to form metal ions which go into the solution. At the negative electrode, metal ions from the solution gain electrons to form metal atoms which are deposited on the object to be plated.

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