Oxidation and Reduction: Electrochemical Series and Electrolysis

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Electrochemical Series

Electrochemical Series the order in which metals tend to lose electrons in chemical reactions.

• The higher a metal is in the electrochemical series, the more reactive it is.
• A metal will displace a less reactive metal from its salt.
• The more reactive metal is oxidised; the less reactive metal is reduced.
• The further apart in the series the elements are, the more readily the reaction will occur.

Mandatory Experiment: Displacement Reactions of Metals

Half fill two test tubes with acidified copper(II) sulfate solution

1. Add magnesium ribbon to one tube.
2. Add zinc powder to the other tube

Results

1. Blue colour of copper sulfate fades; brown precipitate of copper metal formed (copper displaced by magnesium)
2. Blue colour of copper sulfate fades; brown precipitate of copper metal formed (copper displaced by zinc)

Electrolysis

Electrolysis breaking down an aqueous or molten substance by passing electricity through it

Electrolyte a substance that conducts electricity when it is dissolved or melted

Anode (+)

• Negative ions (anions) collect here
• Oxidation

Cathode (-)

• Positive ions (cations) collect here
• Reduction

Factors Affecting Electrolysis

In an aqueous solution, water and anions are capable of being oxidised. Water and cations are capable of being reduced.

Factors affecting electrolysis include:

• the type of electrode used

Electrolysis of Aqueous Copper(II) Sulfate Solution Using Copper Electrodes

1. Electrode at anode starts to dissolve. Cu >>> Cu2+ + 2e-
2. Deposits form on the cathode. Cu2+ +2e- >>>Cu

• the ease of oxidation or reduction of the substance

Electrolysis of acidified water

1. H2O is more easily oxidised than SO4 2-, so H2O is oxidised
2. Anode - H2O >>> 2H+  + ½O2 + 2e-
3. Cathode – 2H+  + 2e- >>> H2

• Species present may be too stable to be oxidised

Electrolysis of aqueous sodium sulfate solution

1. At anode, SO4 2- is too stable to be oxidised  H2O >>> 2H+ +½O2 + 2e-
2. At cathode, Na+ is too stable to be reduced 2H2O + 2e- >>> H2 + 2OH-

Electrolysis of Potassium Iodide

1. Iodine is insoluble in water but forms a soluble complex with potassium iodine formed at anode
2. Hydrogen is more easily oxidised than K+ Hydrogen formed at cathode

Remember!!!

Electrolysis formulas usually follow this rule: the side starting with the non-ion (eg Cu) is the anode and vice versa.

Practical Applications of Electrolysis

Electroplating putting a layer of metal onto the surface of another metal

Used for

• Silver plating cutlery
• Purification of copper