Oxidation and reduction

• Oxidation is the addition of oxygen or removal of hydrogen.
  
• When substances burn in air they react with oxygen.
  
• This is oxidation.
  
• The substances are oxidized by oxygen.
  
• Oxygen is the oxidizing agent.
  

Examples of oxidation reactions:

Carbon, sulphur and magnesium burn in air and are oxidised.

• Carbon + oxygen → carbon dioxide
  
• Sulphur + oxygen → sulphur dioxide
  
• Magnesium + oxygen → magnesium oxide
  

Water is also an oxidising agent because it can add oxygen to other substances. The oxidising agent supplies oxygen. Magnesium + steam → magnesium oxide + hydrogen

Reduction

Metal oxides can be reduced by heating with a reducing agent such as carbon or hydrogen.

Oxidizing agents:     supply oxygen.

Reducing agents:     take away oxygen.

Examples of Oxidizing Agents

• Oxygen
• Water
• Metal oxides

Reducing Agents

• Carbon
• Hydrogen
• Metals

Redox reactions

• Redox reactions are simultaneous oxidation and reduction reactions In a redox reaction one substance is oxidized and the other is reduced e.g.
  
  • Copper oxide is reduced to copper, hydrogen is oxidised to water. Hydrogen is the reducing agent (removes O from CuO). Copper oxide is the oxidising agent (donates O to hydrogen)
    

• The magnesium atom loses 2 electrons (oxidation) to form the magnesium ion, the iron (II) ion gains two electrons (reduced) to form iron atoms.
  

• Chlorine oxidizing iron (II) chloride
  

• A charge on an ion is called its oxidation number. An element has a zero oxidation number because there is no charge on its atoms.
  
• During redox reactions there is a change in oxidation state e.g. when magnesium burns in oxygen, its oxidation state increases from 0 to +2 while oxidation state for oxygen decreases from 0 to – 2 . Magnesium is oxidized and oxygen is reduced.
  
• The number of electrons lost by the reducing agent is equal to the number of electrons gained by the oxidizing agent.
  

Reaction of copper oxide with hydrogen

• Place 3g of black CuO in a test tube and keep the apparatus horizontal and steady with the retort stand and clamps.
• Connect a rubber tube from a gas cylinder to the glass tube and allow a very slow stream of gas to flow through the apparatus.
• Heat the CuO until there is a change in colour and then remove the heat source. After heating, the Copper (II) Oxide turn pink as it was reduced to the element copper.
• Hydrogen removes oxygen from Copper (II) Oxide to form water.
• The copper oxide is reduced while hydrogen is oxidized. Hydrogen is the reducing agent while Copper Oxide is the oxidizing agent