Reactivity Series & Corrosion of Metals

• Metals that are more reactive (easily lose electrons) appear higher up the reactivity series, while less reactive metals appear lower down.
• when metals react, they form positive ions by losing electrons
• the tendency of a metal to form positive ions determines its relative reactivity meaning how reactive it is compared to another metal
• explain reactivity in terms of how easily the metal forms positive ions
• Aluminium does not react with water and reacts slowly with dilute acids but does react with oxygen to form aluminium oxide.

  • Aluminium oxide forms a protective layer, preventing reactions which is why aluminium behaves as if it is unreactive
• what is a displacement reaction?

  is one in which a more reactive metal will displace a less reactive metal from a solution of its salt

  common ex: reaction between magnesium and copper sulfate solution.

  • as magnesium is more reactive than copper, it can displace copper from copper sulfate solution to form copper and magnesium sulfate
  • magnesium is more reactive so will have a greater tendency to lose its electrons and form positive ions and is therefore oxidised
  • copper ions gain these electrons which is reduction

  Equation: Mg(s) + CuSO4 (aq) ----> MgSO4 (aq) + Cu(s)
• corrosion on the surface of metals occurs when they are exposed to the environment for long periods.

  • rusting is a specific type of corrosion that occurs when iron (or steel) reacts with water and oxygen
  • iron + water + oxygen --->hydrated iron (III) oxide (which is also known as rust)
  • you can show that water and oxygen are required for rusting by simple test tube reactions
  • rusting can only happen to iron or steel; if any other metal oxidises in air causing the metal to break down, you should say that the metal has corroded
• Rust is porous, which allows water and oxygen to come into contact with fresh metal. In addition, rust flakes off the surface of iron easily, exposing fresh iron below, which then undergoes rusting.

  • it can be prevented by coating the surface of the iron object with a barrier; preventing water and oxygen coming into contact the iron so rusting cannot occur

  Common barrier methods are:

  • painting
  • greasing
  • coating with plastic or oil

  These barrier methods will only prevent rusting as long as they completely cover the iron
• sacrificial protection is

  is a method where a more reactive metal is attached to a less reactive metal

  • ex: zinc bars being attached to a ship's hull made from steel.
  • Zinc is more reactive than iron, meaning that zinc will lose its electrons more easily than iron to form positive ions and is therefore oxidised more easily that iron so the zinc is oxidised instead of iron
  • the zinc blocks will gradually corrode so the zinc is sacrificed to protect the steel hull
  • they must be replaced before they completely corrodeto provide continuous protection
• what is galvanizing?

  a process where the iron to be protected is coated with a layer of zinc

  • carbon dioxide and oxygen in the air react with the zinc to form zinc carbonate, which provides a protective layer that prevents iron from reacting with oxygen and water and stops rusting.
  • This is similar to other methods; if the coating is scratched the iron is exposed to water and oxygen but is protected by sacrificial protection, zinc will corrode and the iron will not rust