Redox reactions Flashcards
Redox is a word made up from two processes that work together.
The two processes are reduction and oxidation.
Rusting is a redox reaction:
iron + oxygen + water ⇒ hydrated iron(III) oxide
What does galvanising do?
- Galvanising protects iron from rusting by covering it with a layer of zinc.
- The layer of zinc stops water and oxygen from reaching the surface of the iron.
- Zinc also acts as a sacrificial metal, as it is more reactive than iron.
Oxidation and reduction can also be described in terms of electrons.
- A substance is oxidised if it loses electrons. An oxidizing agent takes electrons away from a substance.
- A substance is reduced if it gains electrons. A reducing agent pushes electrons onto anothera substance so that it gains electrons.
What happens during rusting?
- When iron rust, the oxygen is the oxidizing agent; it takes electrons from the iron. The electrons go into the oxygen, so the oxygen itself is reduced.
- Iron loses electrons - it is oxidised.
- Oxygen gains electrons - it is reduced.
Redox reactions can be forced to either direction.
- One direction is oxidation; the other is reduction.
- Iron is normally oxidised, but it can be pushed back.
- Iron ions can be forced from one type to another.
- Chlorine is like oxygen in that it is normally reduced.
Oxidation of Fe
Fe2+ + 2e-
Reduction of Fe2+
Fe
Oxidation of Fe2+
Fe3+ + e-
Reductio of Fe3+
Fe2+
Reduction of Cl2
2Cl-
Oxidation of 2Cl-
Cl2 + 2e-
When a word equation for a displacement reaction is written
- The more reactive metal ‘swaps places’ with the less reactive metal. For example: magnesium is more reactive than zinc, so zinc is replaced.
- Magnesium + zinc sulfate ⇒ magnesium sulfate + zinc
The order of reactivity is needed to work out displacement reactions:
- Magnesium, zinc, iron, tin.
- This means that magnesium displaces zinc, iron, and tin.
- Zinc displaces iron and tin.
- Iron displaces tin.
Displacement happens with
Reactive metals and compounds of a less reactive metal.
How do metals react?
- All metals react by pushing off electrons and turning into ions. This is oxidation.
- These electrons are forced onto the ions of other metals that are not so reactive.
- Metal atoms that gain these electrons are reduced.
Symbol equation for displacement reaction for magnesium and zinc sulfate
Mg + ZnSO4 ⇒ MgSO4 + Zn
Ionic equation for displacement reaction for magnesium and zinc sulfate
Mg + Zn2+ ⇒ Mg2+ +Zn
Tin Barrier
Iron coated with tin which acts as a barrier. It is less reactive so loses electrons even faster causing rusting.
Sacrificial Protection
Iron can be coated in a thin layer of zinc or magnesium. Zinc and magnesium more reactive than iron, so lose electrons in preference to iron. These are sacrificial metals and the process is called sacrificial protection
