AS Chemistry Unit 2 Redox Reactions

Oxidation & Reduction, Oxidation states & Redox Equations

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Redox Reaction

This is a reaction in which both reduction & oxidation take place. OXIDATION is the gain of oxygen and REDUCTION is the loss of oxygen.

Example:- 2PbO + C -----> 2Pb + CO2. Here the lead oxide is reduced to just being lean & the carbon has been oxidised to become carbon dioxide.

Redox Reactions & Electrons - OIL RIG.

OIL - Oxygen is electron loss.

RIG - Reduction is electron gain.

Example:- Al3+ + 3e -----> Al - reduction. The aluminium ion has gained elections.

2Cl- -----> Cl2 + 2e. Oxidised losses electrons.

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Half Equations

This is an equation that shows oxidation or reduction taking place.

Example:- Write a half equation to show how iron(ii) ions Fe2+ become iron(iii) ions Fe3+. 2Fe2+ -----> 2Fe3+ + 2e, OXIDATION.

Example 2:- Write a half equation to show how chlorine Cl2 gas becones chloride ions Cl-. Cl2 + 2e -----> 2Cl-, REDUCTION.

Balanced Redox Equation

2Fe2+ + Cl2 -----> 2Fe3+ + 2Cl-.

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Redox Conditions

Aqueous --> H20 (add O2).

eg:- Show how sulphur dioxide SO2 becomes a sulphate ion SO4+ in aqueous conditions. SO2 + 2H2O -----> SO42- + 4H+ + 2e.

Acidic --> H+ ions are present, oxygen is removed by adding in H+ ions to make H2O.

E.g.:- Show how nitrate ions NO3- becomes nitrogen dioxide NO2 in acidic conditions. NO3- + 2H+ + e -----> NO2 + H2O.

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Oxidation State

This is a number assigned to an element in a compount according to some rules. This number enables us to describe oxidation-reduction reactions,

  • The oxidation state of any elemnt e.g. Fe, H2, O2, Pa, S8 is zero (0).
  • The oxidation state f oxygen in its compound is -2, except for peroxides like H2O2, and Na2O2, in which the oxidation state for O is -1.
  • The oxidation state for hydrogen is +1 in its compound, except for metal hydrides, which as NaH, LiH etc., in which the oxidation state for H is -1.
  • The oxidation states for other element are then assigned to make the algebraic sum of the oxidation states equal to the net charge on the molecule or ion.
  • The following elements usually have the same oxidation states in their compounds:
    • +1 for alkali metals - Li, Na, K, Rb, Cs;
    • +2 for alkaline earth metals - Be, Mg, Ca, Sr, Ba;
    • -1 for halogen except when they form compounds with oxygen or one another.
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