Topic 1.6

  • Created by: Mary
  • Created on: 04-03-13 21:46

Calculating Oxidation State

Rules for Calculating Oxidation State

1. The charge on the molecule gives the total oxidation state for the molecule. Elements have an oxidation state of zero.

2. Group 1 elements have an oxidation state of +1, group 2 elements, +2.

3. Group 7 elements usually have an oxidation state of -1, but this can change depending on what it is bonded to. 

4. Group 6 elements usually have an oxidation state of -2, except for peroxides or when bonded to halogens, where the oxidation states are -1 and +2 respectively. 

5. Hydrogen atoms usually have an oxidation state of +1, except hydrides, which have an oxidation state of -1. 

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Oxidation and Reduction

In terms of electrons: oxidation involves loss of electrons, reduction involves gain of electrons. We can remember this using the mnemonic


If the electrons are on the left hand side it is reduction whereas if they are on the right hand side it is oxidation. 

In terms of oxidation state: oxidation involves an increase in oxidation state, whereas reduction involves a decrease in oxidation state. 

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Writing Balanced Redox Equations

1. Write down the half equations if not given - remember these must balance in terms of number of atoms and charge. 

2. Balance the number of electrons in both equations. 

3. Combine the left hand side of both equations and the right hand side of both equations, not including the electrons. 

4. Remove spectator ions (those on both sides of the equation)

5. Make sure the equation balances in terms of atoms and charges. 

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