Half Equations

Steps to construct and combine half-equations.

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  • Created by: Lauren
  • Created on: 05-01-12 17:34

Constructing simple half equations.

Remember:

Oxidation is loss of electrons

Reduction is gain of electrons

Using the formulae of the ions, for example, Br- , and if you are reducing the atom or ion, you place the electrons on the left hand side, and if you are oxidising, you place the electrons on the right hand side.



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Examples of simple half equations

Br-     -> 2Br  +  2e-          This is an oxidation half equation.

Mg2+  +  2e-  -> Mg           This is a reduction half equation. 

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Complex half equations.

To construct a complex half equation, follow these steps:

1. Write down the formulae for the reactants and products, then balance the atoms undergoing redox.

2. Balance any oxygen atoms using water.

3. Balance hydrogen atoms using H+ 

4. Balance the charges by adding electrons

5. Check the number of atoms are balanced.

6. Check the number of charges are balanced, the number of electrons should be equal to the total change in oxidation states.

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Example:

Nitric acid to nitrogen dioxide gas. 

1. HNO3   ->  NO2

2. Since there are two molecules of water on the right and three on the left, we need to add a molecule of water to the right.

HNO3   ->  NO+ H2O

3. Now we need to balance the hydrogens.

HNO3  + H+  ->  NO+ H2O

4. Now balance the charges by adding an electron. 

HNO3  + H+ + e- ->  NO+ H2O

5. The atoms are balanced 

6. The nitrogen has been reduced from an oxidation state of +5 to +4, one electron has been added, so the equation is balanced. 


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Combining half equations.

Follow these rules. 

1. One half equation must show reduction and the other must show oxidation. 

2. The number of electrons being lost in one equation and the number of electrons being gained in  the other must be the same. This may require you to multiply one or both of the equations. 

3. The equations should then be added together to produce the overall equation. There should be no electrons in the overall equation. 

4. Check that both the atoms and the charges balance in the overall equation. 

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Example

1. Reduction Br2  + 2e-> 2Br

    Oxidation 2I-> I+ 2e-

2. The number of electrons involved in each equation is equal. 

3. Add the equations together, cancelling out the electrons. 

Br2  2I-> 2Br+ I

4. The equation is balanced for both number of charges and number atoms. 

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