Basic Solution:

Separate the reaction into two half-reactions.

Balance each half-reaction separately as in acid:

    1. Balance atoms other than O & H by inspection.

    2. Balance O by adding H2O to the opposite side.

    3. Balance H by addding H+ as appropriate.

    4. Balance the charge by adding electrons (e-) - add to same side as excess of positive charge, or opposite side if excess negative charge.

    5. Balance the charges of the two half-reactions by multiplying appropriately.

Balance as in acid above, then:

    1. Add enough OH- (equal numbers to both sides) to cancel the H+. (This is necessary because there will not be protons present in a basic solution!). (There are a couple of other conventions for balancing in basic solutions. If you are familiar with another and prefer it, you may use it instead.)

    2. Combine the H+ and OH- on the appropriate side of the equation to give waters.

    3. Go back and cancel waters which appear on both sides to give the final equation.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Rules for Assigning Oxidation Numbers

 
  1. In the formula for any substance the sum of the oxidations numbers of all the atoms in the formula is equal to the charge shown. Thus:

    • For elements, such as Ar, O2, S8, etc. in the uncombined state the oxidation number for each atom must be 0, since no charge is shown and the atoms are equal to each other.

    • For monatomic ions the oxidation number equals the charge.

    • For a compound the sum of the oxidation numbers of the atoms equals 0.

    • For a polyatomic ion the sum of the oxidations numbers of the atoms equals the charge on the ion.

  2.  

  3. In compounds fluorine is always assigned an oxidation number of -1.

  4. Alkali metals in compounds will always (for our class) be assigned an oxidation number of +1.

  5. Alkaline-earth metals in compounds will always (for our class) be assigned an oxidation number of +2

  6. In compounds oxygen is usually assigned an oxidation number of -2.

    • Exception 1: in peroxides it is -1 while in superoxides it is -1/2. These will generally be obvious due to other rules (or the names).

    • Exception 2: in combination with fluorine oxygen can be positive due to Rule 2 above, thus for OF2 oxygen is assigned an oxidation number of +2.

  7.  

  8. In compounds hydrogen is usually assigned an oxidation number of +1

    • Exception: in metallic hydrides hydrogen is assigned an oxidation number of -1. These exceptions will be fairly obvious: NaH, CaH2, etc.

  9.  

  10. Aluminum will always (for our class) be assigned an oxidation number of +3, other elements in this Group will usually be assigned an oxidation number of +3.

 

 

© R A Paselk

Last modified 18 February 2013