[theChemCoach]

Originally Posted by Loz_metalhead

My teacher is really bad...I just dont no how you do it, please help. Im in year 11.


does it have anything to do with the valencies...I think it would. Here is a question and the answer...can you tell me how it is worked out. Thanks

Al+0(2)=Al(2)O(3)

then once you balance it its...4Al(2) + 3O(2)---->2Al(2)o(3)

Don't worry, this has nothing to do with valencies.

Firstly, your reaction is
Al(s) + O₂ ---> Al₂O₃(s)

• Now, we have in science this law, called the Conservation of Mass, that states that matter (elements) cannot be created or destroyed

• applied to a chemical reaction... all of the matter that appears in the reactants, MUST appear in the products.

• put even simply, all the atoms that appear on the left hand side (LHS) of the reaction (reactants) must equal all the atoms that appear on the right hand side (RHS) of the reaction

• to do this we BALANCE THE REACTION, by placing numbers (coeffecients) infront of the elements and compounds

• so in our example:
  Al(s) + O₂ ---> Al₂O₃(s)
  on the LHS we have one atom of aluminium and 2 atoms of oxygen,
  and on the RHS we have two atoms of aluminium and 3 atoms of oxygen.
  because they have different amounts on both sides, we have to 'balance' both side, so they have equal atoms of both

STEP ONE:

• place a 2 infront of aluminium on LHS (so now both sides have 2 atoms of aluminium)

2Al(s) + O₂ ---> Al₂O₃(s)
STEP TWO

• place a 1.5 infront of oxygen gas on right hand side, so mathematically we have (1.5x2) 3 atoms of oxygen gas on LHS (and 3 on RHS)
  2Al(s) + 3/2O₂ ---> Al₂O₃(s)
  
  .....BUT...if we double all the coefficients (i.e. the numbers infront of the atoms and compounds, we can get rid of that fraction, and have a whole number instead
  4Al(s) + 3O₂ ---> 2Al₂O₃(s)
  and there you have it, a balanced chemical reaction.

• notice that on the LHS we have 4 Al atoms and 6 oxygen atoms, and on RHS we have 4 Al atoms and 6 oxygen atoms....hence all the matter that appears in the reactants...appears in the products.

Also note, how i balanced the metal (Al) first, then a balanced the non-metal (O) second, you should always follow this rule.


Best of Luck,
George