There are a great number of chemical reactions. In
order to
be a chemical reaction at least one new substance must be
produced.
Recall that there are definite signs that a new substance gets produced
a) a change in colour b) the formation of a gas c) the formation of a precipitate d) the release or absorption of energy (heat). |
You may get only one of these, or a combination of them. |
Chemical reactions are typically written one of three ways. There are word equations, skeleton equations and balanced equations. |
Word Equations |
Word equations are used to describe the reaction in sentence form or in a literally form. As an example we will use the results from a lab that you have already done. Magnesium burns in air to produce a white powder. The white powder has been experimentally determined to be a compound of magnesium and oxygen. The product is magnesium oxide. The word equation would be: |
magnesium + oxygen -------> magnesium oxide |
The "+" means "reacts with", and the arrow means "to produce". The word equation can therefore be read as follows: "Magnesium reacts with oxygen to produce magnesium oxide." Magnesium and oxygen are reactants and the magnesium oxide is the lone product. |
A word equation gives limited information. It identifies only the reactant and products. It does not give their formulae, nor does it tell you the masses of reactants needed or the mass of product produced. |
However at this point in time, you should be able to make up chemical formula based upon the name. |
Elements in Skeleton Equations |
There are 109 elements. A few need to be treated in a special way because of how they bond with each other. You never find elemental oxygen by itself. Elemental oxygen is always O_{2}. Oxygen is one of the diatomic elements. |
The other diatomic elements are: H_{2}, F_{2}, Cl_{2}, Br_{2}, I_{2}, O_{2}, and N_{2}. |
Please note that these are all gases. When substituing these into skeleton equations make sure that you use the correct formulas. All other elements can be treated as if they were monoatomic. (Act like lone atoms.) |
Skeleton Equations |
Skeleton equations are simply the bare bones of a chemical
equation.
The chemical formulae are substituted into the word equation. The
skeleton equation for the reaction above is:
Mg + O_{2} ---------> MgO |
The formulas are written first. Each formula should be checked at this time to make sure that they are correct. If they are not correct then the equation probably will not balance later. |
Balanced Equations |
A skeleton equation again provides limited information. It tells you what the chemical formulas are for the reactants and products, but again it tells you nothing about how much. If you observe the skeleton equation above carefully, you'll see that there is 1 Mg atom on the left and 1 Mg atom on the right. At least that part is balanced. There is however, 2 O atoms on the left and 1 O atom on the right. If even one atom does not match up evenly on both sides then the equation is unbalanced. |
A chemical equation must be balanced. The number of atoms of each type must be the same on both sides of the equation. You must never change the subscripts inside a formula. After all, these have been discovered experimentally and cannot be changed at the whim of a chemistry student or teacher. The only way to balance an equation is to place numbers, called coefficients, in front of whole formulas. A coefficient applies to the entire molecule that follows it. |
Start with a skeleton equation: Mg + O_{2} -------> MgO |
The oxygen atoms did not match so place a 2 in front of
the MgO
so that there are now two O on the right.
Mg + O_{2} --------> 2 MgO |
Now the oxygen atoms balance but the magnesium atoms no
longer match.
We have placed a 2 in front of the MgO which means we have 2 Mg atoms
and
2 O atoms in total. If we place a 2 in front of the Mg on the
left
side then we get a balanced equation.
2 Mg + O_{2} --------> 2 MgO |
If you check the number of atoms on each side of the equation you'll see that it is balanced. The method used above is called balancing by inspection. It is used on the simpler types of equations. |