Writing Chemical Equations


Some Rules for Writing Chemical Equations

 
 
  • The same number of each type of atom must appear on both sides of the equation.  In a chemical process atoms cannot be created or destroyed they can only recombine to form different compounds or revert back to their elemental forms.
  • Chemical formulas show fixed ratios of component elements.  A different ratio shows that a different compound has been formed.
  • Chemical equations must be balanced.  The same number of each element present before the reaction takes place must also be present on the product side of the equation.  Coefficients are placed in front of a chemical formula to show the number of moles of that substances that are necessary for the reaction to occur.  The coefficients can be changed in order to balance the equation provided that a compensating additiion or subtraction of the elements present occurs on the other side of the equation.


 

Let's look at a chemical reaction to practice these rules.  During combustion, magnesium reacts with the oxygen in air to produce a white powder, magnesium oxide.  This is the reaction used in camera flash bulbs because the energy that is released during this reaction takes the form of a bright white flash of light.  You might start by writing an equation for this reaction by writing the symbols for the reactants and products.

On the lefthand side of the equation there is 1 atom of magnesium and there is also 1 atom of Mg on the product side of the equation.  However, there are 2 oxygen atoms on the left and only 1 on the right.  To balance the number of atoms of each type you will need to add coefficients, a number by which all of the elements in the compound the coefficient proceeds will be multiplied.  If you multiply Mg on the lefthand side of the equation by 2 then you must do the same to the Mg atom on the right which will mean doubling the number of oxygen atoms on the right hand side of the equation too since you may not change the ratio of atoms within a molecule because it will change the identity of the substance.  The new equation will look like this:
Now the equation is balanced.  There are the same number of Mg and O atoms on both sides of the equation.  A balanced equation is the source of a great deal of information about both products and reactants during a chemical change. 

Combined with stoichiometry, a chemical equation allows you to move from microscopic interactions between individual atoms and molecules to reactants and products on a larger, more human scale.  The information contained in a balanced equation is summarized in the following table for the combustion of methane, or natural gas, in air.


 


Information Contained in a Balanaced Equation
VIEWED IN TERMS OF REACTANTS
CH4 (g)  +  2O2(g)		 PRODUCTS
 CO2(g)   +  2 H2O(l)
Molecules
1 molecule of methane + 2
molecules of  oxygen		 1 molecule of carbon dioxide  + 2 molecules of water
Moles
1 mol CH4 (g)  + 2 mol  O2(g)		 1 mol CO2 + 2 mol H2O
Mass (g)
16.04 g  CH4 + 64.00 g  O2		 44.01 g CO2  + 36.04 g H2O
Total Mass (g)
80.0  g		  80.0  g
Quick Quiz: Which of the following statements is false?

The total mass of reactants and products can change during a chemical reaction.

The number of atoms or moles of each element on the reactant side of an equation must be the same as the number of that element on the product side.

The compounds on the reactant side of the equation are not necessarily the same as the compounds on the product side of the equation.

A chemical equation can be interpreted in terms of moles of each compound or element in the equition or in terms of atoms.

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