Page 344 - ABCTE Study Guide_Neat

Matter is constantly changing. The food that you eat becomes part of your body; gasoline becomes
carbon dioxide, water, heat, and the energy that runs your car; the ink in your printer cartridge binds with
paper to form a document. The process by which atoms of substances rearrange to form new substances
is called a chemical reaction. Some indications of a chemical reaction include a change in temperature,
such as in fire, a color change that indicates the presence of a different substance, and the generation of
electrical current by a battery. Chemical reactions are written using shorthand similar to a mathematical
equation:

reactant A + reactant B product C + product D

But there are many different types of reactions. For example, burning hydrogen in oxygen can be written
as:

In this reaction, two reactants (hydrogen and oxygen) combine to form one product—water.

You can see that the equation for the reaction of hydrogen and oxygen indicates more than just the
reactants and products. It further shows that two molecules of hydrogen react with a single molecule of
oxygen to produce two molecules of water. The total number of hydrogen atoms (four) and the total
number of oxygen atoms (two) are the same on both sides (reactants and products) of the reaction.
Chemical reactions do not change the identity of any atom. There are always the same numbers and
types of atoms before and after the reaction; they are just arranged differently. Although new
combinations of molecules exist, the equation must bebalanced to show all atoms are accounted for.

Equations are balanced by changing the number of molecules in the equation. Coefficients, which indicate
number of units, called superscripts, can be changed. Subscripts that identify the number of atoms within
a molecule, however, cannot be changed. A general method to balacne chemical reactions is:

Balance metals first

Non-metals except oxygen are balanced next

Balance the hydrogen

Finally, balance oxygen

In order to balance the following equation, we start with a basic equation that shows all the reactants and
the products. For example:

ethane + oxygen carbon dioxide + water

To balance the equation, write the reaction in equation form

Next count the atoms on each side of the equation:

reactant: C – 2 ; H – 6 ; O – 2

product: C – 1 ; H – 2 ; O – 3