Chemistry 0.1 - 0.3 - Formulae and equations

Chemical formulae and identity

Chemists need a representation that says exactly which substance they mean. An element symbol names one element, a chemical formula shows the elements present and their proportions, and a chemical equation shows a reaction. These are related ideas, but they are not interchangeable.

For a molecule, a formula gives the actual number of each type of atom in one molecule. $\ce{H2O}$ therefore contains two hydrogen atoms and one oxygen atom. For an ionic substance, a formula gives the simplest whole-number ratio of ions: $\ce{NaCl}$ gives a 1:1 ratio, not a single “sodium chloride molecule”.

The formula is part of the substance's identity. $\ce{CO}$ and $\ce{CO2}$ are different compounds, and $\ce{O}$ and $\ce{O2}$ do not mean the same thing. Changing a subscript changes the substance.

A core recall bank

This bank is a starting set. As a named substance or ion appears elsewhere in the course, learn its formula in that chemical context rather than trying to invent it from its English name.

Three details prevent many errors:

1. Case matters: $\ce{Co}$ is cobalt, while $\ce{CO}$ is carbon monoxide.
2. Position matters: the small 4 in $\ce{SO4^2-}$ counts oxygen atoms; the raised $2-$ is the ion's overall charge.
3. A formula is not an equation: $\ce{H2O}$ identifies water; it does not say how water was made.

Reactions in words

Suppose magnesium burns in oxygen. At the macroscopic level, there is an intense white light and a white solid forms. These observations are evidence of chemical change. At the sub-microscopic level, magnesium atoms and oxygen molecules are rearranged into particles of magnesium oxide; the atoms are not created or destroyed.

A word equation records the named starting substances and products:

magnesium + oxygen → magnesium oxide

• Reactants are the starting substances, written to the left of the arrow.
• Products are the new substances, written to the right.
• A $+$ separates two substances on the same side; it does not mean ordinary numerical addition.
• The arrow means “reacts to form” or “produces”. It is directional, so it is not an equals sign.

To write a word equation from a description, first identify every starting substance, then every stated product. Use chemical names only. For example, if calcium carbonate decomposes to calcium oxide and carbon dioxide: