Chemical Formulae and Equations

Chemical Formulae and Equations

Writing Balanced Equations

You must always have the same number of atoms of each element on either side of the equation

• Work across the equation from left to right, checking one element after another, except if an element appears in several places in the equation. In that case, leave it until the end.
• If there is a group of atoms (such as nitrate group, NO3), which has not changed from one side to the other, then count the whole groups, rather than counting the individual atoms.
• Check everything at the end

Using State Symbols in Equations

State Symbols are sometimes written after formulae in chemical equations to show which physical state each substance is in.

There are 4 different state symbols that you need to know. They are: 

(s) - solid      (l) - liquid     (g) - gas     (aq) - aqueous

An example of an equation with state symbols is: 

Zn(s) + CuSO4(aq) ----> ZnSO4(aq) + Cu(s)

This equation tells you that when SOLID zinc is added to an AQUEOUS solution of copper(II) sulfate, an AQUEOUS soloution of zinc sulfate and SOLID copper are formed.

State symbols are very useful since they tell you the CONDITIONS required for a reaction to take place. If they were left out in the above equation then you might think that the reaction would take place if you added solid zinc to solid copper(II) sulfate. THIS IS NOT THE CASE; no reaction takes place under these conditions.

Calculating Empirical Formulae

The EMPIRICAL FORMULA of a compound gives the SIMPLEST WHOLE-NUMBER RATIO of atoms of each element in the compound. It can be calculated from knowledge of the ratio of masses of each element in the compound.

For example, a compound that contains 10g of hydrdogen and 80g of oxygen has an empirical formula of H2O. This can be shown by the following calculations:

Amount of hydrogen atoms = mass in grams / RAM of hydrogen = 910/1) = 10mol

Amount of oxygen atoms = mass in grams / RAM of oxygen = (80/16) = 5mol

Therefore, the ratio of moles of hydrogen atoms to moles of oxygen atoms is 10:5

This is 2:1 in its simplest form

Since equal numbers of moles of atoms contain the same number of atoms, it follows that the ratio of hydrogen atoms to oxygen atoms is 2:1

Hence the empirical formula is H2…