Chemical Formulae and Equations
Word Equations
A word equation shows the reactants (left of the arrow) and products (right of the arrow) of a chemical reaction using full chemical names.
Format: reactant 1 + reactant 2 → product 1 + product 2
Example: sodium hydroxide + hydrochloric acid → sodium chloride + water
Reaction conditions or catalyst names are written above the arrow.
Balanced Symbol Equations
A symbol equation uses chemical formulae. It must be balanced so that the same number of atoms of each element appears on both sides of the equation (Law of Conservation of Mass: atoms cannot be created or destroyed).
State Symbols
Always include state symbols unless told otherwise:
| Symbol | Meaning |
|---|---|
| (s) | Solid |
| (l) | Liquid |
| (g) | Gas |
| (aq) | Aqueous (dissolved in water) |
Rules for Balancing
- Write out the unbalanced equation first.
- Count atoms of each element on each side.
- Add large numbers (coefficients) in front of whole formulae to balance. Never change the small numbers inside a formula (that would change the substance).
- Balance elements that appear in only one reactant and one product first; leave elements that appear in multiple places until last.
- Count groups such as NO3 as a single unit if they appear unchanged on both sides.
Example: Balance Al + CuO → Al2O3 + Cu
- There are 2 Al needed on the left: 2Al + CuO → Al2O3 + Cu
- There are 3 O on the right, so need 3 CuO: 2Al + 3CuO → Al2O3 + Cu
- There are now 3 Cu on the left, so need 3 Cu on the right: 2Al + 3CuO → Al2O3 + 3Cu
Balanced equation: 2Al(s) + 3CuO(s) → Al2O3(s) + 3Cu(s)
Relative Atomic Mass (Ar) and Relative Formula Mass (Mr)
Relative atomic mass (Ar): the weighted average mass of one atom of an element relative to 1/12 of a carbon-12 atom. Given on the Periodic Table (the larger of the two numbers).
Relative formula mass (Mr): the sum of the relative atomic masses of all atoms shown in a chemical formula.
How to calculate Mr:
- Identify all atoms in the formula.
- Multiply the number of each type of atom by its Ar.
- Add all the values together.
| Substance | Calculation | Mr |
|---|---|---|
| H2 | 2 x 1 | 2 |
| H2O | (2 x 1) + (1 x 16) | 18 |
| NaCl | 23 + 35.5 | 58.5 |
| K2CO3 | (2 x 39) + 12 + (3 x 16) | 138 |
| Ca(OH)2 | 40 + (2 x 16) + (2 x 1) | 74 |
| Mg(NO3)2 | 24 + (2 x 14) + (6 x 16) | 148 |
Exam Tip
The relative atomic masses of all elements are given on the Periodic Table. The Mr is always the larger of the two numbers next to the symbol.
Writing Formulae for Ionic Compounds
The overall charge of an ionic compound is zero. To find the formula:
Method 1: Direct comparison: if the charges are equal and opposite, one of each ion is needed.
Method 2: Swap and drop: swap the numbers of the charges onto the other ion (ignoring signs), then simplify if necessary.
Example: Copper(II) chloride. Cu has charge 2+, Cl has charge 1-.
- Swap charges: 2 Cl needed for every 1 Cu
- Formula: CuCl2
Example: Magnesium hydroxide. Mg has charge 2+, OH has charge 1-.
- Formula: Mg(OH)2 (brackets needed when there are multiple polyatomic ions)
Common Ion Charges to Learn
| Ion | Charge |
|---|---|
| Group 1 metals (Li, Na, K) | 1+ |
| Group 2 metals (Mg, Ca) | 2+ |
| Group 3 metals (Al) | 3+ |
| Group 5 non-metals (N, P) | 3- |
| Group 6 non-metals (O, S) | 2- |
| Group 7 non-metals (F, Cl, Br, I) | 1- |
| Ag+ | 1+ |
| Cu2+ | 2+ |
| Fe2+ | 2+ |
| Fe3+ | 3+ |
| Pb2+ | 2+ |
| Zn2+ | 2+ |
| H+ | 1+ |
| OH- | 1- |
| NH4+ | 1+ |
| CO32- | 2- |
| NO3- | 1- |
| SO42- | 2- |
Exam Questions
Q: Write a balanced symbol equation for the combustion of magnesium.
2Mg(s) + O2(g) → 2MgO(s)
Q: Balance the following equation: Fe2O3 + C → Fe + CO2
2Fe2O3(s) + 3C(s) → 4Fe(l) + 3CO2(g)
Q: Calculate the relative formula mass of ammonium sulfate, (NH4)2SO4. (H=1, N=14, O=16, S=32)
Mr = (2 x 14) + (8 x 1) + 32 + (4 x 16) = 28 + 8 + 32 + 64 = 132
Q: Write the formula of the compound formed between aluminium (Al3+) and oxide (O2-) ions.
Al2O3 (swap charges: 2 Al for every 3 O)
Q: Why is it incorrect to balance an equation by changing the small numbers within a formula?
Changing the small numbers in a formula changes the identity of the substance. For example, changing H2O to H2O2 would change water into hydrogen peroxide, a completely different compound.
Exam Tip
Non-metals that exist as diatomic molecules must be written as molecules in equations: H2, N2, O2, F2, Cl2, Br2, I2. Writing just O instead of O2 for oxygen gas is a common mistake.