When writing balanced chemical equations it is essential that you first get the individual chemical formulas correct. These formulas are based on the type of bonding that the chemicals undergo and the number of electrons that are required to give the atom a complete outer shell.
Metallic bonding →
Ionic bonding →
Covalent bonding →
Between Metals →
Between metals & non-metals →
Between metals & non-metals →
Sea of electrons
Donate and accept electrons
Share electrons
Example:
1. Sodium Chloride – Sodium has a positive +1 charge and Chloride has a -1 charge. The two charges cancel each other out in order to make a neutral molecule – NaCl
2. Barium Chloride – Barium has a charge of +2 and Chlorine has a charge of -1. Therefore to make it a neutral we need two chlorine ions. The formula will be BaCl2 to make it a neutral molecule.
3. Aluminium Hydroxide – Aluminium has a charge of +3 and Hydroxide has a charge of -1 therefore we need 3 hydroxides to balance out the aluminium Al(OH)3. Brackets are placed around the hydroxide ion as both the oxygen and the hydrogen is multiplied by 3.
| Positive Ions | Negative Ions |
|---|---|
| Hydrogen H+ | Chlorine Cl – |
| Sodium Na+ | Bromine Br – |
| Silver Ag+ | Fluoride Fl – |
| Potassium K+ | Iodine I – |
| Lithium Li+ | Hydroxide OH – |
| Barium Ba2+ | Nitrate NO3- |
| Calcium Ca2+ | Oxide O2- |
| Copper Cu2+ | Sulphide S2- |
| Magnesium Mg2+ | Carbonate CO32- |
| Zinc Zn2+ | |
| Lead Pb2+ | |
| Iron (III) Fe3+ | |
| Aluminium Al3+ |
Steps to Write a Chemical Equation
1. The first step in writing a chemical equation is to get the worded equation established first. Some common chemical equations are:
Combustion
Hydrocarbon + Oxygen gas → carbon dioxide + water
Neutralisation
Acid + base → salt + water
Acid + metal → hydrogen gas + salt
Acid + carbonate → carbon dioxide + salt
Decomposition reactions – breakdown of a chemical
Combination reactions – joining of two chemicals
Metal + oxygen → metal oxide
Metal oxide + water → base
2. Using the notes above, write out the individual formulas for each of the chemicals involved in the reaction.
3. Now it is necessary to balance an equation. When a chemical reaction takes place, all of the atoms on the left-hand side need to be accounted for on the right-hand side. That is everything that was there at the beginning of the reaction is still there at the end of the reaction, it is just in a different form.
Balancing Equations
When balancing equations, it is important to only alter the coefficients, that is the BIG NUMBERS out the front of each chemical. By altering the small numbers, you are changing the chemical formula
Example 1:
H2 + O2 → H2O
This reaction shows that we have two hydrogen atoms on the left-hand side and two hydrogen atoms on the right-hand side of the arrow. On the left-hand side of the arrow, we have two oxygen atoms but only one oxygen on the right-hand side. We need to add a two in front of the water molecule to balance the oxygen, however this causes problems with the hydrogen. A two also needs to be added in front of the hydrogen molecule making the equation:
2H2 + O2 → 2H2O
Now it is important to put in the states:
(s) – solid
(g) – gas
(l) – liquid
(aq) – aqueous (dissolved in water)
Hence the final equation is:
2H2(g) + O2(g) → 2H2O(g)
Need more help?
If you need more help on this topic, you can book a lesson with one of our Chemistry tutors. Pay-as-you-go lessons with no minimum number of sessions.
Lessons are held over Skype and FaceTime. Click on the button below to book a lesson