Balanced chemical equations
A balanced chemical equationA chemical equation written using the symbols and formulae of the reactants and products, so that the number of units of each element present is the same on both sides of the arrow. represents a chemical reaction using the formulae of the reactantA substance that reacts together with another substance to form products during a chemical reaction. and productA substance formed in a chemical reaction.. It shows the number of units of each substance involved.
State symbols
Balanced chemical equations sometimes include state symbolA symbol used in chemical equations to show if a substance is a solid, a liquid, a gas, or an aqueous solution. in brackets after each formula. They show the physical state of that substance.
| State symbol | Meaning |
| (s) | Solid |
| (l) | Liquid |
| (g) | Gas |
| (aq) | Aqueous solution |
| State symbol | (s) |
|---|---|
| Meaning | Solid |
| State symbol | (l) |
|---|---|
| Meaning | Liquid |
| State symbol | (g) |
|---|---|
| Meaning | Gas |
| State symbol | (aq) |
|---|---|
| Meaning | Aqueous solution |
An aqueous solution forms when a substance dissolves in water.
State symbols are useful because they show what a substance is like. For example:
- H2O(l) is liquid water but H2O(g) is steam and H2O(s) is ice
- HCl(g) is hydrogen chloride gas but HCl(aq) is hydrochloric acid
Balancing an equation
The law of conservation of massThe total mass of reactants before a reaction is equal to the total mass of products after a reaction, because no atoms can be created or destroyed so the mass must remain unchanged. states that no atomThe smallest part of an element that can exist. are lost or made during a chemical reaction, so the total massThe amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). of the productA substance formed in a chemical reaction. is equal to the total mass of the reactantA substance that reacts together with another substance to form products during a chemical reaction..
This means that chemical reactions can be represented by symbol equations. A balanced symbol equation has the same number of atoms of each elementA substance made of one type of atom only. on both sides of the arrow.
To balance an equation, add numbers to the left of one or more formulae. Here is one way to work out how to do this for the reaction between nitrogen and hydrogen.
| Step | Result |
| Check to see if there are an equal number of atoms of each element on both sides. There aren't. | N2 + H2 → NH3 |
| There are two nitrogen atoms on the left but only one on the right, so put a big 2 on the left of the NH3. | N2 + H2 → 2NH3 |
| Check again. There are two hydrogen atoms on the left but (2 × 3) = 6 on the right, so put a big 3 in front of the H2. | N2 + 3H2 → 2NH3 |
| Check again to see if there are equal numbers of each element on both sides. There are. | (Two nitrogen atoms and six hydrogen atoms) |
| Add the state symbols if asked to do so. | N2(g) + 3H2(g) → 2NH3(g) |
| Step | Check to see if there are an equal number of atoms of each element on both sides. There aren't. |
|---|---|
| Result | N2 + H2 → NH3 |
| Step | There are two nitrogen atoms on the left but only one on the right, so put a big 2 on the left of the NH3. |
|---|---|
| Result | N2 + H2 → 2NH3 |
| Step | Check again. There are two hydrogen atoms on the left but (2 Ă— 3) = 6 on the right, so put a big 3 in front of the H2. |
|---|---|
| Result | N2 + 3H2 → 2NH3 |
| Step | Check again to see if there are equal numbers of each element on both sides. There are. |
|---|---|
| Result | (Two nitrogen atoms and six hydrogen atoms) |
| Step | Add the state symbols if asked to do so. |
|---|---|
| Result | N2(g) + 3H2(g) → 2NH3(g) |