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. models a chemical reaction using the formulaA combination of symbols that indicates the chemical composition of a substance. 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.
Balancing an equation
If you just write an equation replacing names with formulae, it may not be balanced. The numbers of atomThe smallest part of an element that can exist. of each elementA substance made of one type of atom only. on the left must be the same as they are on the right.
To balance an unbalanced equation, you need to 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 equal numbers of atoms of each element on both sides. Here there aren’t. | N2 + H2 → NH3 |
| There are two nitrogen atoms on the left but only one on the right, so a big 2 is added to the left of the NH3. | N2 + H2 → 2NH3 |
| There are two hydrogen atoms on the left but (2 × 3) = 6 on the right, so a big 3 is placed in front of the H2. | N2 + 3H2 → 2NH3 |
| Check to see if there are equal numbers of each element of both sides. There are. | (two nitrogen atoms and six hydrogen atoms) |
| Add the state symbols if they are requested. | N2(g) + 3H2(g) → 2NH3(g) |
| Step | Check to see if there are equal numbers of atoms of each element on both sides. Here there aren’t. |
|---|---|
| Result | N2 + H2 → NH3 |
| Step | There are two nitrogen atoms on the left but only one on the right, so a big 2 is added to the left of the NH3. |
|---|---|
| Result | N2 + H2 → 2NH3 |
| Step | There are two hydrogen atoms on the left but (2 × 3) = 6 on the right, so a big 3 is placed in front of the H2. |
|---|---|
| Result | N2 + 3H2 → 2NH3 |
| Step | Check to see if there are equal numbers of each element of both sides. There are. |
|---|---|
| Result | (two nitrogen atoms and six hydrogen atoms) |
| Step | Add the state symbols if they are requested. |
|---|---|
| Result | N2(g) + 3H2(g) → 2NH3(g) |
State symbols
Balanced equations often 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 aqueousDissolved in water to form a solution. Shown as (aq) in chemical equations. 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
- HCl(g) is hydrogen chloride gas, but HCl(aq) is hydrochloric acid
Question
Sodium metal reacts with water to form sodium hydroxide solution and hydrogen gas. Write a balanced equation for the reaction, including state symbols.
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g) or
Na(s) + H2O(l) → NaOH(aq) + ½H2(g)
