To balance a chemical equation in chemistry, you must follow the law of conservation of mass, which stipulates that the total molar mass of the reactants must be equal to the total molar mass of the products. This is done by writing numerical coefficients to indicate how many moles of each substance are present during a chemical reaction.
It’s important to note that you can only use whole numbers when balancing equations in chemistry. A balanced chemical equation that has whole numbers as coefficients serves as the standard for calculating the proportion of the same substances with variable amounts. This is very useful in laboratory analytical experiments and in chemical manufacturing.
Read on to learn more about chemical equations and how to balance an equation in chemistry.
In this post:
What are chemical equations?
Chemical equations are symbolic representations of ideal chemical reactions. They typically include the symbols of the elements in chemical formulas, which are usually written with numerical coefficients preceding them to ensure the chemical equation is balanced.
Chemical equations also contain plus signs where applicable, such as when there are two or more reactants or two or more products. The reactants are on the left-hand side of the equation while the products are on the right.
An arrow symbol (rather than an equals symbol) is normally used to separate the reactants from the products. Although the arrow usually points to the direction of the products, a chemical equation sometimes contains two arrows that point in either direction. This indicates dynamic equilibrium, which means the reaction goes both ways.
Other notations may also be written above the arrow to indicate the presence of a catalyst, while small letters in parentheses after the subscripts signal the state or phase of matter e.g solid, liquid or gas.
Here is a simple example of a chemical equation representing the reaction between hydrogen and oxygen, which forms water.
2H2 + O2 → 2H2O
The balanced chemical equation indicates that two moles of diatomic hydrogen must react with one mole of diatomic oxygen to produce two moles of water.
Balancing equations in chemistry
Balancing equations in chemistry is akin to accounting or bookkeeping in finance; the credit side must equal the debit side. This is crucial for establishing the standard ideal proportional relationship between the reactants and products based on the law of conservation of mass.
The basic assumptions in balancing chemical equations are the following:
- The law of conservation of mass – this states that “mass can neither be created nor destroyed” in chemical reactions, so the total mass of the reactants is equal to the total mass of the products
- The substances involved are 100% pure – in actual experiments, it would be very difficult or even virtually impossible to obtain 100% pure reactants; there will always be some contaminants or impurities. In chemical equations, however, you can theoretically assume that the reactants are absolutely pure
- The reaction occurs under standard conditions – the standard conditions refer to the temperature, pressure, and concentration of the reactants. These are 25°C, 1 atm and 1M, respectively.
Examples of chemical equations
Not all chemical equations are written as balanced chemical reactions when it comes to elements or molecules. Some are half equations, such as those representing electrochemical reactions in batteries.
Here are some examples:
- Zn = Zn2+ + 2e– (oxidation reaction)
- 2H+ + 2e– = H2 (reduction reaction)
- HCl + NaOH → NaCl + H2O (acid-base neutralisation)
- 6CO2 + 6H2O + light → C6H12O6 + 6O2 (photosynthesis)
- C12H22O11 (s) + H2SO4 (aq) + 1/2 O2 (g) → 11C (s) + CO2 (g) + 12H2O (g) + SO (dehydration of sucrose)
- 2Na + Cl2 → 2NaCl
- CH4 + 2O2 → 2H2O + CO2 (complete combustion of methane)
Steps to balance a chemical equation
In most cases, balancing chemical equations is a case of trial and error. It may involve some degree of intuition, although you can also be methodical in your approach. Put simply, it’s about ensuring that the corresponding coefficients would result in equal numbers of atoms on both sides of the equation.
Here are some steps that you can follow to balance a chemical equations:
- Step 1: Write the reactants and the products.
- Step 2: Count the number of atoms on both sides of the equation. If they are already equal then there’s no need to balance the equation. However, if they aren’t, you’ll need to proceed to the next step.
- Step 3: You may want to tabulate the number of atoms for easy comparison. It’s important that you don’t alter the subscripts as this will automatically change the chemical identities of the substances involved.
- Step 4: Change the numerical coefficients relating to the atoms or groups of atoms. There may be some cases when atoms are grouped together. Instead of treating them individually, you can treat them as groups.
- Step 5: Repeat step 2. If the equation is still not balanced, you’ll need to try other coefficients.
The steps outlined above follow the traditional “trial and error” method. However, another way to balance a chemical equation is by using the algebraic method. Although similar to the trial and error method, it involves some basic algebra of assigning symbols to the unknown coefficients.
Balancing chemical equations is important for two main reasons – it allows us to make comparisons in analytical experiments and it helps with efficiency in manufacturing. The main methods of balancing chemical equations are the traditional “trial and error” technique and the algebraic method.
The blog on chemicals.co.uk and everything published on it is provided as an information resource only. The blog, its authors and affiliates accept no responsibility for any accident, injury or damage caused in part or directly from following the information provided on this website. We do not recommend using any chemical without first consulting the Material Safety Data Sheet which can be obtained from the manufacturer and following the safety advice and precautions on the product label. If you are in any doubt about health and safety issues please consult the Health & Safety Executive (HSE).