Guidelines for Balancing Chemical Equations

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Balancing the chemical equation for the reaction between phosphorus pentoxide and water to make phosphoric acid.
Balancing the chemical equation for the reaction between phosphorus pentoxide and water to make phosphoric acid involves balancing number of atoms and moles so they are the same on both sides of the reaction. (Ephert)

One of the basic skills you develop while studying chemistry is balancing chemical equations. Writing a balanced equation puts you in a good position to perform all manner of calculations!

What Is a Balanced Chemical Equation?

A balanced chemical equation is an equation that accurately represents a chemical reaction while following the Law of Conservation of Mass, which states that matter cannot be created or destroyed in a chemical reaction. In a balanced equation, the number of atoms of each element is the same on both sides of the reaction.

Components of a Balanced Chemical Equation

Here is a closer look at the parts of a balanced equation:

  1. Reactants – The starting substances that undergo a chemical change. These appear on the left side of the equation.
  2. Products – The substances formed as a result of the chemical reaction. These appear on the right side of the equation.
  3. Coefficients – Whole numbers placed in front of chemical formulas to ensure that the equation is balanced.
  4. Subscripts – Whole number subscripts immediately following element symbols that give the number of atoms.
  5. State Symbols (Optional but Useful) – Indicate the physical state of the substances, such as:
    • (s) = solid
    • (l) = liquid
    • (g) = gas
    • (aq) = aqueous (dissolved in water)
  6. Reaction Arrow (e.g., → or =) – Separates reactants from products and shows the direction of the reaction.

Example of a Balanced Chemical Equation

2H2 + O2 → 2H2O2

  • Reactants: H2​ (hydrogen gas) and O2​ (oxygen gas).
  • Products: H2O (water).
  • Coefficients: 2 before H2 and H2​O ensure that there are equal numbers of hydrogen and oxygen atoms on both sides.

Balancing equations is essential in chemistry to accurately represent chemical reactions and perform stoichiometric calculations. The quantities are expressed as moles.

Steps for Balancing Chemical Equations

It takes practice to be able to write balanced equation. There are essentially three steps to the process:

  • Write the unbalanced equation.
    • Chemical formulas of reactants are listed on the lefthand side of the equation.
    • Products are listed on the righthand side of the equation.
    • Reactants and products are separated by putting an arrow between them to show the direction of the reaction. Reactions at equilibrium will have arrows facing both directions.
  • Balance the equation.
    • Apply the Law of Conservation of Mass to get the same number of atoms of every element on each side of the equation. Tip: Start by balancing an element that appears in only one reactant and product.
    • Once one element is balanced, proceed to balance another, and another, until all elements are balanced.
    • Balance chemical formulas by placing coefficients in front of them. Do not add subscripts, because this will change the formulas.
  • Indicate the states of matter of the reactants and products.
    • Use (g) for gaseous substances.
    • Use (s) for solids.
    • Use (l) for liquids.
    • Use (aq) for species in solution in water.
    • Write the state of matter immediately following the formula of the substance it describes.

Balancing Chemical Equations Worked Example Problem

Tin oxide is heated with hydrogen gas to form tin metal and water vapor. Write the balanced equation that describes this reaction.

  1. Write the unbalanced equation.

    SnO2 + H2 → Sn + H2O
  2. Balance the equation. Look at the equation and see which elements are not balanced. In this case, there are two oxygen atoms on the left side of the equation and only one on the righthand side. Correct this by putting a coefficient of 2 in front of water:

    SnO2 + H2 → Sn + 2 H2O

  3. This puts the hydrogen atoms out of balance. Now there are two hydrogen atoms on the left and four hydrogen atoms on the right. To get four hydrogen atoms on the right, add a coefficient of 2 for the hydrogen gas. Remember, coefficients are multipliers, so if we write 2 H2O it denotes 2×2=4 hydrogen atoms and 2×1=2 oxygen atoms.


    SnO2 + 2 H2 → Sn + 2 H2O

    The equation is now balanced. Be sure to double-check your math! Each side of the equation has 1 atom of Sn, 2 atoms of O, and 4 atoms of H.

  4. Indicate the physical states of the reactants and products. To do this, you need to be familiar with the properties of various compounds or you need to be told what the phases are for the chemicals in the reaction. Oxides are solids, hydrogen forms a diatomic gas, tin is a solid, and the term ‘water vapor’ indicates that water is in the gas phase:

    SnO2(s) + 2 H2(g) → Sn(s) + 2 H2O(g)


    There you go! This is the balanced equation for the reaction. Remember, no elements appear on one side of the reaction and not on the other. Check your work by counting up the number of atoms of each element just to be sure you have balanced your equation correctly. This was a straightforward example, using conservation of mass. For reactions involving ions, conservation of charge also comes into play.

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Redox Reactions Terminology

Balancing Redox Reactions

Sometimes the reactants or products are ions. Learn how to balance equations for both mass and charge,

Common Mistakes and Misconceptions

Changing Subscripts Instead of Coefficients

One of the most frequent mistakes students make is altering the subscripts of a compound instead of adjusting the coefficients. The subscripts define the chemical identity of a molecule, and changing them creates a different substance. Always adjust only the coefficients when balancing.

Forgetting to Count Polyatomic Ions as a Whole

When balancing equations with polyatomic ions that remain unchanged on both sides of the reaction, it’s often easier treating them as a unit instead of breaking them into individual atoms. This simplifies the balancing process.

Ignoring the Law of Conservation of Mass

Every balanced chemical equation must obey the Law of Conservation of Mass, meaning the number of atoms of each element must be the same on both sides. Failing to verify this is a common mistake.

Not Balancing Oxygen and Hydrogen Last

When balancing combustion reactions or reactions involving oxygen and hydrogen, it’s often easier balancing other elements first and leave oxygen and hydrogen for the final steps, as they are commonly found in multiple compounds.

Forgetting to Check the Final Answer

After balancing an equation, always double-check by counting the atoms on both sides. Even minor miscalculations can lead to an incorrect answer.

References

  • Brady, James E.; Senese, Frederick; Jespersen, Neil D. (2007). Chemistry: Matter and Its Changes. John Wiley & Sons. ISBN 9780470120941.
  • Marshall, Hugh (1902). “Suggested Modifications of the Sign of Equality for Use in Chemical Notation”. Proceedings of the Royal Society of Edinburgh. 24: 85–87. doi:10.1017/S0370164600007720
  • Thorne, Lawrence R. (2010). “An Innovative Approach to Balancing Chemical-Reaction Equations: A Simplified Matrix-Inversion Technique for Determining the Matrix Null Space”. Chem. Educator. 15: 304–308. doi:10.48550/arXiv.1110.4321