Guidelines for Balancing Chemical Equations

Balancing the equation for the reaction between phosphorus pentoxide and water to make phosphoric acid.
Balancing the 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 will develop as you study chemistry is the ability to balance a chemical equation. One you have written a balanced equation, you will be in a good position to perform all manner of calculations! Balancing a chemical equation refers to establishing the mathematical relationship between the quantity of reactants and products. The quantities are expressed as grams or moles.

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.

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. It’s always a good idea to check 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 will also come into play.