Balancing Chemical Equations

Introductory Stoichiometry and Mass Relations in Chemical Equations

A balanced chemical equation states the reactants and products in a chemical reaction.
A balanced chemical equation states the reactants and products in a chemical reaction. Jeffrey Coolidge / Getty Images

A chemical equation describes what happens in a chemical reaction. The equation identifies the reactants (starting materials) and products (resulting substances), the formulas of the participants, the phases of the participants (solid, liquid, gas), the direction of the chemical reaction, and the amount of each substance. Chemical equations are balanced for mass and charge, meaning the number and type of atoms on the left side of the arrow is the same as the number of type of atoms on the right side of the arrow.

The overall electrical charge on the left side of the equation is the same as the overall charge on the right side of the equation. In the beginning, it's important to first learn how to balance equations for mass.

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 equations. There are essentially three steps to the process:

3 Steps for Balancing Chemical Equations

  1. 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.
    • Use the one- and two-letter element symbols to identify elements.
    • When writing a compound symbol, the cation in the compound (positive charge) is listed before the anion (negative charge). For example, table salt is written as NaCl and not ClNa.
  1. 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.
  1. 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.
    • Generally, there isn't a space between the compound and the state of matter.
    • Write the state of matter immediately following the formula of the substance it describes.

Balancing Equation: 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

Refer to Table of Common Polyatomic Ions and Formulas of Ionic Compounds if you have trouble writing the chemical formulas of the products and reactants.

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 lefthand 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

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.

The coefficient is a number that goes in front of a chemical formula. Remember, coefficients are multipliers, so if we write 2 H2O it denotes 2x2=4 hydrogen atoms and 2x1=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.

3. 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)

This is the balanced equation for the reaction. Be sure to check your work!

Remember Conservation of Mass requires the equation to have the same number of atoms of each element on both sides of the equation. Multiply the coefficient (number in front) times the subscript (number below an element symbol) for each atom. For this equation, both sides of the equation contain:

  • 1 Sn atom
  • 2 O atoms
  • ​4 H atoms

If you'd like more practice, review another example of balancing equations. If you think you're ready, try a quiz to see if you can balance chemical equations.

Worksheets for Practicing Balancing Equations

Here are some worksheets with answers you can download and print to practice balancing equations:

Balance Equations with Mass and Charge

Some chemical reactions involve ions, so you need to balance them for charge as well as mass. Similar steps are involved.

How to Balance Ionic Equations
How to Balance Redox (Oxidation-Reduction) Reactions

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Helmenstine, Anne Marie, Ph.D. "Balancing Chemical Equations." ThoughtCo, May. 1, 2017, thoughtco.com/balancing-chemical-equations-introduction-602380. Helmenstine, Anne Marie, Ph.D. (2017, May 1). Balancing Chemical Equations. Retrieved from https://www.thoughtco.com/balancing-chemical-equations-introduction-602380 Helmenstine, Anne Marie, Ph.D. "Balancing Chemical Equations." ThoughtCo. https://www.thoughtco.com/balancing-chemical-equations-introduction-602380 (accessed December 15, 2017).