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