A chemical equation is a symbolic representation of a chemical reaction, indicating the reactants and products in a reaction and the direction in which the reaction proceeds. French chemist Jean Beguin gets credit for formulating the first chemical equation in 1615.
Here is an explanation of the types of chemical equations, the parts they contain, and examples.
Parts of a Chemical Equation
The three essential parts of a chemical equation are the reactants, the products, and the reaction arrow, but many equations contain much more information.
- Chemical Formulas: Reactants and products are written as chemical formulas, using element symbols and subscripts.
- Reactants: By convention, chemical reactions are read from left to right. The reactants are written on the left side of the equation. Drawing the chemical structures of the reactants is fine, but the usual method gives the reactant chemical formulas (e.g., H2, O2).
- Products: Products of a chemical reaction are written on the right side of the reaction arrow.
- Reaction Arrow: The reaction arrow tells which direction the chemical reaction proceeds.
- Plus Signs: When more than one reactant or product occurs, use a plus sign (+) between them.
- Coefficients: Coefficients are numbers written in front of a chemical formula. They indicate the stoichiometry of the reactants and products, so you can see at a glance the mole ratio between them in a balanced equation. For example, in the formula 4H2O2, “4” is the coefficient. By convention, the number “1” is omitted. So, you write O2 and not 1O2. Usually, coefficients are whole numbers, but sometimes you’ll see equations using fractions or decimal points.
- Subscripts: Subscripts follow the element symbol in a chemical formula, indicating how many atoms of that element are in a compound. For example, the “2” in H2O is a subscript.
- State of Matter: Not all chemical equations list the state of matter of reactants and products. But, when given, the state of matter follows the chemical formula of a reactant or product, enclosed within parentheses. The abbreviations for the states of matter are (s) for solid, (l) for liquid, (g) for gas, and (aq) for when a species is dissolved in aqueous solution.
Reaction Arrow in a Chemical Equation
The type of reaction arrow describes the direction in which the chemical reaction proceeds:
- The most common reaction arrow points from left to right. The symbol → means the reaction proceeds in a net forward direction, where reactants react and yield products.
- Some equations illustrate equilibrium. The symbol ⇌ indicates a condition of chemical equilibrium between the reactants and products. The reaction proceeds in both directions, with both sides of the equation acting simultaneously as reactants and products. If one arrow is shorter than the other, then the longer arrow represents the primary direction the reaction proceeds, while the shorter arrow indicates the reaction still occurs in this direction, but is not favored.
- The symbol ⇄ means that the reaction proceeds in both the forward and backward directions.
- An equal sign or = indicates a stoichiometric relationship.
Unbalanced and Balanced Chemical Equations
Chemical equations are either unbalanced or balanced.
- An unbalanced chemical equation lists the reactants and products and the direction the reaction proceeds, but does not specify a mole ratio between reactants and products.
Example: H2 + O2 → H2O
- A balanced chemical equation includes coefficients before chemical formulas and indicates the stoichiometric ratio between reactants and products. A balanced chemical equation contains equal numbers and types of atoms on both sides of the reaction arrow. It is balanced for both mass and charge.
Example: 2H2 + O2 → 2H2O
Ionic Chemical Equations
Ionic chemical equations indicate the net electrical charge on reactants and products. In a balanced ionic equation, the net electrical charge is the same on both sides of the reaction arrow.
Example: 2Ag+(aq) + Cr2O72−(aq) → Ag2Cr2O7(s)
Note that you multiply the coefficient by the charge. In this example, there are two “+” charges for the silver ion and two “-” charges for the chromate ion. These charges cancel one another out, leaving a net neutral charge on the reactants side of the equation [2(+1) + 1(−2) = 0]. The product, silver chromate, is electrically neutral.
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- IUPAC (1997). “Chemical reaction equation.” Compendium of Chemical Terminology (2nd ed.) (the “Gold Book”). Oxford: Blackwell Scientific Publications. ISBN 0-9678550-9-8. doi:10.1351/goldbook
- 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
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