Look at this rusty bike. It has been left outside in damp weather too many times, so the iron in the metal parts has rusted. Iron rusts when it combines with oxygen in the air. Iron rusting is an example of a chemical reaction. In a chemical reaction, substances change into entirely different substances. For example, the iron in the bike and the oxygen in the air have changed into rust.
Q: How could you represent this reaction, besides just describing it in words?
A: Scientists use a standard method to represent a chemical reaction, called a chemical equation.
What Is A Chemical Equation?
A chemical equation is a shorthand way to sum up what occurs in a chemical reaction. The general form of a chemical equation is:
- Reactants → Products
The reactants in a chemical equation are the substances that begin the reaction, and the products are the substances that are produced in the reaction. The reactants are always written on the left side of the equation and the products on the right. The arrow pointing from left to right shows that the reactants change into the products during the reaction. This happens when chemical bonds break in the reactants and new bonds form in the products. As a result, the products are different chemical substances than the reactants that started the reaction.
Q: What is the general equation for the reaction in which iron rusts?
A: Iron combines with oxygen to produce rust, which is the compound named iron oxide. This reaction could be represented by the general chemical equation below. Note that when there is more than one reactant, they are separated by plus signs (+). If more than one product were produced, plus signs would be used between them as well.
- Iron + Oxygen → Iron Oxide
Using Chemical Symbols and Formulas
When scientists write chemical equations, they use chemical symbols and chemical formulas instead of names to represent reactants and products. Look at the chemical reaction illustrated in the Figure below. In this reaction, carbon reacts with oxygen to produce carbon dioxide. Carbon is represented by the chemical symbol C. The chemical symbol for oxygen is O, but pure oxygen exists as diatomic (“two-atom”) molecules, represented by the chemical formula O2. A molecule of the compound carbon dioxide consists of one atom of carbon and two atoms of oxygen, so carbon dioxide is represented by the chemical formula CO2.
Q: What is the chemical equation for this reaction?
A: The chemical equation is:
- C + O2 → CO2
Q: How have the atoms of the reactants been rearranged in the products of the reaction? What bonds have been broken, and what new bonds have formed?
A: Bonds between the oxygen atoms in the oxygen molecule have been broken, and new bonds have formed between the carbon atom and the two oxygen atoms.
Is It Balanced?
All chemical equations, like equations in math, must balance. This means that there must be the same number of each type of atom on both sides of the arrow. That’s because matter is always conserved in a chemical reaction. This is the law of conservation of mass.
Look at the equation above for the reaction between carbon and oxygen in the formation of carbon dioxide. Count the number of atoms of each type. Are the numbers the same on both sides of the arrow? The answer is yes, so the equation is balanced.
Let’s return to the chemical reaction in which iron (Fe) combines with oxygen (O2) to form rust, or iron oxide (Fe2O3). The equation for this reaction is:
- 4Fe+ 3O2 → 2Fe2O3
This equation illustrates the use of coefficients to balance chemical equations. A coefficient is a number placed in front of a chemical symbol or formula that shows how many atoms or molecules of the substance are involved in the reaction. From the equation for rusting, you can see that four atoms of iron combine with three molecules of oxygen to form two molecules of iron oxide.
Q: Is the equation for the rusting reaction balanced? How can you tell?
A: Yes, the equation is balanced. You can tell because there is the same number of each type of atom on both sides of the arrow. First count the iron atoms. There are four iron atoms in the reactants. There are also four iron atoms in the products (two in each of the two iron oxide molecules). Now count the oxygen atoms. There are six on each side of the arrow, confirming that the equation is balanced in terms of oxygen as well as iron.
- Scientists use chemical equations to summarize what happens in chemical reactions. Reactants are placed on the left side of the equation and products are placed on the right. An arrow is used to indicate the direction in which the reaction occurs. Plus signs (+) are placed between multiple reactants or products.
- In chemical equations, reactants and products are represented by chemical symbols and formulas. Numbers called coefficients are placed in front of the symbols and formulas to show how much of each substance is involved in the reaction.
- Chemical equations must be balanced. A balanced equation has the same number of each type of atom on both sides of the equation.
- What is a chemical equation? Identify the parts of a chemical equation.
- Write a chemical equation for the chemical reaction in which calcium carbonate (CaCO3) produces calcium oxide (CaO) and carbon dioxide (CO2).
- Describe in words the chemical reaction represented by the following chemical equation: 2NO2 → 2O2 + N2
- When is a chemical equation balanced?