What is a Chemical Equation?
A chemical equation represents a chemical reaction using symbols and numbers. Chemical equations show the relative molar quantities and identities of different reactants and products. Balancing chemical equations can help you determine the accurate ratio of compounds involved in a reaction!
Fundamentals of a Chemical Equation
1. Basic parts of a chemical equation
There are 3 basic parts to a chemical equation: reactants, products, and the “yields” arrow.
|Definition||Reactants are compounds or elements that are needed in the reaction and that undergo chemical change during the reaction.||In reactions, “yield” means to “produce” or “form” and is usually represented by an arrow. When reactants undergo a chemical change, they yield products.||Products are the end results, or compounds produced, in a chemical reaction.|
|Example||CH4 (g) + 2O2 (g)||→||CO2 (g) + 2H2O (g)|
The reactants and products are listed as multiple molecules added together, and this is the case for any chemical equation. The yields arrow, however, can come in a few different styles:
- Forward arrow (→) shows a general reaction.
- Equilibrium arrow (⇌) shows a reversible process.
- Resonance arrow (↔) shows that two or more species are resonance structures of each other.
- Theoretical arrow (⇢) shows a theoretical process.
- Retrosynthetic arrow (⇒) shows potentially ways to get from reactants to products.
2. Numbers in chemical equations
There are two types of numbers in chemical equations: subscripts and coefficients. Subscripts represent how many atoms of each element are present in a molecule. Coefficients represent how many molecules of a specific chemical compound are present in the reaction. Only coefficients of molecules can be changed in balancing equations.
Molecules in which the subscripts of atoms of the same elements are different (for example H2O vs. H2O2) have different chemical compositions, meaning they are not the same compound. For this reason, when balancing chemical equations, subscripts of molecules cannot be changed. The same molecules having different coefficients (for example 2H2O and 5H2O) are the same compounds, present in different amounts.
In a molecule 3NO2, the coefficient is 3 (there are 3 molecules of NO2), and the subscript of oxygen is 2 (there are 2 oxygen atoms in 1 molecule of NO2). In total, there are 6 oxygen atoms (3 molecules of NO2 and 2 oxygen atoms per 1 molecule of NO2.)
The coefficients of each molecule or compound represent the relative quantity of the element in the reaction in moles or numbers of molecules. Because they are relative quantities, the numbers can also be interpreted as ratios; in the example above, the ratio would be 1 CH4 : 2O2 : 1 CO2 : 2H2O. As long as the ratios are constant, the equation can be correctly rewritten with any proportional coefficients (such as 2:4:2:4, 3:6:3:6, etc.), though it is common practice to use the smallest whole number coefficients.
3. The Law of Conservation of Mass
The law of conservation of mass states that matter (or atoms) are neither created nor destroyed in chemical reactions. This means that there must be equal amounts of each compound before and after a chemical reaction takes place. The law applies to balancing chemical equations because there must be the same relative amounts of each compound on both sides of the equation.
Here’s an example of a reaction in which methane burns in air to produce carbon dioxide and water vapor:
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)
As you can see, both sides of the equation each have 1 carbon atom, 4 hydrogen atoms, and 4 oxygen atoms. Thus the equation is balanced.
Steps to Writing & Balancing Chemical Equations
- Identify the names of the reactants and products. It can be helpful to write a word equation to list out all the compounds in the reaction. In a word equation, the reactants and products are represented by their names, meaning they are written in word form, instead of as a molecular formula.
- It is important to remember that word equations only show the names of compounds, and not the quantities. This means that word equations do not show the full picture of the reaction.
- Using the word equation, write a formula equation by rewriting all of the compounds into their chemical formula.
- Formula equations are not balanced. They only show the compounds in their chemical formula.
- Balance the formula equation using the law of conservation of mass to write the chemical equation.
- Start by counting the atoms of elements that only appear once on each side of the equation and balance those first. Balance atoms that appear multiple times on each side of the equation last.
- Usually, balance any hydrogen or oxygen atoms last.
- After balancing all the atoms, you have a balanced formula equation, or chemical equation.
Balancing Chemical Equations – Example
Balance the reaction: methane burns in air and combines with oxygen to produce carbon dioxide and water vapor
1. Write the Word Equation
Write the reaction as a word equation: methane + oxygen → carbon dioxide + water
2. Rewrite the Word Equation as a Formula Equation
Rewrite as formula equation: CH4 (g) + O2 (g) → CO2 (g) + H2O (g)
3. Balance the formula equation using the law of conservation of mass.
- Count the atoms of elements that appear once in the equation. In this example, carbon and hydrogen atoms appear once, and oxygen appears twice on the right side.
- Balance hydrogen and oxygen atoms last. Since oxygen appears twice, you would balance oxygen last. In the example, you would start by balancing carbon, then hydrogen, and oxygen last.
- There is one carbon atom on the left side of the equation (CH4), and one on the right side (CO2). Therefore, carbon is already balanced. Both compounds containing carbon (CH4 and CO2) should have a coefficient of 1, because they must exist equally to have an equal number of carbon atoms.
- There are 4 hydrogen atoms on the left side of the equation (CH4), and 2 on the right side (H2O). You will need 2 more hydrogen atoms on the right side to balance the hydrogen atoms. Multiplying the coefficient of H2O by 2 gives 2H2O, which has 4 hydrogen atoms.
- It is important not to try to change H2O to H4O in an effort to balance the hydrogen atoms. Doing so changes the chemical identity, and H4O is not water. Multiplying the entire compound (H2O) changes the number of water molecules in the reaction, without altering its chemical identity.
- The formula equation is now: 1CH4 (g) + 1O2 (g) → 1CO2 (g) + 2H2O (g)
- There are now 2 oxygen atoms on the left side (O2) and 4 on the right side (2H2O and CO2). Multiplying the coefficient of O2 by 2 gives 2O2, which has 4 oxygen atoms.
- After all the atoms are balanced, you have a chemical equation: CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)
4. Check the balanced equation by counting all the atoms.
|Left Side||Right Side|
|Carbon atoms – 1 (CH4)||Carbon atoms – 1 (CO2)|
|Hydrogen atoms – 4 (CH4)||Hydrogen atoms – 4 (2H2O)|
|Oxygen atoms – 4 (2O2)||Oxygen atoms – 4 (2 from CO2, 2 from 2H2O)|