READ: Balancing Equations

READ: Balancing Equations

Introduction

Even though chemical compounds are broken up to form new compounds during a chemical reaction, atoms in the reactants do not disappear, nor do new atoms appear to form the products. In chemical reactions, atoms are never created or destroyed. The same atoms that were present in the reactants are present in the products. The atoms are merely re-organized into different arrangements. In a complete chemical equation, the two sides of the equation must be balanced. That is, in a balanced chemical equation, the same number of each atom must be present on the reactant and product sides of the equation.

A chemical reaction occurs when some substances change chemically to other substances. Chemical reactions are represented by chemical equations. Consider a simple chemical reaction, the burning of methane. In this reaction, methane (CH₄) combines with oxygen (O₂) in the air and produces carbon dioxide (CO₂) and water vapor (H₂O). The reaction is represented by the following chemical equation:

CH4+2O2→CO2+2H2O

This balanced chemical equation can be read as 1 mole of CH₄ plus 2 moles of O₂transforms into 1 mole of CO₂ and 2 moles of H₂O.

This equation shows that one molecule of methane combines with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water vapor. All chemical equations must be balanced. This means that the same number of each type of atom must appear on both sides of the arrow. This chemical equation is balanced because there are one carbon atom, four hydrogen atoms, and four oxygen atoms on the left side of the arrow AND there are one carbon atom, four hydrogen atoms, and four oxygen atoms on the right side of the arrow.

Write these as chemical reactions:

1. Aqueous sodium hydroxide is mixed with gaseous chlorine to produce aqueous solutions of sodium chloride and sodium hypochlorite plus liquid water.
2. Solid xenon hexafluoride is mixed with liquid water to produce solid xenon trioxide and gaseous hydrogen fluoride.

Balancing Equations

The process of writing a balanced chemical equation involves three steps. As a beginning chemistry student, you will not know whether or not two given compounds will react or not. Even if you saw them react, you would not know what the products are without running any tests to identify them. Therefore, for the time being, you will be told both the reactants and products in any equation you are asked to balance.

• Step 1: Know what the reactants and products are, and write a word equation for the reaction.
• Step 2: Write the formulas for all the reactants and products.
• Step 3: Adjust the coefficients to balance the equation.

There are two types of numbers that appear in chemical equations. There are subscripts, which are part of the chemical formulas of the reactants and products, and there are coefficients that are placed in front of the formulas to indicate how many molecules of that substance are used or produced. In the chemical formula shown below, the coefficients and subscripts are labeled.

The equation above indicates that one mole of solid copper is reacting with two moles of aqueous silver nitrate to produce one mole of aqueous copper(II) nitrate and two moles of solid silver. Recall that a subscript of 1 is not written - when no subscript appears for an atom in a formula, it is understood that only one atom is present. The same is true in writing coefficients in balanced chemical equations. If only one atom or molecule is present, the coefficient of 1 is omitted.

The subscripts are part of the formulas, and once the formulas for the reactants and products are determined, the subscripts may not be changed. The coefficients indicate the ratio of each substance involved in the reaction and may be changed in order to balance the equation. Coefficients are inserted into the chemical equation in order to make the total number of each atom on both sides of the equation equal. Note that equation balancing is accomplished by changing coefficients, never by changing subscripts.

Example 1

Write a balanced equation for the reaction that occurs between chlorine gas and aqueous sodium bromide to produce liquid bromine and aqueous sodium chloride.

Step 1: Write the word equation (keeping in mind that chlorine and bromine refer to the diatomic molecules).

chlorine+sodium bromide yields bromine+sodium chloride

Step 2: Substitute the correct formulas into the equation.

Cl2+NaBr→Br2+NaCl

Step 3: Insert coefficients where necessary to balance the equation.

By placing a coefficient of 2 in front of NaBr, we can balance the bromine atoms. By placing a coefficient of 2 in front of the, NaCl we can balance the chlorine atoms.

Cl2+2NaBr→Br2+2NaCl

A final check (always do this) shows that we have the same number of each atom on the two sides of the equation. We have also used the smallest whole numbers possible as the coefficients, so this equation is properly balanced.

Example 2

Write a balanced equation for the reaction between aluminum sulfate and calcium bromide to produce aluminum bromide and calcium sulfate. Recall that polyatomic ions usually remain together as a unit throughout a chemical reaction.

Step 1: Write the word equation.

aluminum sulfate+calcium bromide→aluminum bromide+calcium sulfate

Step 2: Replace the names of the substances in the word equation with formulas.

Al2(SO4)3+CaBr2→AlBr3+CaSO4

Step 3: Insert coefficients to balance the equation.

In order to balance the aluminum atoms, we must insert a coefficient of 2 in front of the aluminum compound in the products.

Al2(SO4)3+CaBr2→2AlBr3+CaSO4

In order to balance the sulfate ions, we must insert a coefficient of 3 in front of the product CaSO_4.

Al2(SO4)3+CaBr2→2AlBr3+3CaSO4

In order to balance the bromine atoms, we must insert a coefficient of 3 in front of the reactant CaBr₂.

Al2(SO4)3+3CaBr2→2AlBr3+3CaSO4

The insertion of the 3 in front of the reactant CaBr₂ also balances the calcium atoms in the product CaSO₄. A final check shows that there are two aluminum atoms, three sulfur atoms, twelve oxygen atoms, three calcium atoms, and six bromine atoms on each side. This equation is balanced.

Note that this equation would still have the same number of atoms of each type on each side with the following set of coefficients:

2Al2(SO4)3+6CaBr2→4AlBr3+6CaSO4

Count the number of each type of atom on either side of the equation to confirm that this equation is “balanced”. While this set of coefficients does “balanced” the equation, they are not the lowest set of coefficients possible. Chemical equations should be balanced with the simplest whole number coefficients. We could divide each of the coefficients in this equation by 2 to get another set of coefficients that still balance the equation and are whole numbers. Since it is required that an equation be balanced with the lowest whole number coefficients, the equation above is not properly balanced. When you have finished balancing an equation, you should not only check to make sure it is balanced, you should also check to make sure that it is balanced with the simplest set of whole number coefficients possible.

Summary

• A chemical reaction is the process in which one or more substances are changed into one or more new substances.
• Chemical reactions are represented by chemical equations.
• Chemical equations have reactants on the left, an arrow that symbolizes “yields”, and the products on the right.
• Chemical equations are balanced to represent the concept that atoms are neither created nor destroyed.

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