3.9: How to Write Balanced Chemical Equations

Learning Objectives

• Explain the roles of subscripts and coefficients in chemical equations.
• Balance a chemical equation when given the unbalanced equation.
• Explain the role of the Law of Conservation of Mass in a chemical reaction.

Even though chemical compounds are broken up and new compounds are formed 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. This is known as the Law of Conservation of Mass. The same atoms that were present in the reactants are present in the products—they are merely reorganized into different arrangements.

Coefficients and Subscripts

There are two types of numbers that appear in chemical equations. There are subscripts (CO2) which are part of the chemical formulas of the reactants and products; and there are coefficients, ex. 2CO2, that are placed in front of the formulas to indicate how many molecules of that substance is used or produced. 2COindicates there are two molecules of carbon dioxide.

$2 \ce{H_2} \left( g \right) + \ce{O_2} \left( g \right) \rightarrow 2 \ce{H_2O} \left( g \right)$

In the formation of water,  the coefficients are 2H2, and 2H2O and the subscripts for example O2 , H2 and H2O and You cannot change subscripts in a chemical formula to balance a chemical equation; you can change only the coefficients. Changing subscripts changes the ratios of atoms in the molecule and the resulting chemical properties. H2O is not the same as H2OThe 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 number of each substance involved in the reaction and may be changed in order to balance the equation.

Chemical reactions are done on the macroscopic scale (we can see them), and we cannot see 2 molecules of carbon dioxide, 2CO, but we can see a mole of a substance. What does this mean? A mole is a unit for an amount of something. A mole is defined as 6.02 x 1023 items. So in this case, in 2 moles of carbon dioxide, 2CO, there are 1.2 x 1024 molecules of carbon dioxide. Remember molecules are submicroscopic. We can detect moles of substance in the lab and even hold it, but we cannot hold 2 molecules.

Balancing a Chemical Equation

The simplest and most generally useful method for balancing chemical equations is “inspection,” better known as trial and error. The following is an efficient approach to balancing a chemical equation using this method.

Steps in Balancing a Chemical Equation

1. Identify the most complex substance.
2. Beginning with that substance, choose an element(s) that appears in only one reactant and one product, if possible. Adjust the coefficients to obtain the same number of atoms of this element(s) on both sides.
3. Balance the remaining atoms, usually ending with the least complex substance and using fractional coefficients if necessary. If a fractional coefficient has been used, multiply both sides of the equation by the denominator to obtain whole numbers for the coefficients.
4. Count the numbers of atoms of each kind on both sides of the equation to be sure that the chemical equation is balanced. This means that you have conserved mass in the chemical reaction.

Example $$\PageIndex{1}$$: Combustion of Heptane

Balance the chemical equation for the combustion of Heptane ($$\ce{C_7H_{16}}$$).

$\ce{C_7H_{16} (l) + O_2 (g) → CO_2 (g) + H_2O (g) } \nonumber$

Solution

Steps Example
1. Identify the most complex substance. The most complex substance is the one with the largest number of different atoms, which is $$C_7H_{16}$$. We will assume initially that the final balanced chemical equation contains 1 molecule or formula unit of this substance.

a. Because one molecule of n-heptane contains 7 carbon atoms, we need 7 CO2 molecules, each of which contains 1 carbon atom, on the right side:

$\ce{C7H16 (l) + O2 (g) → } \underline{7} \ce{CO2 (g) + H2O (g) } \nonumber$

• 7 carbon atoms on both reactant and product sides

b. Because one molecule of n-heptane contains 16 hydrogen atoms, we need 8 H2O molecules, each of which contains 2 hydrogen atoms, on the right side:

$\ce{C7H16 (l) + O2 (g) → 7 CO2 (g) + } \underline{8} \ce{H2O (g) } \nonumber$

• 16 hydrogen atoms on both reactant and product sides
3. Balance polyatomic ions as a unit. There are no polyatomic ions to be considered in this reaction.
4. Balance the remaining atoms.

The carbon and hydrogen atoms are now balanced, but we have 22 oxygen atoms on the right side and only 2 oxygen atoms on the left. We can balance the oxygen atoms by adjusting the coefficient in front of the least complex substance, O2, on the reactant side:

$\ce{C7H16 (l) + }\underline{11} \ce{ O2 (g) → 7 CO2 (g) + 8H2O (g) } \nonumber$

• 22 oxygen atoms on both reactant and product sides
5. Check your work. The equation is now balanced, and there are no fractional coefficients: there are 7 carbon atoms, 16 hydrogen atoms, and 22 oxygen atoms on each side. Always check to be sure that a chemical equation is balanced.

Example $$\PageIndex{2}$$: Combustion of Isooctane

Combustion of Isooctane ($$\ce{C_8H_{18}}$$)

$\ce{C8H18 (l) + O2 (g) -> CO_2 (g) + H_2O(g)} \nonumber$

Solution

The assumption that the final balanced chemical equation contains only one molecule or formula unit of the most complex substance is not always valid, but it is a good place to start. The combustion of any hydrocarbon with oxygen produces carbon dioxide and water.

Steps Example
1. Identify the most complex substance. The most complex substance is the one with the largest number of different atoms, which is $$\ce{C8H18}$$. We will assume initially that the final balanced chemical equation contains 1 molecule or formula unit of this substance.

a. The first element that appears only once in the reactants is carbon: 8 carbon atoms in isooctane means that there must be 8 CO2 molecules in the products:

$\ce{C8H18 (l) + O2 (g) -> }\underline{8} \ce{ CO2 (g) + H2O(g)}\nonumber$

• 8 carbon atoms on both reactant and product sides

b. 18 hydrogen atoms in isooctane means that there must be 9 H2O molecules in the products:

$\ce{C8H18 (l) + O2 (g) -> 8CO2 (g) + }\underline{9} \ce{ H2O(g)} \nonumber$

• 18 hydrogen atoms on both reactant and product sides
3. Balance the remaining atoms.

The carbon and hydrogen atoms are now balanced, but we have 25 oxygen atoms on the right side and only 2 oxygen atoms on the left. We can balance the least complex substance, O2, but because there are 2 oxygen atoms per O2 molecule, we must use a fractional coefficient ($$\dfrac{25}{2}$$) to balance the oxygen atoms:

$\ce{C8H18 (l) + } \underline{ \dfrac{25}{2} } \ce{O2 (g)→ 8CO2 (g) + 9H2O(g) }\nonumber$

• 25 oxygen atoms on both reactant and product sides

The equation is now balanced, but we usually write equations with whole number coefficients. We can eliminate the fractional coefficient by multiplying all coefficients on both sides of the chemical equation by 2:

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$\underline{2} \ce{C8H18(l) + } \underline{25} \ce{O2(g) ->} \underline{16} \ce{CO2(g) + } \underline{18} \ce{H2O(g)} \nonumber$

The balanced chemical equation has 16 carbon atoms, 36 hydrogen atoms, and 50 oxygen atoms on each side.

Balancing equations requires some practice on your part as well as some common sense. If you find yourself using very large coefficients or if you have spent several minutes without success, go back and make sure that you have written the formulas of the reactants and products correctly.

Exercise $$\PageIndex{1}$$

Is each chemical equation balanced?

1. $$\ce{2Hg(ℓ)+ O_2(g) \rightarrow Hg_2O_2(s)}$$
2. $$\ce{C_2H_4(g) + 2O_2(g)→ 2CO_2(g) + 2H_2O(g)}$$
yes
no
yes

Exercise $$\PageIndex{2}$$

Balance the following chemical equations.

1. $$\ce{N2 (g) + O2 (g) → NO2 (g) }$$
2. $$\ce{C6H14(l) + O2(g)→ CO2(g) + H2O(g)}$$
N2 (g) + 2O2 (g) → 2NO2 (g)
2C6H14(l) + 19O2(g)→ 12CO2(g) + 14H2O(g)

Summary

• To be useful, chemical equations must always be balanced. Balanced chemical equations have the same number and type of each atom on both sides of the equation.
• The coefficients in a balanced equation must be the simplest whole number ratio. Mass is always conserved in chemical reactions.

Vocabulary

• Chemical reaction - The process in which one or more substances are changed into one or more new substances.
• Reactants - The starting materials in a reaction.
• Products - Materials present at the end of a reaction.
• Balanced chemical equation - A chemical equation in which the number of each type of atom is equal on the two sides of the equation.
• Subscripts - Part of the chemical formulas of the reactants and products that indicate the number of atoms of the preceding element.
• Coefficient - A small whole number that appears in front of a formula in a balanced chemical equation.