Name: reading chemical equations and balancing them
Uploaded: 2020-09-03T13:38:26.000Z
Duration: 3 min 10 s
Channel: JoVE

K12 Standards

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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.4.2

Chemical Equations

Chemical reactions involve chemical transformations of one or more substances into different materials. A reaction is conventionally represented using a chemical equation, which utilizes chemical formulas to represent atoms, molecules, or compounds involved.

The substances present at the start of a reaction and consumed during the process are called reactants. Reactants are specified on the left side of the equation. The substances formed during the reaction are called products and are indicated on the right side of the equation.

An arrow separating the reactants and products shows the direction of the reaction. The different states of matter are depicted using parenthesized abbreviations. 

A chemical reaction observes the law of conservation of mass, where the total masses of all participating elements are conserved. Therefore, the cumulative number of atoms of each element will match in both the reactants and products. 

Chemical equations are balanced using numerical coefficients to reflect the actual number of atoms involved in the reaction. It is important to only change the numerical coefficients and not numerical subscripts that define the identity of a compound while balancing any chemical equation. 

General guidelines help illustrate and adjust a chemical equation efficiently.

For example, the combustion of liquid benzoic acid, forming gaseous carbon dioxide and water is represented initially using a skeletal chemical equation. The atoms are balanced next by first adjusting those within the compounds, followed by the atoms of free elements. 

The inclusion of coefficients seven and three before the formulas for carbon dioxide and water balances the carbon and hydrogen atoms. A fractional coefficient of 15/2 adjusts the oxygen atoms. 

Fractional coefficients are converted to whole numbers by multiplying the complete equation by the denominator. The collective number of atoms of each element equals on both sides in a balanced equation.

In chemical equations representing reactions involving ionic compounds, the cations are balanced first, followed by the anions. Polyatomic ions are always balanced as a unit.

For example, aqueous aluminum sulfate reacts with aqueous calcium hydroxide forming solid aluminum hydroxide and solid calcium sulfate. Using a skeletal equation, the cations of aluminum and calcium are balanced, followed by the anions of polyatomic hydroxide and sulfate ions. Lastly, the coefficients are adjusted to get an overall balanced equation.

chemical equations

Chemical equations represent the identities and relative quantities of substances involved in a chemical reaction. The substances undergoing reaction are called reactants, and their formulas are placed on the left side of the equation. The substances generated by the reaction are called products, and their formulas are placed on the right side of the equation. Plus signs (+) separate individual reactant and product formulas, and an arrow (&#8594;) separates the reactant and product (left and right) sides of the equation. The relative numbers of reactant and product species are represented by numerical coefficients, placed immediately to the left of each formula. A coefficient of 1 is typically not usually shown. The smallest possible whole-number coefficients are commonly used in a chemical equation, and they are interpreted as ratios. For example, methane and oxygen react to yield carbon dioxide and water in a 1:2:1:2 ratio. 

<img alt="Methane combustion reaction equation: CH&#8324; + 2O&#8322; &#8594; CO&#8322; + 2H&#8322;O." src="/files/core/CDNSource/lessons/1019/1019_Equations-01.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq1" />

The ratio indicates that the smallest possible coefficients of methane, oxygen, carbon dioxide, and water are 1, 2, 1, and 2, respectively. The coefficients may be interpreted with regard to any amount (number) unit, and so this equation may be correctly read in many ways, including:

i. One methane molecule and two oxygen molecules react to yield one carbon dioxide molecule and two water molecules.

ii. One mole of methane molecules and 2 moles of oxygen molecules react to yield 1 mole of carbon dioxide molecules and 2 moles of water molecules. 

The physical states of reactants and products in chemical equations very often are indicated with a parenthetical abbreviation following the formulas. Standard abbreviations include &#8216;s&#8217; for solids, &#8216;l&#8217; for liquids, &#8216;g&#8217; for gases, and &#8216;aq&#8217; for substances dissolved in water.

<h4>Balancing equations</h4>

In a balanced equation, the numbers of atoms for each element involved in the reaction are the same on the reactant and product sides, thereby satisfying the law of conservation of matter. A balanced equation can be confirmed by adding the numbers of atoms on either side of the arrow and comparing these sums to ensure they are equal. Note that the number of atoms for a given element is calculated by multiplying the coefficient of any formula containing that element by the element&#8217;s subscript in the formula. If an element appears in more than one formula on a given side of the equation, the number of atoms represented in each must be computed and then added together.

To balance the equation, the coefficients of the equation may be changed as needed. It is sometimes convenient to use fractions instead of integers as intermediate coefficients in the process of balancing a chemical equation. When balance is achieved, all the equation&#8217;s coefficients may then be multiplied by a whole number to convert the fractional coefficients to integers without upsetting the atom balance. 

For example, the reaction of ethane (C2H6) with oxygen yields water and carbon dioxide, which can be represented by the following unbalanced equation:

<img alt="Ethane combustion formula, C2H6+O2&#8594;CO2+H2O, chemical reaction equation." src="/files/core/CDNSource/lessons/1019/1019_Equations-02.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq2" />

The unbalanced equation contains: 

<table style="margin-left: auto; margin-right: auto;">
	<tbody style="border: 1px solid black;">
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Atoms</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Reactant</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Product</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Balanced?</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">C</td>
			<td style="color: black; border: 1px solid black; text-align: center;">2</td>
			<td style="color: black; border: 1px solid black; text-align: center;">1</td>
			<td style="color: black; border: 1px solid black; text-align: center;">No</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">H</td>
			<td style="color: black; border: 1px solid black; text-align: center;">6</td>
			<td style="color: black; border: 1px solid black; text-align: center;">2</td>
			<td style="color: black; border: 1px solid black; text-align: center;">No</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">O</td>
			<td style="color: black; border: 1px solid black; text-align: center;">2</td>
			<td style="color: black; border: 1px solid black; text-align: center;">3</td>
			<td style="color: black; border: 1px solid black; text-align: center;">No</td>
		</tr>
	</tbody>
</table>

To balance the number of carbon atoms and hydrogen atoms, multiply CO2 by the coefficient 2 and multiply H2O by the coefficient 3, respectively. This changes the total number of oxygen atoms on the product to 7. To balance the number of oxygen atoms, multiply oxygen by the fractional coefficient 7/2. This initial balancing of the carbon, hydrogen, and oxygen atoms by changing the coefficients for reactants and products, gives the provisionally balanced equation: 

<img alt="Ethane combustion reaction equation, showing C2H6 + 7/2O2 &#8594; 2CO2 + 3H2O." src="/files/core/CDNSource/lessons/1019/1019_Equations-03.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq3" />

A conventional balanced equation with integer-only coefficients is derived by multiplying each coefficient by 2, to generate the equation:

<img alt="Combustion equation, ethane reacts with oxygen forming carbon dioxide and water, formula representation." src="/files/core/CDNSource/lessons/1019/1019_Equations-04.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq4" />

This text is adapted from <a href="https://openstax.org/books/chemistry-2e/pages/4-1-writing-and-balancing-chemical-equations">Openstax, Chemistry 2e, Section 4.1: Writing and Balancing Chemical Equations</a>.

Chemical equations represent the identities and relative quantities of substances involved in a chemical reaction. The substances undergoing reaction are ...

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Molecules, Compounds, and Chemical Equations

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KEY TERMS AND CONCEPTS

reading chemical equations and balancing them

Chemical equations show the identities and relative amounts of substances in a chemical reaction. The substances that react are called reactants, and their formulas are written on the left side of the equation. The substances formed are called products, and their formulas are written on the right side. A plus sign (+) separates formulas on the same side, and an arrow (→) shows the change from reactants to products.

The numbers in a chemical equation are shown with coefficients. A coefficient is the number written in front of a formula. A coefficient of 1 is usually not written. The smallest whole-number coefficients are commonly used, and they work as ratios. For example, methane and oxygen react to form carbon dioxide and water in a 1:2:1:2 ratio.

That ratio can be read in more than one way. It can mean 1 methane molecule and 2 oxygen molecules make 1 carbon dioxide molecule and 2 water molecules. It can also mean 1 mole of methane molecules and 2 moles of oxygen molecules make 1 mole of carbon dioxide molecules and 2 moles of water molecules. Chemical equations may also include physical states after the formulas. Common state labels are s for solid, l for liquid, g for gas, and aq for substances dissolved in water.

A balanced chemical equation has the same number of atoms for each element on both sides. This follows the law of conservation of matter. To check balance, count the atoms on the reactant side and the product side and compare the totals. To find the number of atoms in a formula, multiply the coefficient by the subscript. If an element appears in more than one formula on one side, add all of those atoms together.

Balancing a chemical equation means changing coefficients, not the formulas themselves. Fractions can be used as temporary coefficients during balancing. For the reaction of ethane with oxygen, carbon and hydrogen are balanced first by placing 2 in front of carbon dioxide and 3 in front of water. That leaves 7 oxygen atoms on the product side, so oxygen gets a fractional coefficient of 7/2. The final equation is then multiplied by 2 to turn all coefficients into whole numbers without changing the balance.

Reading Chemical Equations and Balancing Them

reading-chemical-equations-and-balancing-them

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Chemical equations show the identities and relative amounts of substances in a chemical reaction. The substances that react are called reactants, and their formulas are written on the left side of the equation. The substances formed are called products, and their formulas are written on the right side. A plus sign (+) separates formulas on the same side, and an arrow (→) shows the change from reactants to products. The numbers in a chemical equation are shown with coefficients. A coefficient...

key terms and concepts

Learn how to read chemical equations, identify reactants and products, use coefficients and state symbols, and balance equations with whole-number ratios.

Reading Chemical Equations and Balancing Them | JoVE K12

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Physical Science

PS.PSc.2.3

Pennsylvania State Academic Standards

Chemical Reactions

3.2.9-12.C

3.2.9-12.G

Louisiana Student Standards for Science

High School

Matter And Its Interactions

HS-PS1-2

HS-PS1-7

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Ohio’s Learning Standards and Model Curriculum

Study Of Matter

Reactions of matter

Interactions Of Matter

Chemical reactions

Stoichiometry

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CHEM1.PS1.2

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Virginia  Standards of Learning

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CH.3

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2.2.1 9C.1.2.1.2

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SC.912.P.8

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Reading Chemical Equations and Balancing Them

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