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3.17: Balancing Chemical Equations

Difficulty Level: Basic Created by: CK-12
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The little boy on this seesaw weighs more than the little girl on the other side. That’s why his side of the seesaw is on the ground and her side is up in the air. For a seesaw to balance, the two riders must be the same weight. A chemical equation that represents a chemical reaction is a little bit like a seesaw. For a chemical equation to balance, there must be the same number of each type of atom on both sides of the equation.

Writing Chemical Equations

A chemical equation represents the changes that occur during a chemical reaction. A chemical equation has the general form:

Reactants → Products

An example of a simple chemical reaction is the reaction in which hydrogen (H 2 ) and oxygen (O 2 ) combine to produce water (H 2 O). In this reaction, the reactants are hydrogen and oxygen and the product is water. To write the chemical equation for this reaction, you would start by writing the reactants on the left and the product on the right, with an arrow between them to show the direction in which the reaction occurs:

Equation 1:

H 2 + O 2 → H 2 O

Q: Look closely at equation 1. There’s something wrong with it. Do you see what it is?

A: All chemical equations must be balanced. This means that there must be the same number of each type of atom on both sides of the arrow. That’s because mass is always conserved in chemical reactions. Count the number of hydrogen and oxygen atoms on each side of the arrow. There are two hydrogen atoms in both reactants and products. There are two oxygen atoms in the reactants but only one in the product. Therefore, equation 1 is not balanced.

Using Coefficients

Coefficients are used to balance chemical equations. A coefficient is a number placed in front of a chemical symbol or formula. It shows how many atoms or molecules of the substance are involved in the reaction. For example, two molecules of hydrogen would be written as 2 H 2 , and two molecules of water would be written 2 H 2 O. A coefficient of 1 usually isn’t written. Coefficients can be used to balance equation 1 (above) as follows:

Equation 2:

2 H 2 + O 2 → 2 H 2 O

Equation 2 shows that two molecules of hydrogen react with one molecule of oxygen to produce two molecules of water. The two molecules of hydrogen each contain two hydrogen atoms and so do the two molecules of water. Therefore, there are now four hydrogen atoms in both reactants and products.

Q: Is equation 2 balanced?

A: Count the oxygen atoms to find out. There are two oxygen atoms in the one molecule of oxygen in the reactants. There are also two oxygen atoms in the products, one in each of the two water molecules. Therefore, equation 2 is balanced.

Steps in Balancing a Chemical Equation

Balancing a chemical equation involves a certain amount of trial and error. In general, however, you should follow these steps:

  1. Count each type of atom in reactants and products. Does the same number of each atom appear on both sides of the arrow? If not, the equation is not balanced, and you need to go to step 2.
  2. Place coefficients, as needed, in front of the symbols or formulas to increase the number of atoms or molecules of the substances. Use the smallest coefficients possible. Warning! Never change the subscripts in chemical formulas. Changing subscripts changes the substances involved in the reaction. Change only the coefficients.
  3. Repeat steps 1 and 2 until the equation is balanced.

Q: Balance this chemical equation for the reaction in which nitrogen (N 2 ) and hydrogen (H 2 ) combine to form ammonia (NH 3 ):

N 2 + H 2 → NH 3

A: First count the nitrogen atoms on both sides of the arrow. There are two nitrogen atoms in the reactants so there must be two in the products as well. Place the coefficient 2 in front of NH 3 to balance nitrogen:

N 2 + H 2 → 2 NH 3

Now count the hydrogen atoms on both sides of the arrow. There are six hydrogen atoms in the products so there must also be six in the reactants. Place the coefficient 3 in front of H 2 to balance hydrogen:

N 2 + 3 H 2 → 2 NH 3

If you’re still not sure how to balance chemical equations, watch the excellent video at this URL: http://www.youtube.com/watch?v=RnGu3xO2h74 .

Summary

  • A chemical equation represents the changes that occur during a chemical reaction. It has the general form: Reactants → Products. All chemical equations must be balanced. This means that there must be the same number of each type of atom on both sides of the arrow.
  • Coefficients are used to balance chemical equations. A coefficient is a number placed in front of a chemical symbol or formula. It shows how many atoms or molecules of the substance are involved in the reaction.
  • To balance a chemical equation, place coefficients as needed in front of the symbols or formulas so the same number of each type of atom occurs in both reactants and products.

Practice

Practice balancing chemical equations by completing the worksheet at this URL:

http://chemistry.about.com/library/formulabalance.pdf

Review

  1. Which of the following chemical equations is balanced?
    1. Zn + HCl → ZnCl 2 + H 2
    2. 2 Zn + 2 HCl → ZnCl 2 + H 2
    3. 2 Zn + HCl → ZnCl 2 + H 2
    4. Zn + 2 HCl → ZnCl 2 + H 2
  2. Balance this chemical equation: CH 4 + O 2 → CO 2 + H 2 O
  3. Sam was given the following equation to balance: H 2 O → H 2 + O 2 . She balanced it as follows: 2 HO → H 2 + O 2 . What did she do wrong? What is the correct way to balance the equation?

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Difficulty Level:

Basic

Grades:

7 , 8

Date Created:

Oct 31, 2012

Last Modified:

Jun 27, 2014
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