Download Report

Balancing Chemical Equations
Jean Brainard, Ph.D.
Say Thanks to the Authors
Click http://www.ck12.org/saythanks
(No sign in required)
To access a customizable version of this book, as well as other
interactive content, visit www.ck12.org
CK-12 Foundation is a non-profit organization with a mission to
reduce the cost of textbook materials for the K-12 market both
in the U.S. and worldwide. Using an open-content, web-based
collaborative model termed the FlexBook®, CK-12 intends to
pioneer the generation and distribution of high-quality educational
content that will serve both as core text as well as provide an
adaptive environment for learning, powered through the FlexBook
Platform®.
Copyright © 2013 CK-12 Foundation, www.ck12.org
The names “CK-12” and “CK12” and associated logos and the
terms “FlexBook®” and “FlexBook Platform®” (collectively
“CK-12 Marks”) are trademarks and service marks of CK-12
Foundation and are protected by federal, state, and international
laws.
Any form of reproduction of this book in any format or medium,
in whole or in sections must include the referral attribution link
http://www.ck12.org/saythanks (placed in a visible location) in
addition to the following terms.
Except as otherwise noted, all CK-12 Content (including CK-12
Curriculum Material) is made available to Users in accordance
with the Creative Commons Attribution-Non-Commercial 3.0
Unported (CC BY-NC 3.0) License (http://creativecommons.org/
licenses/by-nc/3.0/), as amended and updated by Creative Commons from time to time (the “CC License”), which is incorporated
herein by this reference.
Complete terms can be found at http://www.ck12.org/terms.
Printed: October 28, 2013
AUTHOR
Jean Brainard, Ph.D.
www.ck12.org
C ONCEPT
Concept 1. Balancing Chemical Equations
1
Balancing Chemical
Equations
• Describe a balanced chemical equation.
• Explain the meaning of coefficients in chemical equations.
• Outline how to balance chemical equations.
The little girl on this seesaw weighs more than the little boy on the other side. That’s why her side of the seesaw
is on the ground and his 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 (H2 ) and oxygen (O2 ) combine to
produce water (H2 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:
H2 + O2 → H2 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.
1
www.ck12.org
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 H2 , and two molecules of water would be written 2 H2 O. A coefficient
of 1 usually isn’t written. Coefficients can be used to balance equation 1 (above) as follows:
Equation 2:
2 H2 + O2 → 2 H2 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 (N2 ) and hydrogen (H2 ) combine to form
ammonia (NH3 ):
N2 + H2 → NH3
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 NH3 to balance nitrogen:
N2 + H2 → 2 NH3
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 H2 to balance hydrogen:
N2 + 3 H2 → 2 NH3
If you’re still not sure how to balance chemical equations, watch the excellent video at this URL: http://www.youtu
be.com/watch?v=RnGu3xO2h74.
2
www.ck12.org
Concept 1. Balancing Chemical Equations
MEDIA
Click image to the left for more content.
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/libra
ry/formulabalance.pdf.
Review
1. Which of the following chemical equations is balanced?
a.
b.
c.
d.
Zn + HCl → ZnCl2 + H2
2 Zn + 2 HCl → ZnCl2 + H2
2 Zn + HCl → ZnCl2 + H2
Zn + 2 HCl → ZnCl2 + H2
2. Balance this chemical equation: CH4 + O2 → CO2 + H2 O
3. Sam was given the following equation to balance: H2 O → H2 + O2 . She balanced it as follows: 2 HO → H2
+ O2 . What did she do wrong? What is the correct way to balance the equation?
3