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216

L O O K I N G A H E A D

7.1 Evidence for a Chemical Reaction

7.2 Chemical Equations

7.3 Balancing Chemical Equations

Chapter 7

Chemical Reactions: An Introduction

Aluminum reacting with bromine

7I N Y O U R L I F E

Chemical Reactions: An Introduction • Chapter 7 • 217

Prereading Questions

W H A T D O Y O U K N O W ?

1. How can you tell that a chemical reaction has taken place?

2. Hydrogen gas reacts with oxygen gas to form water. Write a chemical equation for this reaction.

3. Write the law of conservation of matter in your own words.

Chemistry is about change. Grass grows. Steel rusts. Hair is bleached, dyed, “permed,” or straightened. Natural gas burns to heat houses. Nylon is produced for jackets, swimsuits,

and pantyhose. Water is decomposed to hydrogen and oxygen gas by an electric current. The bombardier beetle concocts a toxic spray to shoot at its enemies.

These are just a few examples of chemical changes that affect each of us. Chemical reactions are the heart and soul of chemistry, and in this chapter we will discuss fundamental ideas about chemical reactions.

Nylon jackets are sturdy and dry quickly. These characteristics make them ideal for athletic wear.

218 • Chapter 7 • Chemical Reactions: An Introduction

SECTION 7.1

Evidence for a Chemical ReactionObjective• To learn the signals that show a chemical reaction has occurred

How do we know when a chemical change (a reaction) has occurred? That is, what are the clues that a chemical change has taken place? A glance back at the processes in the introduction suggests that chemical reactions often give a visual signal. Steel changes from a smooth, shiny material to a reddish-brown, flaky substance when it rusts. Hair changes color when it is bleached. Solid nylon is formed when two particular liquid solutions are brought into contact. A blue flame appears when natural gas reacts with oxygen. Chemical reac-tions, then, often give visual clues:

• A color changes.

• A solid forms.

• Bubbles are produced.

• A flame occurs.

However, reactions are not always visible. Sometimes the only signal that a reaction is occurring is a change in temperature as heat is produced or absorbed (see Figure 7.1).

Figure 7.1(a) An injured girl uses a cold pack to help prevent swelling. The pack is activated by breaking an ampule; which starts a chemical reaction that absorbs heat rapidly, lowering the temperature of the area to which the pack is applied. (b) A hot pack is used to warm hands and feet in winter. When the package is opened, oxygen from the air penetrates a bag containing solid chemicals. The resulting reaction produces heat.

(a)

(b)

7

TABLE 7.1

Some Clues That a Chemical Reaction Has Occurred1. The color changes.

2. A solid forms.

3. Bubbles form.

4. Heat and/or a fl ame is produced, or heat is absorbed.

7.1 • Evidence for a Chemical Reaction • 219

RESEARCH LINKS

1 What types of evidence indicate that a chemicalreaction has taken place?

2 Give an example of each type of clue from Table 7.1 that a chemical reaction has taken place.

3 Look at the following photographs and identify any clues that tell you a chemical reaction has taken place.

a. b. c. d.

R E V I E W Q U E S T I O N SS E C T I O N 7 . 1

Evidence for a Chemical Reaction

heat or flame occurs

solid forms

bubbles form

color change


Objectives• To learn to identify the characteristics of a chemical reaction• To learn the information given by a chemical equation

Chemists have learned that a chemical change always involves arearrangement of the ways in which the atoms are grouped. For example, when the methane, C H4, in natural gas combines with oxygen, O2, in the air and burns, carbon dioxide, C O2, and water, H2O, are formed. Such a chemical change is called a chemical reaction. We represent a chemical reaction by writing a chemical equation in which the chemicals present before the reaction, the reactants, are shown to the left of an arrow and the chemicals formed by the reaction, the products, are shown to the right of an arrow. The arrow indicates the direction of the change and is read as “yields” or “produces”:

Reactants n Products

For the reaction of methane with oxygen, we have

CO2O2CH4 H2O!!

Reactants

reacts with to produce and

Products

Carbondioxide WaterOxygenMethane

!

Note from this equation that the products contain the same atoms as the reactants but that the atoms are associated in different ways. That is, a chemical reaction involves changing the ways the atoms are grouped.

In a chemical reaction, atoms are neither created nor destroyed. All atoms present in the reactants must be accounted for among the products.

In other words, there must be the same number of each type of atom on the product side as on the reactant side of the arrow. Making sure that the equation for a reaction obeys this rule is called balancing thechemical equation for a reaction.

The equation that we have shown for the reaction between C H4 and O 2 is not balanced. We can see that it is not balanced by taking apart the reactants and products.

CH H

H H

CO

O

OH

H

O O

1 C 2 H2 O 1 O1 C 4 H 2 O

Totals: 1 C 4 H 2 O 1 C 3 O2 H

CH4 + ! +O2 CO2 H2O

The reaction cannot happen this way because, as it stands, this equation states that one oxygen atom is created and that two hydrogen atoms are destroyed. A reaction is only a rearrangement of the way the atoms are grouped; atoms are not created or destroyed. The total number of each type of atom must be

Chemical EquationsSECTION 7.2

Key Terms• Chemical reaction

• Chemical equation

• Reactants

• Products

• Balancing the chemical equation

Chemical reactionOne or more substances are changed into one or more new substances by the reorganization of component atoms.

Chemical equationA representation of a chemical reaction using the formulas of the starting substances that react and the new substances that are formed

ReactantThe starting substance of a chemical reaction, shown to the left of the reaction arrow

ProductThe new substance formed by a chemical reaction, shown to the right of the reaction arrow

7

7.2 • Chemical Equations • 221

the same on both sides of the arrow. We can fix the imbalance in this equation by involving one more O2 molecule on the left and by showing the production of one more H2O molecule on the right.

CH H

H H

OH

H

O O

1 C 4 H 4 O 4 O 4 HTotals:

CH4 + + + +!O2

O O

O2

CO

O

1 C

CO2 H2O

OH

H

H2O

This balanced chemical equation shows the actual numbers of molecules involved in this reaction (see Figure 7.2).

When we write the balanced equation for a reaction, we group like molecules together:

CH4 ! O2 ! O2 n CO2 ! H 2 O ! H 2 O

is written

CH4 ! 2O2 n CO2 ! 2H2 O

Active Reading Question

Why must a chemical equation be balanced?

The chemical equation for a reaction provides us with two important types of information:• The identities of the reactants and products• The relative numbers of each

Let’s Review

Besides specifying the compounds involved in the reaction, we often indicate in the equation the physical states of the reactants and products by using the following symbols.

Physical StatesSymbol State

(s) solid(l) liquid

(g) gas

(aq) dissolved in water (in aqueous solution)

For example, when solid potassium reacts with liquid water, the prod-ucts are hydrogen gas and potassium hydroxide; the latter remains dissolved in the water. From this information about the reactants and products, we can write the equation for the reaction.

• Solid potassium is represented by K(s); liquid water is written as H2O(l);

• hydrogen gas contains diatomic molecules and is represented as H2 (g);

• potassium hydroxide dissolved in water is written as KOH(aq).

+

+

= C = O = H

Figure 7.2The reaction between methane and oxygen yields water and carbon dioxide. Note that there are four oxygen atoms in the products and in the reactants; none has been gained or lost in the reaction. Similarly, there are four hydrogen atoms and one carbon atom in the reactants and in the products. The reaction simply changes the way the atoms are grouped.


Figure 7.3The reactants (a) potassium metal (stored in mineral oil to prevent oxidation) and (b) water. (c) The reaction of potassium with water. The flame occurs because the hydrogen gas, H2 (g),produced by the reaction burns in air, reacts with O2 (g),at the high temperatures caused by the reaction.

So the unbalanced equation for the reaction is Solid Hydrogen Potassium hydroxide potassium Water gas dissolved in water

K(s) ! H2 O(l) n H2 (g) ! KOH(aq)

This reaction is shown in Figure 7.3.The hydrogen gas produced in this reaction then reacts with the oxygen

gas in the air, producing gaseous water and a flame. The unbalanced equation for this second reaction is

H 2 (g) ! O2 (g) n H2 O(g)

Both of these reactions produce a great deal of heat. In Example 7.1 we will practice writing the unbalanced equations for reactions. Then, in the next section, we will discuss systematic procedures for balancing equations.

Chemical Equations: Recognizing Reactants and Products

Write the unbalanced chemical equation for each of the following reactions:

a. Solid mercury(II) oxide decomposes to produce liquid mercury metal and gaseous oxygen.

b. Solid carbon reacts with gaseous oxygen to form gaseous carbon dioxide.

c. Solid zinc is added to an aqueous solution containing dissolved hydrogen chloride to produce gaseous hydrogen that bubbles out of the solution and zinc chloride that remains dissolved in the water.

Solution

a. Reactants In this case we have only one reactant, mercury(II) oxide. The name mercury(II) oxide means that the H g2! cation is present, so one O2" ion is required for a zero net charge. The formula is HgO, which is written as HgO(s) in this case because it is given as a solid.

Products The products are liquid mercury, written as Hg(l), and gaseous oxygen, written as O2 (g). Remember that under normal conditions, oxygen exists as a diatomic molecule.

EXAMPLE 7.1

(a) (b) (c)

Because Zn forms only the Z n2! ion, a Roman numeral is usually not used. Thus ZnC l2 is commonly called zinc chloride.

i n f o r m a t i o n

7.2 • Chemical Equations • 223

RESEARCH LINKS

1 Identify the reactants and products and write chemical equations for each of the following reactions:

a. Gaseous chlorine reacts with an aqueous solution of potassium bromide to form liquid bromine and an aqueous solution of potassium chloride.

b. Solid aluminum reacts with solid iodine to produce solid aluminum iodide.

c. Solid magnesium reacts with an aqueous solution of hydrochloric acid to form an aqueous solution of magnesium chloride and bubbles of hydrogen gas.

2 List several pieces of information you can determine about a chemical reaction just by reading the chemical equation.

3 What is the difference between a coefficient and a subscript in a chemical equation? Provide an example of each.

4 What is the difference between an aqueous solution and a liquid? Write the symbols for each.


Chemical Equation The unbalanced equation is

HgO(s) n Hg(l) ! O2 (g) Reactant Products

b. Reactants In this case solid carbon, written as C(s), reacts with gaseous oxygen, written as O2 (g).

Products The product is gaseous carbon dioxide, written as C O2 (g).

Chemical Equation The equation (which happens to be balanced) is

C(s) ! O2 (g) n CO2 (g) Reactants Products

c. Reactants In this reaction solid zinc, Zn(s), is added to an aqueous solution of hydrogen chloride, which is written as HCl(aq) and called hydrochloric acid.

Products The products are gaseous hydrogen, written as H2 (g), andaqueous zinc chloride. The name zinc chloride means that the Z n2!

ion is present, so two C l" ions are needed to achieve a zero net charge. Thus zinc chloride dissolved in water is written as ZnC l2 (aq).

Chemical Equation The unbalanced equation is

Zn(s) ! HCl(aq) n H 2 (g) ! ZnCl2 (aq) Reactants Products

Practice Problem • Exercise 7.1

Identify the reactants and products and write the unbalanced equation (including symbols for states) for each of the following chemical reactions:

a. Solid magnesium metal reacts with liquid water to form solid magnesium hydroxide and hydrogen gas.

b. Solid ammonium dichromate (review Table 4.4 if this compound is unfamiliar) decomposes to solid chromium(III) oxide, gaseous nitrogen, and gaseous water.

c. Gaseous ammonia reacts with gaseous oxygen to form gaseous nitrogen monoxide and gaseous water.

Zinc metal reacts with hydrochloric acid to produce bubbles of hydrogen gas.


Objective• To learn to write a balanced equation for a chemical reaction

As we saw in the previous section, an unbalanced chemical equation is not an accurate representation of the reaction that occurs. Whenever you see an equation for a reaction, you should ask yourself whether it is balanced.

Atoms are conserved in a chemical reaction.

That is, atoms are neither created nor destroyed. They are just grouped differently. The same number of each type of atom is found among the reactants and among the products.

Chemists determine the identity of the reactants and products of a reac-tion by experimental observation. For example, when methane (natural gas) is burned in the presence of sufficient oxygen gas, the products are always carbon dioxide and water.

The identities (formulas) of the compounds must never be changed in balancing a chemical equation.

In other words, the subscripts in a formula cannot be changed, nor can atoms be added to or subtracted from a formula.

Most chemical equations can be balanced by trial and error—that is, by inspection. Keep trying until you find the numbers of reactants and products that give the same number of each type of atom on both sides of the arrow.

The Combustion of Hydrogen Consider the reaction of hydrogen gas and oxygen gas to form liquid water. First, we write the unbalanced equation from the description of the reaction.

H 2 (g) ! O2 (g) n H2 O(l)

We can see that this equation is unbalanced by counting the atoms on both sides of the arrow.

OH

H

H H

2 H 2 O 2 H, 1 O

+ !H2(g)

O O

O2(g) H2O(l)

We have one more oxygen atom in the reactants than in the products. Because we cannot create or destroy atoms and because we cannot change the formulas of the reactants or products, we must balance the equation by adding more molecules of reactants and/or products. In this case we need one more oxygen atom on the right, so we add another water molecule (which contains one O atom). Then we count all of the atoms again.

Key Term• Coefficients

Balancing Chemical EquationsSECTION 7.3

Reactants Products

2 H 2 H

2 O 1 O

Trial and error is often useful for solving problems. It’s okay to make a few wrong turns before you get to the right answer.

D I D Y O U K N O W

7H H O

H

H

O O

2 H 2 O

Totals:

H2(g) + +!O2(g)

2 H, 1 O 2 H, 1 O

2 H 2 O 4 H

H2O(l) H2O(l)

OH

H

2 O

We have balanced the oxygen atoms, but now the hydrogen atoms have become unbalanced. There are more hydrogen atoms on the right than on the left. We can solve this problem by adding another hydrogen molecule (H2) to the reactant side.

H H OH

H

O O

4 H 2 O 4 H 2 OTotals:

H2(g)

H H

H2(g) ++ +!O2(g) H2O(l) H2O(l)

OH

H

The equation is now balanced. We have the same numbers of hydrogen and oxygen atoms represented on both sides of the arrow. Collecting like molecules, we write the balanced equation as

2H2 (g) ! O2 (g) n 2 H2 O(l)

Consider next what happens if we multiply every part of this balanced equation by 2,

2 " [2 H2 (g) ! O2 (g) n 2 H2 O(l)]

to give

4H2 (g) ! 2 O2 (g) n 4 H2 O(l)

This equation is balanced (count the atoms to verify this). In fact, we can multiply or divide all parts of the original balanced equation by any number to give a new balanced equation. Thus each chemical reaction has many possible balanced equations. Is one of the many possibilities preferred over the others? Yes.

The accepted convention is that the “best” balanced equation is the one with the smallest whole numbers. These whole numbers are called the coefficients for the balanced equation. Therefore, for the reaction of hydrogen and oxygen to form water, the “correct” balanced equation is

2H2 (g) ! O2 (g) n 2 H2 O(l)

The coefficients 2, 1 (never written), and 2, respectively, are the smallest whole numbers that give a balanced equation for this reaction.


When hydrogen gas reacts with oxygen gas to form water, why must two water molecules be formed for every oxygen molecule reacted?

7.3 • Balancing Chemical Equations • 225

Reactants Products

2 H 4 H

2 O 2 O

Reactants Products

4 H 4 H

2 O 2 O

What if mass was not conserved? How would it change how you balance equations?

C R I T I C A LT H I N K I N G

Coeffi cientThe number written in front of the chemical formulas in a balanced chemical equation—coeffi cients indicate the relative numbers of reactants and products in the reaction.


Combustion of Ethanol Next we will balance the equation for the reaction of liquid ethanol, C2 H 5OH, with oxygen gas to form gaseous carbon dioxide and water. This reaction, among many others, occurs in engines that burn a gasoline–ethanol mixture called gasohol.

The first step in obtaining the balanced equation for a reaction is always to identify the reactants and products from the description given for the reaction. In this case we are told that liquid ethanol, C2 H 5 OH(l), reacts with gaseous oxygen, O2 (g), to produce gaseous carbon dioxide, C O2 (g),and gaseous water, H2 O(g). Therefore, the unbalanced equation is

C2 H 5 OH(l) ! O2 (g) n C O2 (g) ! H2 O(g) Liquid Gaseous Gaseous Gaseous ethanol oxygen carbon dioxide water

When one molecule in an equation is more complicated (contains more elements) than the others, it is best to start with that molecule. The most complicated molecule here is C2 H 5OH, so we begin by considering the products that contain the atoms in C2 H 5OH. We start with carbon. The only product that contains carbon is C O2. Because C2 H 5OH contains two carbon atoms, we place a 2 before the C O2 to balance the carbon atoms.

C2H5OH(l) ! O2(g) 2CO2(g) ! H2O(g)

2 C atoms 2 C atoms

!

We cannot change the formula of any reactant or product when we balance an equation. We can only place coefficients in front of the formulas.

Next we consider hydrogen. The only product containing hydrogen is H2O. C2 H5OH contains six hydrogen atoms, so we need six hydrogen atoms on the right. Because each H2O contains two hydrogen atoms, we need threeH2O molecules to yield six hydrogen atoms. So we place a 3 before the H2O.

2CO2(g) ! 3H2O(g)!C2H5OH(l) ! O2(g)

(5 ! 1) H6 H

(3 " 2) H6 H

Finally, we count the oxygen atoms. On the left we have three oxygen atoms (one in C2 H 5OH and two in O2), and on the right we have seven oxygen atoms (four in 2C O2 and three in 3 H2O). We can correct this imbalance if we have three O2 molecules on the left. That is, we place a coefficient of 3 before the O2 to produce the balanced equation.

1 O

7 O 7 O

(3 " 2) O (2 " 2) O 3 O

2CO2(g) ! 3H2O(g)!C2H5OH(l) ! 3O2(g)

At this point you may have a question: why did we choose O2 on the left when we balanced the oxygen atoms? Why not use C2 H 5OH, which has an oxygen atom? The answer is that if we had changed the coefficient in front of C2 H 5OH, we would have unbalanced the hydrogen and carbon atoms.

When balancing equations, start by looking at the most complicated molecule.

i n f o r m a t i o n

7Now we count all of the atoms as a check to make sure the equation is balanced.

2 C 6 H 2 C 7 O 6 H7 OTotals:

C2H5OH(l)

O

O

OOOO

H

H

OOH

H

HH

O

3O2(g)

C OOC

OO

2CO2(g) 3H2O(g)+ +!

HHHHH

OHC

C

The equation is now balanced. We have the same numbers of all types of atoms on both sides of the arrow. Notice that these coefficients are the smallest whole numbers that give a balanced equation.


Why can we not change the formulas for reactants and products to balance a chemical equation?

How to Write and Balance Equations

Step 1 Read the description of the chemical reaction.

• What are the reactants, the products, and their states?

• Write the appropriate formulas.

Step 2 Write the unbalanced equation that summarizes the information from step 1.

Step 3 Balance the equation by inspection, starting with the most complicated molecule.

• Proceed element by element to determine what coefficients are necessary so that the same number of each type of atom appears on both the reactant side and the product side.

• Do not change the identities (formulas) of any of the reactants or products.

Step 4 Check to see that the coefficients used give the same number of each type of atom on both sides of the arrow.

• Note that an “atom” may be present in an element, a compound, or an ion.

• Check to see that the coefficients used are the smallest whole numbers that give the balanced equation. This can be done by determining whether all coefficients can be divided by the same whole numbers to give a set of smaller whole number coefficients.


How do we determine which substance to begin with when balancing a chemical equation?


Reactants Products

2 C 2 C

6 H 6 H

7 O 7 O

To check that an equation is balanced, count the number of atoms on both sides of the equation.

D I D Y O U K N O W


Reactants Products

2 K 2 K

4 H 4 H

2 O 2 O

Balancing Chemical Equations I

For the following reaction:

Solid potassium reacts with liquid water to form gaseous hydrogen and potassium hydroxide that dissolves in the water.

• Write the unbalanced chemical equation.

• Balance the equation.

Solution

Step 1 From the description given for the reaction, we know that the reactants are solid potassium, K(s), and liquid water, H2 O(l).The products are gaseous hydrogen, H2(g), and dissolved potassium hydroxide, KOH(aq).

Step 2 The unbalanced equation for the reaction is

K(s) ! H2 O(l) n H2 (g) ! KOH(aq)

Step 3 Although none of the reactants or products is very complicated, we will start with KOH because it contains the most elements (three). We will arbitrarily consider hydrogen first. Note that on the reactant side of the equation in step 2, there are two hydrogen atoms, but on the product side, there are three. If we place a coefficient of 2 in front of both H2O and KOH, we now have four H atoms on each side.

H2(g) ! 2KOH(aq)

4 Hatoms

2 Hatoms

2 Hatoms

!K(s) ! 2H2O(l)

Also note that the oxygen atoms balance.

H2(g) ! 2KOH(aq)

2 Oatom

2 Oatom

!K(s) ! 2H2O(l)

However, the K atoms do not balance; we have one on the left and two on the right. We can fix this easily by placing a coefficient of 2 in front of K(s) to give the balanced equation:

2K(s) ! 2H2 O(l) n H2 (g) ! 2KOH(aq)

Step 4

Check: There are 2 K, 4 H, and 2 O on both sides of the arrow, and the coefficients are the smallest whole numbers that give a balanced equation. We know this because we cannot divide through by a given whole number to give a set of smaller whole-number coefficients. For example, if we divide all of the coefficients by 2, we get

K(s) ! H2 O(l) n 1__2 H 2 (g) ! KOH(aq)

This is not acceptable because the coefficient for H2 is not a whole number.

EXAMPLE 7.2

What if a friend is balancing chemical equations by changing the values of the subscripts instead of using the coefficients? How would you explain to your friend that this tactic is the wrong approach?

C R I T I C A LT H I N K I N G

7

Connection to BiologyC H E M I S T R Y I N Y O U R W O R L D


H A N D S - O N C H E M I S T R Y • M I N I - L A B •

Materials• four different colors of modeling clay• toothpicks

Procedure1. Obtain four different colors of modeling clay.

2. With the clay and toothpicks, make models of each of the reactants and products in the following equations, and use these models to balance the equations.

a. N2 ! H2 n N H3

b. C2 H 6 ! O2 n C O2 ! H2 O

c. H2 ! O2 n H2 O

d. O2 n O3

Results/Analysis1. How do your models show the difference

between a subscript and a coefficient?

2. How do your models show why we cannot change a subscript to balance an equation?

Modeling Equations

If someone said to you, “Name something that protects itself by spraying its enemies,” your answer would almost certainly be “a skunk.” Of course, you would be correct, but there is another correct answer—the bombardier beetle. When threatened, this beetle shoots a boiling stream of toxic chemicals at its enemy. How does this clever beetle accomplish this? Obviously, the boiling mixture cannot be stored inside the beetle’s body all the time. Instead, when endangered, the beetle mixes chemicals that produce the hot spray. The chemicals involved are stored in two compartments. One compartment contains the chemicals hydrogen peroxide ( H2 O2) and methylhydroquinone ( C7 H8 O2). The key reaction is the decomposition of hydrogen peroxide to form oxygen gas and water:

2H2 O2(aq) n 2H2O(l) ! O2(g)

Hydrogen peroxide also reacts with the hydroquinones to produce other compounds that become part of the toxic spray.

However, none of these reactions occurs very fast unless certain enzymes are present. (Enzymes are natural substances that speed up biological reactions by means we will not discuss here.) When the beetle mixes the hydrogen peroxide and hydroquinones with the enzyme, the decomposition of H2 O2 occurs rapidly, producing a hot mixture pressurized by the formation of oxygen gas. When the gas pressure becomes high enough, the hot spray is ejected in one long stream or in short bursts. The beetle has a highly accurate aim and can shoot several attackers with one batch of spray.

The Bombardier Beetle

A bombardier beetle defending itself


Balancing Chemical Equations II

Under appropriate conditions at 1000 °C, ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide (common name, nitric oxide) and gaseous water.

• Write the unbalanced equation.

• Balance the equation for this reaction.

Solution

Step 1 The reactants are gaseous ammonia, NH3(g), and gaseous oxygen, O 2 (g). The products are gaseous nitrogen monoxide, NO(g), and gaseous water, H2 O(g).

Step 2 The unbalanced equation for the reaction is

NH3 (g) ! O2 (g) n NO(g) ! H2 O(g)

Step 3 In this equation there is no molecule that is obviously the most complicated. Three molecules contain two elements, so we arbitrarily start with N H3. We arbitrarily begin by looking at hydrogen. A coefficient of 2 for N H3 and a coefficient of 3 for H2O give six atoms of hydrogen on both sides.

NO(g) ! 3H2O(g)

6 H 6 H

!2NH3(g) ! O2(g)

We can balance the nitrogen by giving NO a coefficient of 2.

2NO(g) ! 3H2O(g)

2 N 2 N

!2NH3(g) ! O2(g)

Finally, we note that there are two atoms of oxygen on the left and five on the right. The oxygen can be balanced with a coefficient

of 5__2 for O2, because 5__

2 " O2 gives five oxygen atoms.

O2(g) 2NO(g) ! 3H2O(g)

5 O 3 O2 O

!2NH3(g) ! 25

However, the convention is to have whole-number coefficients, so we multiply the entire equation by 2.

2 " [2N H3 (g) ! 5__2 O 2 (g) n 2NO(g) ! 3 H2 O(g)]

or

2 " 2N H3 (g) ! 2 " 5__2 O 2 (g) n 2 " 2NO(g) ! 2 " 3 H2O(g)

4N H3 (g) ! 5 O2 (g) n 4NO(g) ! 6 H2 O(g)

Step 4

Check: There are 4 N, 12 H, and 10 O atoms on both sides, so the equation is balanced. These coefficients are the smallest whole numbers that give a balanced equation.

EXAMPLE 7.3

Reactants Products

1 N 1 N

3 H 2 H

2 O 1 O

Reactants Products

4 N 4 N

12 H 12 H

10 O 10 O

Decorations on glass are produced by etching with hydrofluoric acid.



Propane, C 3 H 8, a liquid at 25 °C under high pressure, is often used for gas grills and as a fuel in rural areas where there is no natural gas pipeline. When liquid propane is released from its storage tank, it changes to propane gas that reacts with oxygen gas (it “burns”) to give gaseous carbon dioxide and gaseous water. Write and balance the equation for this reaction.

READING T IP

This description of a chemical process contains many words, some of which are crucial to solving the problem and some of which are not. First sort out the important information and use symbols to represent it.

Balancing Chemical Equations III

Glass is sometimes decorated by etching patterns on its surface. Etching occurs when hydrofluoric acid, an aqueous solution of HF, reacts with the silicon dioxide in the glass to form gaseous silicon tetrafluoride and liquid water.

• Write the equation for the reaction.

• Balance the equation.

Solution

Step 1 From the description of the reaction we can identify the reactants:

hydrofluoric acid HF(aq)

solid silicon dioxide Si O2 (s)

and the products:

gaseous silicon tetrafluoride Si F4 (g)

liquid water H2 O(l)

Step 2 The unbalanced equation is

SiO2 (s) ! HF(aq) n Si F4 (g) ! H2 O(l)

Step 3 There is no clear choice here for the most complicated molecule. We arbitrarily start with the elements in SiF4. The silicon is bal-anced (one atom on each side), but the fluorine is not. To balance the fluorine, we need a coefficient of 4 before the HF.

SiO2 (s) ! 4HF(aq) n Si F4 (g) ! H2 O(l)

Hydrogen and oxygen are not balanced. Because we have four hydrogen atoms on the left and two on the right, we place a 2 before the H2 O:

SiO2 (s) ! 4HF(aq) n Si F4 (g) ! 2 H 2 O(l)

This balances the hydrogen and the oxygen (two atoms on each side).

Step 4Check: SiO2(s) ! 4HF(aq) SiF4(g) ! 2H2O(l)

1 Si, 2 O, 4 H, 4 F 1 Si, 4 F, 4 H, 2 O

!

!Totals:

All atoms check, so the equation is balanced.

EXAMPLE 7.4


RESEARCH LINKS

1 Write and balance each of the following chemical equations.

a. Gaseous chlorine reacts with an aqueous solution of potassium bromide to form liquid bromine and an aqueous solution of potassium chloride.

b. Solid aluminum reacts with solid iodine to produce solid aluminum iodide.

c. Solid magnesium reacts with an aqueous solution of hydrochloric acid to form an aqueous solution of magnesium chloride bubbles of hydrogen gas.

2 Which can be changed when balancing a chemical equation, the coefficients or the subscripts? Why?

3 Why does each chemical reaction have many possible balanced equations? Of all possible balanced equations, which is preferred?

4 Explain what each of the following symbols (shown in red) represents:

CH4 (g) ! 2 O 2 (g) n C O2 (g) ! 2H2O(g)

5 When balancing the equation

C 2 H 5 OH(l) ! O 2 (g) n CO2 (g) ! H2 O(g),

which substance should you start with? Why? Balance the chemical equation.



Give the balanced equation for each of the following reactions.

a. When solid ammonium nitrite is heated, it produces nitrogen gas and water vapor.

b. Gaseous nitrogen monoxide (common name, nitric oxide) decomposes to produce dinitrogen monoxide gas (common name, nitrous oxide) and nitrogen dioxide gas.

c. Liquid nitric acid decomposes to reddish-brown nitrogen dioxide gas, liquid water, and oxygen gas. (This is why bottles of nitric acid become yellow upon standing.)

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Chapter 7 • Review • 233

Chapter 7 Review

Key Terms

7.1

7.2 Chemical reaction

Chemical equation

Reactants

Products

Balancing the chemical equation

7.3 Coefficients

Key Ideas

7.1 Evidence for a Chemical Reaction! A chemical reaction produces a signal that it has occurred.

These signals include

• Color change

• Solid formation

• Bubble formation

• Heat

• Flame

7.2 Chemical Equations! The physical states of reactants and products in a reaction are

indicated by the following symbols.

Physical States

Symbol State

(s) solid

(l) liquid

(g) gas

(aq)dissolved in water

(in aqueous solution)

! Chemical reactions involve a rearrangement of the ways atoms are grouped together.

! A chemical equation represents a chemical reaction.

• Reactants are shown to the left of an arrow.

• Products are shown to the right of the arrow.

! In a chemical reaction atoms are neither created nor destroyed. A balanced chemical equation must have the same number of each type of atom on the reactant and product sides.

! A balanced chemical equation uses numbers (coefficients) in front of the reactant and product formulas to show the relative numbers of each.

7.3 Balancing Chemical Equations! A chemical reaction is balanced by using a systematic approach.

• Write the formulas of the reactants and products to give the unbalanced chemical equation.

• Balance by trial and error, starting with the most complicated molecule(s).

• Check to be sure the equation is balanced (same numbers of all types of atoms on the reactant and product sides).

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Chapter 7 Assessment

All exercises with blue numbers have answers in the back of this book.

7.1 Evidence for a Chemical Reaction

1. Review how we defined chemical change in Chapter 3. Explain how the visual clues mentioned in this chapter are indicators of chemical change as defined in Chapter 3.

2. If a piece of blackboard chalk is heated strongly, the mass of the chalk decreases substantially, and the chalk is converted into a fine powder. What evidence is there for a chemical reaction having taken place when the chalk is heated?

3. If you have had a clogged sink drain at your home, your parents may have have tried using a commercial drain cleaner to dissolve the clog. What evidence is there that such drain cleaners work by chemical reaction?

4. If a bottle of aspirin is left open to the air, even-tually one of the products is acetic acid (vinegar is a mixture of acetic acid and water). Is there evidence that this change represents a chemical reaction?

7.2 Chemical Equations

5. The substances present before a chemical reaction takes place are called the , and the substances present after the reaction takes place are called the .

6. In an ordinary chemical reaction, are neither created nor destroyed.

7. The notation “(g)” after a substance’s formula indicates it exists in the state.

8. In a chemical equation for a reaction, the nota-tion “(aq)” after a substance’s formula means that the substance is dissolved in .

Note: In some of the following problems, you will need to write a chemical formula from the name of the compound. It may help to review Chapter 4.

9. If calcium carbonate is heated strongly, carbon dioxide gas is driven off, leaving a residue of calcium oxide. Write the unbalanced chemical equation for this process.

10. Liquefied propane gas is often used for cooking in rural areas away from natural gas lines. Propane ( C3 H 8) burns in oxygen gas, producing carbon dioxide gas, water vapor, and heat. Write the unbalanced chemical equation for this process.

11. If a sample of pure hydrogen gas is ignited very carefully, the hydrogen burns gently, combining with the oxygen gas of the air to form water vapor. Write the unbalanced chemical equation for this reaction.

12. Solid ammonium carbonate, (N H4 ) 2 C O3, is used as the active ingredient in “smelling salts.” When solid ammonium carbonate is heated, it decomposes into ammonia gas, carbon dioxide gas, and water vapor. Write the unbalanced chemical equation for this process.

13. Silver oxide may be decomposed by strong heat-ing into silver metal and oxygen gas. Write the unbalanced chemical equation for this process.

14. Methanol (C H3OH, wood alcohol) is an impor-tant industrial chemical. Methanol may be synthesized from carbon monoxide gas and hydrogen gas under certain conditions of temperature and pressure. Write the unbalanced chemical equation for this process.

15. Elemental boron is produced in one industrial process by heating diboron trioxide with magne-sium metal, also producing magnesium oxide. Write the unbalanced chemical equation for this process.

16. Calcium metal is moderately reactive. If pieces of calcium are added to water, the metal begins to bubble as hydrogen gas is formed. The water begins to turn cloudy as solid calcium hydrox-ide begins to form. Write the unbalanced chemical equation for the reaction of calcium metal with water.

17. Phosphorus trichloride is used in the manufac-ture of certain pesticides and may be synthesized directly from its elements. Write the unbalanced chemical equation for this process.

18. Magnesium hydroxide has been used for many years as an antacid (“milk of magnesia”) because it reacts with the hydrochloric acid in the stomach, producing magnesium chloride and water. Write the unbalanced chemical equation for this process.

19. Nitrous oxide gas (systematic name: dinitrogen monoxide) is used by some dentists as an anes-thetic. Nitrous oxide (and water vapor) can be produced in small amounts in the laboratory by careful heating of ammonium nitrate. Write the unbalanced chemical equation for this reaction.

20. Hydrogen sulfide gas is responsible for the odor of rotten eggs. Hydrogen sulfide burns in air, producing sulfur dioxide gas and water vapor. Write the unbalanced chemical equation for this process.

21. Acetylene gas ( C2 H 2) is often used by plumbers, welders, and glass blowers because it burns in oxygen with an intensely hot flame. The prod-ucts of the combustion of acetylene are carbon dioxide and water vapor. Write the unbalanced chemical equation for this process.

22. If ferric oxide is heated strongly in a stream of carbon monoxide gas, it produces elemental iron and carbon dioxide gas. Write the unbal-anced chemical equation for this process.

23. The Group 2 metals (Ba, Ca, Sr) can be produced in the elemental state by the reaction of their oxides with aluminum metal at high tempera-tures, also producing solid aluminum oxide. Write the unbalanced chemical equations for the reactions of barium oxide, calcium oxide, and strontium oxide with aluminum.

24. Ozone gas is a form of elemental oxygen containing molecules with three oxygen atoms,O3. Ozone is produced from atmospheric oxygen gas, O2, by the high-energy outbursts found in lightning storms. Write the unbalanced equation for the formation of ozone gas from oxygen gas.

25. Carbon tetrachloride may be prepared by the reaction of natural gas (methane, C H4) and chlorine gas in the presence of ultraviolet light. Write the unbalanced chemical equation for this process. (HCl is also a product.)

26. Ammonium nitrate is used as a “high-nitrogen” fertilizer, despite the fact that it is quite explo-sive if not handled carefully. Ammonium nitrate can be synthesized by the reaction of ammonia gas and nitric acid. Write the unbalanced chemi-cal equation for this process.

27. The principal natural ore of lead is galena, which is primarily lead(II) sulfide. Lead(II) sulfide can be converted to lead(II) oxide and sulfur dioxide gas by heating strongly in air. Write the unbalanced chemical equation for the reaction of lead(II) sulfide with oxygen gas.

28. At high temperatures, xenon gas will combine directly with fluorine gas to produce solid xenon tetrafluoride. Write the unbalanced chemical equation for this process.

29. Ammonium nitrate is highly explosive if not handled carefully, breaking down into nitrogen gas, oxygen gas, and water vapor. The expansion of the three gases produced yields the explosive force in this case. Write the unbalanced chemi-cal equation for this process.

30. Silver nitrate is used in “styptic pencils,” which help to cauterize small nicks and cuts that occur during shaving. Silver nitrate can be prepared by dissolving metallic silver in concentrated nitric acid, with hydrogen gas being an additional product of the reaction. Write the unbalanced chemical equation for this process.

7.3 Balancing Chemical Equations

31. When balancing a chemical equation, one must never change the of any reactant or product.

32. After balancing a chemical equation, we ordinar-ily make sure that the coefficients are the smallest possible.

33. Balance each of the following chemical equations.

a. H2 O 2 (aq) n H2 O(l) ! O2 (g)

b. Ag(s) ! H2 S(g) n Ag2 S(s) ! H2 (g)

c. FeO(s) ! C(s) n Fe(l) ! C O2 (g)

d. C l2 (g) ! KI(aq) n KCl(aq) ! I2 (s)


a. Ca F2 (s) ! H2SO4 (l) n CaSO4 (s) ! HF(g)

b. KBr(s) ! H3PO4 (aq) n K3PO4 (aq) ! HBr(g)

c. TiC l4 (l) ! Na(s) n NaCl(s) ! Ti(s)

d. K2CO3 (s) n K2O(s) ! C O2 (g)


a. Si I4 (s) ! Mg(s) n Si(s) ! Mg I2 (s)

b. Mn O2 (s) ! Mg(s) n Mn(s) ! MgO(s)

c. Ba(s) ! S8 (s) n BaS(s)

d. N H3 (g) ! C l2 (g) n NH4 Cl(s) ! NC l3 (g)


a. Ba(N O3 ) 2 (aq) ! N a2 Cr O4 (aq) n

BaCr O4 (s) ! NaN O3 (aq)

b. C2 H 5 OH(l) ! O2 (g) n CO2 (g) ! H2 O(l)

c. Ca C2 (s) ! H2O(l) n Ca(OH)2 (s) ! C2 H2 (g)

Chapter 7 • Assessment • 235


Critical Thinking37. Many ships are built with aluminum superstruc-

tures to save weight. Aluminum, however, burns in oxygen if there is a sufficiently hot ignition source, which has led to several tragedies at sea. Write the unbalanced chemical equation for the reaction of aluminum with oxygen, producing aluminum oxide as the product.

38. Crude gunpowders often contain a mixture of potassium nitrate and charcoal (carbon). When such a mixture is heated until reaction occurs, a solid residue of potassium carbonate is produced. The explosive force of the gunpowder comes from the fact that two gases are also produced (carbon monoxide and nitrogen), which increase in volume with great force and speed. Write the unbalanced chemical equation for the process.

39. The sugar sucrose, which is present in many fruits and vegetables, reacts in the presence of certain yeast enzymes to produce ethyl alcohol (ethanol) and carbon dioxide gas. Balance the following equation for this reaction of sucrose.

C12 H22 O11 (aq) ! H2O(l) n C2 H5OH(aq) ! C O2 (g)

40. Methanol (methyl alcohol), C H3OH, is a very important industrial chemical. Today, methanol is synthesized from carbon monoxide and elemental hydrogen. Write the balanced chemi-cal equation for this process.

41. The Hall process is an important method by which pure aluminum is prepared from its oxide (alumina, A l2 O3) by indirect reaction with graphite (carbon). Balance the following equation, which is a simplified representation of this process.

Al2 O 3 (s) ! C(s) n Al(s) ! C O2 (g)

42. Iron oxide ores, commonly a mixture of FeO and F e2 O 3, are given the general formula F e3 O 4 . They yield elemental iron when heated to a very high temperature with either carbon monoxide or elemental hydrogen. Balance the following equations for these processes.

Fe3 O 4 (s) ! H2 (g) n Fe(s) ! H2 O(g)

Fe3 O 4 (s) ! CO(g) n Fe(s) ! C O2 (g)

43. The elements of Group 1 all react with sulfur to form the metal sulfides. Write balanced chemical equations for the reactions of the Group 1 elements with sulfur.

44. A common experiment in chemistry classes involves heating a weighed mixture of potas-sium chlorate, KCl O3, and potassium chloride. Potassium chlorate decomposes when heated, producing potassium chloride and oxygen gas. By measuring the volume of oxygen gas produced in this experiment, students can calculate the relative percentage of KCl O3 and KCl in the original mixture. Write the balanced chemical equation for this process.

45. A common demonstration in chemistry classes involves adding a tiny speck of manganese(IV) oxide to a concentrated hydrogen peroxide,H2 O 2, solution. Hydrogen peroxide decomposes quite spectacularly under these conditions to produce oxygen gas and steam (water vapor). Manganese(IV) oxide speeds up the decomposi-tion of hydrogen peroxide but is not consumed in the reaction. Write the balanced equation for the decomposition reaction of hydrogen peroxide.

46. Glass is a mixture of several compounds, but a major constituent of most glass is calcium silicate, CaSiO3. Glass can be etched by treatment with hydrogen fluoride: HF attacks the calcium silicate of the glass, producing gaseous and water-soluble products that can be removed by washing the glass. For example, the volumetric glassware in chemistry laboratories is often graduated by using this process. Balance the following equation for the reaction of hydrogen fluoride with calcium silicate.

CaSiO3 (s) ! HF(g) n Ca F2 (aq) ! Si F4 (g) ! H2 O(l)

47. If you had a “sour stomach,” you might try an antacid tablet to relieve the problem. Can you think of evidence that the action of such an antacid is a chemical reaction?

48. If solutions of potassium chromate and barium chloride are mixed, a bright yellow solid (barium chromate) forms and settles out of the mixture, leaving potassium chloride in solution. Write a balanced chemical equation for this process.

49. When a strip of magnesium metal is heated in oxygen, it bursts into an intensely white flame and produces a finely powdered dust of magne-sium oxide. Write the unbalanced chemical equation for this process.

Standardized Test Practice C h a p t e r 7

1 Which of the following is not a clue that a chemical reaction has taken place?

A A solid is formed when two clear solutions are mixed.

B A clear solution is added to a red solution, and the result is a blue solution.

C A solid is added to water, and bubbles form.

D A pure solid is heated and turns into a pure liquid.

2 What does it mean for a chemical equation to be balanced?

A The sums of the coefficients on each side of the equation are equal.

B The same number of each type of atom appears on both sides of the equation.

C The formulas of the reactants and the products are the same.

D The sums of the subscripts on each side of the equation are equal.

3 Which of the following physical states is not used for H2O in a chemical equation?

A (s)

B (l)

C (aq)

D (g)

4 Which of the following is the coefficient forO2 when the equation is balanced?

C 4 H 10 (l) ! O 2 (g) n C O2 (g) ! H 2 O(g)

A 4

B 8

C 10

D 13

5 What is the sum of the coefficients of the following equation when it is balanced?

FeS(s) ! HCl(g) n FeC l2 (s) ! H 2 S(g)

A 5

B 4

C 3

D 2

6 When the equation representing the reaction between iron(II) oxide and carbon to form iron and carbon dioxide is balanced, the coefficient for iron is

A 1

B 2

C 3

D 4

7 The reaction of an element X ( ) with element Y ( ) is represented in the following diagram. Which of the equations best describes this reaction?

A 3X ! 8Y n X3 Y 8B 3X ! 6Y n X3 Y 6C X ! 2Y n X Y2

D 3X ! 8Y n 3X Y2 ! 2Y

8 Explain the law of conservation of mass and discuss how it applies to balancing a chemical equation.

For questions 9–12, write and balance the chemical equation as described. Include the state of each substance.

9 When magnesium metal is added to water, hydrogen gas is produced along with a solution of magnesium hydroxide.

10 When an aqueous solution of lead(II) nitrate is added to an aqueous solution of sodium iodide, the yellow solid lead(II) iodide is produced along with a solution of sodium nitrate.

11 Propane gas ( C3 H 8) reacts with oxygen in the air to form carbon dioxide and water vapor.

12 Calcium metal reacts with oxygen gas to produce solid calcium oxide.

Chapter 7 • Standardized Test Practice • 237

chemical reactions: an introductionrapisura.weebly.com/uploads/1/5/9/0/15901848/zch7.pdf · 218 •...

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