Download chapter 6: chemical reactions: an introduction

CHAPTER 6: CHEMICAL REACTIONS: AN
INTRODUCTION
INTRODUCTION
Knowing how to write chemical equations and how to interpret what they mean is an important
part of chemistry. If you have learned the symbols for the elements, and can write the formulas
of compounds from their names, then learning to write chemical equations will be easier. The
Answers to Learning Review reviews the logic used when balancing a chemical equation by trial
and error. If you are having trouble balancing equations, go over the steps used to balance the
equations in the Answers to Learning Review.
GOALS FOR THIS CHAPTER
1.
2.
3.
Know the clues that help tell you that a chemical reaction has occurred. (Section 6.1)
Given a word description of a reaction, be able to write chemical formulas for reactants
and products. (Section 6.2)
Be able to balance an equation so that there are the same numbers of atoms on both sides.
(Section 6.3)
QUICK DEFINITIONS
Chemical reaction
A change during which atoms form new associations with other
atoms. (Section 6.2)
Chemical equation
Shows compounds before a chemical reaction takes place on the
left, and compounds formed from a chemical reaction on the
right (Section 6.2)
Reactants
Compounds shown on the left of a chemical equation. They are
the compounds which react to form new compounds.
(Section 6.2)
Products
Compounds shown on the right of a chemical equation. They
are the compounds which are formed by the reaction.
(Section 6.2)
Coefficients
The number in front of a chemical formula that tells how many
of each atom or molecule is present. (Section 6.3)
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PRETEST
1.
When a solid metal is placed in an aqueous solution, bubbles rise to the surface. Could
this observation indicate that a chemical reaction has occurred?
2.
Is the equation below balanced?
3.
Is the equation below balanced?
4.
Use the word description for the reaction that follows to write a balanced equation.
Aqueous silver nitrate reacts with aqueous ammonium phosphate to form aqueous
ammonium nitrate and solid silver phosphate.
5.
Balance the equation for the reaction of aqueous copper(II) sulfate with solid iron to
produce aqueous iron(II) sulfate and solid copper.
6.
Balance the equation below.
PbO(s) + NH/aq)
7.
Balance the equation below.
8.
Balance the equation below.
9.
Balance the equation below.
10.
Balance the equation below.
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~
6 Chemical Reactions: An Introduction
Pb(s) + N2(g) + HzO(l)
PRETEST ANSWERS
1.
Yes, the formation of bubbles of gas is one indication that a chemical reaction has taken
place. (6.1)
2.
The equation, below is balanced. There are the same numbers of atoms on both sides.
(6.2)
CaCz + 2 H 20
1 Ca 2C
3.
-.
4H 20
C2~
+ Ca(OH)2
2 C 4H
1 Ca 20
The equation below is not balanced. The numbers of K and H are not the same on both
sides. (6.2)
KOH + H 2S04
1 K5 0
-.
3 HIS
~S04
+ ~O
2K50
2HlS
4.
Aqueous silver nitrate reacts with aqueous ammonium phosphate to form aqueous
ammonium nitrate and solid silver phosphate can be written in equation form as
5.
Aqueous copper(II) sulfate reacts with solid iron to produce aqueous iron(II) sulfate and
solid copper can be written in equation form as
CuS04(aq) + Fe(s) - . FeSOiaq) + Cu(s)
6.
(6.3)
The balanced equation is
3 PbO(s) + 2 NH3(aq) .........
3Pb30
7.
(6.2)
2N6H
3 Pb(s) + Nig) + 3 ~O(l)
3Pb
2N
6H 30
(6.3)
The balanced equation is
2 AI(s) + 3 CI2(g) ......... 2 AICI3(s)
2Al
6Cl
2Al 6Cl
8.
(6.3)
The balanced equation is
Ba(CI03h(s) .........
1 Ba 2 Cl 6 0
BaC~(s)
1 Ba 2 Cl
+ 3 02(g)
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Pretest Answers
105
9.
The balanced equation is
6.3)
C3Hg(g) + 50Z<g) ......... 3 COz(g) + 4 Hz0(g)
3 C 8H
10.
10 0
3 C 10 0
8H
The balanced equation is
(6.3)
NzOs(g) + Hz0(l) ......... 2 HN03(aq)
2N60
2H
2H2N60
CHAPTER REVIEW
6.1 EVIDENCE FOR A CHEMICAL REACTION
WhatIs the Evidence for a Chemical Reaction?
Any or all of the changes below often indicate that a chemical reaction has taken place.
1.
The color changes.
2.
A solid forms.
3.
Bubbles form.
4.
Heat and/or flame is produced, or heat is absorbed.
6.2 CHEMICAL EQUATIONS
The changes which happen during a chemical reaction are shown by writing a chemical equation.
The starting materials are called reactants and are shown on the left side of the chemical
equation. The substances formed in a reaction are called products and are shown on the right
side of the equation.
The same kinds of atoms must be present before and after a chemical reaction because atoms are
neither created nor destroyed during a reaction.
The same number of each kind of atom must be present before and after a chemical reaction. In
other words, the equation must be balanced.
For example, when hydrogen gas and oxygen gas react to form water vapor, the chemical
equation must have hydrogen and oxygen atoms on both sides of the chemical equation, and it
must have the same number of hydrogen and oxygen atoms on both sides.
Sometimes, the physical state of each of the products and reactants is indicated by a small letter
in parentheses after the chemical symbol. Fe(s) indicates that iron is a solid reactant. HzOO)
indicates liquid water. 0z(g) indicates gaseous oxygen and NaCI(aq) indicates a solution of
sodium chloride in water (aqueous).
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6 Chemical Reactions: An Introduction
Example:
20Z<g) + Hz(g) ..... 2Hz0(l)
This equation reads hydrogen gas reacts with oxygen gas to produce liquid water.
6.3 BALANCING CHEMICAL EQUATIONS
How Can You Balance a Chemical Equation?
1.
2.
3.
From the word description of the reaction, write formulas for all of the reactants and all of
the products. Separate the reactants and products with an arrow, which means, "reacts to
form". If you are having trouble writing the formulas from the word descriptions, you
need to review the guidelines in Section 5.7. Put in the physical states of each of the
reactants and products if they are given in the word description.
Count the number of each kind of atom on both sides of the equation. Usually, the number
of one or more atoms is not balanced.
Inspect the chemical equation you have written and pick out the most complex chemical
formula. Begin balancing the equation with the atoms of this formula. Try to begin
balancing equations with atoms other than oxygen and hydrogen. If you save oxygen and
hydrogen for last they can often be easily balanced by adjusting the coefficient of water, if
it is present in the reaction.
Example:
Under the proper conditions, solid potassium chlorate will decompose to form solid
potassium chloride and oxygen gas. Write a balanced chemical equation for this reaction.
1.
2.
First, write correct chemical formulas for reactants and products.
KCl0 3(s) ..... KCI(s) + 0z(g)
Count the atoms.
KCl0 3(s) ..... KCl(s) + 0z(g)
1 K, 1 Cl, 3 0 atoms
1 K, 1 Cl, 2 0 atoms
The number of potassium and chlorine atoms on each side of the equation is already
the same, but the left side has three oxygen atoms, while the right side has only two.
We will need to balance the equation by adjusting the number of oxygen atoms.
3.
Pick out the most complex formula, KCl03 and adjust the coefficients of any
molecules containing these atoms. If we change the coefficient in front of 0z to 2,
we increase the number of oxygen atoms on the right to 4, which still does not
balance the three oxygen atoms on the left. If we change the coefficient in front of
0z to a 3, there will be six oxygen atoms on the right and three on the left. We can
Chapter Review
107
balance the number of oxygen atoms by changing the coefficient of KCI03 from 1 to
2. Now, there are six oxygen atoms on both sides.
2KCI03(s)
--lIIo--
KCI(s) + 30z(g)
But now the number of K and CI atoms are not equal on both sides. There are 2 K
and CI atoms on the left, but only one of each on the right. Put a coefficient of 2 in
front of KCI on the right. Now, there are equal numbers and kinds of atoms on both
sides of the equation.
2KCI03(s)
--lIIo--
2KCI(s) + 0z(g)
Important!
Remember that you can change coefficients when you are balancing an equation, but you
can never change the subscript number. For example, to balance an equation for the
breakdown of water into oxygen and hydrogen you need two oxygen atoms on the left side
of the equation, but you start with ~O, which contains one oxygen atom.
The correct way to adjust the number of oxygen atoms is to change the coefficient to
2H zO. If you had adjusted the number of oxygen atoms by changing the subscript number
to Hz0z you would have changed the formula of the reactant from water to hydrogen
peroxide, which does not participate in this reaction. The balanced equation is
LEARNING REVIEW
1.
Which of the following indicates that a chemical reaction has occurred?
a.
b.
c.
d.
2.
108
Liquid water boils to produce steam.
Burning firewood gives off heat.
Mixing two colorless liquids produces a bright yellow solid.
Solid NaHC03 dissolves in water.
Why is it important that chemical equations be balanced?
6 Chemical Reactions: An Introduction
3.
Count the number of each kind of atom on both sides of the equation and decide which
reactions are balanced, and which are not.
a.
b.
c.
d.
e.
4.
c.
d.
KOH(aq) + ~S(aq) -.. ~S(aq) + ~O(l)
HN02(aq) -.. N203(g) + H20(aq)
NaOH(aq) + ~SOiaq) -.. N~S04(aq) + ~O(l)
(NH4)2S(aq) + Pb(N03)2(aq) -.. PbS(s) + NH4N03(aq)
Al(s) + 02(g) -.. A~03(s)
Balance these chemical equations.
a.
b.
c.
d.
7.
Solid iron metal reacts with oxygen in the atmosphere to form rust, iron(III) oxide.
Solid magnesium metal reacts with aqueous hydrochloric acid to produce hydrogen
gas and an aqueous solution of magnesium chloride.
Solid silver oxide decomposes upon heating to produce solid silver metal and
oxygen gas.
Aqueous sodium hydroxide reacts with aqueous nitric acid to produce aqueous
sodium nitrate and liquid water.
Balance these chemical equations. Check your work by counting the number of each kind
of atom on both sides of the equation.
a.
b.
c.
d.
e.
6.
KCl03 -.. KCl + 02
2NaOH + CO2 -.. N~C03 + ~O
C2HsOH + CO2 -.. 2C02 + 3~0
3Cu + HN03 -.. 3Cu(N03)2 + NO + ~O
Use the following word descriptions to write unbalanced chemical equations showing the
formulas of reactants and products. Make sure you include the physical states of reactants
and products.
a.
b.
5.
H, +Br2 -.. HBr
S02(g) + 02(g) -.. S03(g)
C4HlO(g) + 02(g) -.. CO2(g) + ~O(g)
Fe203(s) + C(s) -.. Fe(s) + CO2(g)
TiCl4(l) + H20(l) -.. Ti02(s) + HCl(aq)
Balance these chemical equations.
a.
b
c.
d.
KI(aq) + Br2(l) -.. KBr(aq) + lis)
Pb02(s) -.. PbO(s) + 02(g)
Fe(OH)3(s) + H2S04(aq) -.. Fe 2(S04h(s) + ~O(l)
~P04(aq) + BaCliaq) -.. KCl(aq) + Ba3(P04)2(s)
Learning Review
109
PRACTICE EXAM
1.
Which of the following is not evidence of a chemical reaction?
a.
b.
c.
d.
e.
2.
Which of the statements about chemical equations is true?
a.
b.
c.
d.
e.
3.
The arrow means "reacts to form".
The products are always shown on the left side.
Often there are atoms present in the products not present in the reactants.
When a reaction occurs, atoms do not recombine with other elements.
The physical state of an element cannot change during a chemical reaction.
Which of the following statements about the physical states of reactants and products is
not true?
a.
b.
c.
d.
e.
4.
A bowl of soup is warmed from room temperature to the boiling point of water.
A yellow solid forms when two clear yellow liquids are mixed.
After dropping a strip of metal into a colorless liquid, bubbles form on the surface of
the metal.
When two solutions are mixed, the temperature of the mixture drops.
When a white solid is dissolved in a colorless liquid, the solution turns blue.
BaCrOz(s) on the product side indicates the formation of a solid.
HCI(aq) on the reactant side indicates that there is water present.
Brz(l) indicates that the bromine is dissolved in liquid water.
COz(g) on the product side indicates that at least one product is a gas.
Both products and reactants can be present as solids, liquids or gases.
Solid calcium carbide, CaCz, will react with water to produce acetylene gas, CzHz, and
aqueous calcium hydroxide. What is the correct equation for this reaction?
a.
Ca(OH)z(aq) + CzHz(g) -... CaCz(s) + 2Hz0(I)
b.
CaCz(s) + 2Hz0(g) -... CzHz(g) + Ca(OH)Z<g)
c.
CaCzes) + 2H zO(l) -... CzHz(g) + Ca(OH}z(aq)
d.
CaCz(s) + 2Hz0(aq) -... CzHz(l) + Ca(OH}z(aq)
e.
CaCz(s) + 2HzO(I) -... CzHz(s) + Ca(OH)z(1)
Practice Exam
113
5.
Solid lead(II) oxide reacts with ammonia gas to form solid lead, nitrogen gas and liquid
water. What is the correct balanced equation for this reaction?
a.
b.
d.
3PbO(s) + 2NH 3(g) --.. 3Pb2(s) + N2(g) + 3H 20(g)
3PbO(s) + 2NH3(g) --.. 3Pb(s) + Nz<g) + 3Hz0(l)
e.
3Pb0Z<s) + 2NH 3(l) --.. 3Pb(g) + N2(1) + 3H 20(s)
c.
6.
3Pb02(s) + 2~(g) --.. 3Pb(s) + N2(g) + 32° (1)
3PbO(s) + 2NH/(g) --.. 3Pb(s) + N(g) + 3HzO(l)
Balance the equation below.
equation?
What is the sum of the coefficients for the balanced
HN02 --.. NO + HN03 + Hz0
7.
8.
114
a.
4
b.
5
c.
d.
e.
6
7
9
What is the correct balanced equation for the reaction of solid phosphorus with solid iodine
to produce liquid phosphorus triiodide?
a.
P(s) + 2Iz<s) --.. PI3(s)
b.
Pis) + 6Iz<s) --.. 4PI 3(l)
c.
d.
2P(s) + 3I2(s) --.. P2I3(l)
Pis) + 3I 2(s) --.. 2P 2I3(1)
e.
3P(s) + I2(g) --.. P 3I2(1)
Which equation is not balanced?
a.
2MgO --.. 2Mg + 02
b.
c.
4AI + 302 --.. 2Al203
2Na + Hz0 --.. 2NaOH + Hz
d.
Mg(OH)2 + 2HCI --.. MgClz + 2Hz0
e.
2S02 + 02 --.. 2S03
6 Chemical Reactions: An Introduction
9.
Which is the correct balanced equation for the reaction below?
a.
b.
c.
d.
e.
10.
K3P04 + 3BaCl 2 --- 3KCI + B~(P04)2
4K 3P04 + 6BaCl 2 --- 12KCI + 2B~(P04)2
K3P04 + BaCl 2 --- KCI + B~(POJ2
2K 3P04 + 3BaCl2 --- 6KCI + B~(P04)2
2K3P04 + 3BaC~ --- 2KCI + B~(POJ2
Which is the correct balanced equation for the reaction below?
a.
b.
c.
d.
e.
C6H14(l) + 902(g) --- 6C02(g) + 7~O(l)
C6H14(l) + 9.502(g) --- 6C02(g) + 7H 20(1)
2C6H1il) + 190ig) --- 12C02(g) + 14~O(l)
C6H14(1) + 3.502(g) --- 3C02(g) + 7~0(1)
C6H14(l) + 902(g) --- 6C02(g) + 6H 20(1)
PRACTICE EXAM ANSWERS
1.
a (6.1)
2.
3.
4.
a (6.2)
c (6.2)
c (6.3)
d (6.3)
d (6.3)
b (6.3)
c (6.3)
d (6.3)
c (6.3)
5.
6.
7.
8.
9.
10.
Practice Exam Answers
115