Chapter 7
Pretest
1. Which of the following is an example of a physical change?
a. Wood burns and becomes ash.
b. A steel nail rusts over time.
c. Ice melts and becomes water.
d. Milk curdles when acid is added to it.
2. Which of the following characteristics can you determine about a
substance based on its chemical formula?
a. the number and types of atoms that make up the substance
b. the mass of an unknown sample of the substance
c. the melting point of the substance
d. the density and state of the substance at room temperature
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Chapter 7
Pretest (continued)
3. How do you find the atomic mass of an element?
4. Which conversion factor would you multiply 0.020 m by in
order to express the quantity in centimeters?
a. 1000 m/1 km
b. 1 km/1000 m
c. 1 m/100 cm
d. 100 cm/1 m
5. Which is the correct chemical formula for potassium
hydroxide?
a. POH b. KOH
c. P5OH d. K2OH
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• A chemical reaction is the process by which one or
more substances are changed into one or more
different substances.
• In any chemical reaction, the original substances are
known as the reactants and the resulting substances
are known as the products.
• According to the law of conservation of mass, the
total mass of reactants must equal the total mass of
products for any given chemical reaction.
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Section 7.1
Interest Grabber
Equation Analogy
Imagine that you work at a skateboard shop and you are in
charge of assembling the skateboards. Every skateboard
requires one deck (the board), two trucks (the mounted axles),
and four wheels.
1. Your boss asks you to make five skateboards. How many
trucks do you need?
2. The following diagram shows the “recipe” for one
skateboard. What do you notice about the relative
amounts of each part on either side of the arrow?
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• A chemical equation represents, with symbols and
formulas, the identities and relative molecular or
molar amounts of the reactants and products in a
chemical reaction.
• example: The following chemical equation shows
that the reactant ammonium dichromate yields
the products nitrogen, chromium(III) oxide, and
water.
(NH4)2Cr2O7(s) N2(g) + Cr2O3(s) + 4H2O(g)
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Indications of a Chemical Reaction
• Certain easily observed changes usually indicate
that a chemical reaction has occurred.
1. Evolution of energy as heat and light
2. Production of a gas
3. Formation of a precipitate.
• A solid that is produced as a result of a
chemical reaction in solution and that
separates from the solution is known as a
precipitate.
4. Color change
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Characteristics of Chemical Equations
• The following requirements will aid you in writing and
reading chemical equations correctly.
1. The equation must represent known facts.
2. The equation must contain the correct formulas
for the reactants and products.
3. The law of conservation of mass must be satisfied.
• A coefficient is a small whole number that
appears in front of a formula in a chemical
equation.
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Elements That Normally Exist as
Diatomic Molecules
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Characteristics of Chemical Equations,
continued
Word and Formula Equations
• The first step in writing a chemical equation is to
identify the facts to be represented.
• A word equation is an equation in which the reactants
and products in a chemical reaction are represented by
words.
• A word equation is qualitative
• example: methane + oxygen carbon dioxide + water
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Characteristics of Chemical Equations,
continued
Word and Formula Equations, continued
• The next step in writing a correct chemical equation is
to replace the names of the reactants and products with
appropriate symbols and formulas.
• A formula equation represents the reactants and
products of a chemical reaction by their symbols or
formulas.
• example: The formula equation for the reaction of methane
and oxygen is
CH4(g) + O2(g) CO2(g) + H2O(g) (not balanced)
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Characteristics of Chemical Equations,
continued
Word and Formula Equations, continued
• To complete the process of writing a correct equation,
the law of conservation of mass must be taken into
account.
• The relative amounts of reactants and products
represented in the equation must be adjusted so
that the numbers and types of atoms are the same
on both sides of the equation.
• This process is called balancing an equation
and is carried out by inserting coefficients.
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Characteristics of Chemical Equations,
continued
Word and Formula Equations, continued
• To balance the equation, begin by counting atoms of
elements that are combined with atoms of other
elements and that appear only once on each side of the
equation.
CH4(g) + O2(g) CO2(g) + 2H2O(g) (not balanced)
• Begin by counting carbon atoms.
• Carbon is already balanced in the equation.
• Two additional hydrogen atoms are needed on the right
side of the equation.
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Characteristics of Chemical Equations,
continued
Word and Formula Equations, continued
CH4(g) + O2(g) CO2(g) + 2H2O(g) (partially balanced)
• Now consider the number of oxygen atoms.
• Increase the number of oxygen atoms on the left side to four
by placing the coefficient 2 in front of the molecular formula
for oxygen.
• The correct chemical equation, or balanced formula
equation, for the burning of methane in oxygen is
CH4(g) + 2O2(g) CO2(g) + 2H2O(g)
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Characteristics of Chemical Equations, continued
Additional Symbols Used in Chemical Equations
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Characteristics of Chemical Equations, continued
Additional Symbols Used in Chemical Equations
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Symbols Used in Chemical Equations
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Methane Combustion
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• A chemical formula indicates:
– the elements present in a compound
– the relative number of atoms or ions of each element
present in a compound
• Chemical formulas also allow chemists to calculate a
number of other characteristic values for a compound:
– formula mass
– molar mass
– percentage composition
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Characteristics of Chemical Equations,
continued
Sample Problem A
Write word and formula equations for the chemical
reaction that occurs when solid sodium oxide is added to
water at room temperature and forms sodium hydroxide
(dissolved in the water). Include symbols for physical
states in the formula equation. Then balance the formula
equation to give a balanced chemical equation.
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Characteristics of Chemical Equations,
continued
Sample Problem A Solution
• The word equation must show the reactants, sodium oxide and
water, to the left of the arrow.
• The product, sodium hydroxide, must appear to the right of the
arrow.
sodium oxide + water sodium hydroxide
• Sodium has an oxidation state of +1, that oxygen usually has an
oxidation state of 2, and that a hydroxide ion has a charge of 1.
The unbalanced formula equation is
Na2O + H2O NaOH (not balanced)
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Characteristics of Chemical Equations,
continued
Sample Problem A Solution, continued
Adding symbols for the physical states of the reactants and products
and the coefficient 2 in front of NaOH produces a balanced chemical
equation.
Na2O(s) + H2O(l) 2NaOH(aq)
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Significance of a Chemical Equation
• Some of the quantitative information revealed by a
chemical equation includes
1. The coefficients of a chemical reaction indicate
relative, not absolute, amounts of reactants and
products.
H2(g) + Cl2(g) 2HCl(g)
1 molecule H2 : 1 molecule Cl2 : 2 molecules HCl
• This ratio shows the smallest possible relative
amounts of the reaction’s reactants and products.
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Significance of a Chemical Equation
2. The relative masses of the reactants and products
of a chemical reaction can be determined from the
reaction’s coefficients.
• An amount of an element or compound in moles can be
converted to a mass in grams by multiplying by the
appropriate molar mass.
• example:
2.02 g H2
1 mol H2 2.02 g H2
mol H2
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Interpreting a Chemical Reaction
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Significance of a Chemical Equation
3. The reverse reaction for a chemical equation has
the same relative amounts of substances as the
forward reaction.
• An equation gives no indication of whether a reaction
will actually occur.
• Chemical equations give no information about the
speed at which reactions occur.
• Equations do not give any information about how
the bonding between atoms or ions changes
during the reaction.
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Balancing Chemical Equations
• The following procedure demonstrates how to master
balancing equations by inspection using a step-by-
step approach.
1. Identify the names of the reactants and the
products, and write a word equation.
water hydrogen + oxygen
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Balancing Chemical Equations, continued
• balancing equations by inspection, continued
2. Write a formula equation by substituting correct
formulas for the names of the reactants and the
products.
H2O(l) H2(g) + O2(g) (not balanced)
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Balancing Chemical Equations, continued
• balancing equations by inspection, continued
3. Balance the formula equation according to the law
of conservation of mass.
• Balance the different types of atoms one at a time.
• First balance the atoms of elements that are combined
and that appear only once on each side of the equation.
• Balance polyatomic ions that appear on both sides of
the equation as single units.
• Balance H atoms and O atoms after atoms of all other
elements have been balanced.
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Balancing Chemical Equations, continued
• balancing equations by inspection, continued
4. Count atoms to be sure that the equation is
balanced.
2H2O(l) 2H2(g) + O2(g)
(4H + 2O) = (4H) + (2O)
• If the coefficients do not represent the smallest possible
whole-number ratio of reactants and products, divide the
coefficients by their greatest common factor in order to
obtain the smallest possible whole-number coefficients.
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Balancing Chemical Equations, continued
Sample Problem C
The reaction of zinc with aqueous hydrochloric acid
produces a solution of zinc chloride and hydrogen gas.
Write a balanced chemical equation for the reaction.
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Balancing Chemical Equations, continued
Sample Problem C Solution
• Write the word equation.
zinc + hydrochloric acid zinc chloride + hydrogen
• Write the formula equation.
Zn(s) + HCl(aq) ZnCl2(aq) + H2(g) (not balanced)
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Section 7.1
Reading Strategy
Monitoring Your Understanding
a. and c. How to balance chemical equations and how to convert
from mass to moles.
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Formula Masses
• The formula mass of any molecule, formula unit, or ion is
the sum of the average atomic masses of all atoms
represented in its formula.
– example: formula mass of water, H2O
average atomic mass of H: 1.01 amu
average atomic mass of O: 16.00 amu
1.01 amu
2 H atoms 2.02 amu
H atom
16.00 amu
1 O atom 16.00 amu
O atom
average mass of H2O molecule: 18.02 amu
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Formula Masses
• The mass of a water molecule can be referred to as a
molecular mass.
• The mass of one formula unit of an ionic compound, such
as NaCl, is not a molecular mass.
• The mass of any unit represented by a chemical formula
(H2O, NaCl) can be referred to as the formula mass.
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Formula Masses, continued
Sample Problem F
Find the formula mass of potassium chlorate, KClO3.
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Formula Masses, continued
Sample Problem F Solution
The mass of a formula unit of KClO3 is found by adding
the masses of one K atom, one Cl atom, and three O
atoms.
Atomic masses can be found in the periodic table in the
back of your book.
In your calculations, round each atomic mass to two
decimal places.
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Formula Masses, continued
Sample Problem F Solution, continued
39.10 amu
1 K atom 39.10 amu
K atom
35.45 amu
1 Cl atom 35.45 amu
Cl atom
16.00 amu
3 O atoms 48.00 amu
O atom
formula mass of KClO3 = 122.55 amu
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Figure 2
Burning of Carbon
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Figure 2
Burning of Carbon
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Figure 2
Burning of Carbon
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Go to section
Balancing Chemical Section 7.1
Equations
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Balancing Chemical Section 7.1
Equations
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Balancing Chemical Section 7.1
Equations
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Balancing Chemical Section 7.1
Equations
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Calculations With Figure 8
Chemical Equations
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Calculations With Figure 8
Chemical Equations
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Calculations With Figure 8
Chemical Equations
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Calculations With Figure 8
Chemical Equations
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Calculations With Figure 8
Chemical Equations
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Molar Masses
• The molar mass of a substance is equal to the mass in
grams of one mole, or approximately 6.022 1023
particles, of the substance.
– example: the molar mass of pure calcium, Ca, is 40.08 g/mol
because one mole of calcium atoms has a mass of 40.08 g.
• The molar mass of a compound is calculated by adding the
masses of the elements present in a mole of the molecules
or formula units that make up the compound.
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Molar Masses, continued
• One mole of water molecules contains exactly two moles
of H atoms and one mole of O atoms. The molar mass of
water is calculated as follows.
1.01 g H
2 mol H 2.02 g H
mol H
16.00 g O
1 mol O 16.00 g O
mol O
molar mass of H2O molecule: 18.02 g/mol
• A compound’s molar mass is numerically equal to its
formula mass.
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Calculating Molar Masses for Ionic Compounds
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Molar Masses, continued
Sample Problem G
What is the molar mass of barium nitrate, Ba(NO3)2?
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Molar Masses, continued
Sample Problem G Solution
One mole of barium nitrate, contains one mole of Ba, two moles of
N (1 2), and six moles of O (3 2).
137.33 g H
1 mol Ba 137.33 g Ba
mol Ba
14.01 g
2 mol N 28.02 g N
mol N
16.00 g O
6 mol O 96.00 g O
mol O
molar mass of Ba(NO3)2 = 261.35 g/mol
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Molar Mass as a Conversion Factor
• The molar mass of a compound can be used as a
conversion factor to relate an amount in moles to a mass
in grams for a given substance.
• To convert moles to grams, multiply the amount in moles
by the molar mass:
Amount in moles molar mass (g/mol)
= mass in grams
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Mole-Mass Calculations
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Molar Mass as a Conversion Factor, continued
Sample Problem H
What is the mass in grams of 2.50 mol of oxygen gas?
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Molar Mass as a Conversion Factor, continued
Sample Problem H Solution
Given: 2.50 mol O2
Unknown: mass of O2 in grams
Solution:
moles O2 grams O2
amount of O2 (mol) molar mass of O2 (g/mol) = mass of
O2 (g)
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Molar Mass as a Conversion Factor, continued
Sample Problem H Solution, continued
Calculate the molar mass of O2.
16.00 g O
2 mol O 32.00 g
mol O
Use the molar mass of O2 to convert moles to mass.
32.00 g O2
2.50 mol O2 80.0 g O2 80.0 g O2
mol O2
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Converting Between Amount in Moles and
Number of Particles
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Molar Mass as a Conversion Factor, continued
Sample Problem I
Ibuprofen, C13H18O2, is the active ingredient in many
nonprescription pain relievers. Its molar mass is
206.31 g/mol.
a. If the tablets in a bottle contain a total of 33 g of
ibuprofen, how many moles of ibuprofen are in the
bottle?
b. How many molecules of ibuprofen are in the bottle?
c. What is the total mass in grams of carbon in 33 g
of ibuprofen?
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Molar Mass as a Conversion Factor, continued
Sample Problem I Solution
Given: 33 g of C13H18O2
molar mass 206.31 g/mol
Unknown: a. moles C13H18O2
b. molecules C13H18O2
c. total mass of C
Solution: a. grams moles
1 mol C13H18O2
g C13H18O2 mol C13H18O2
206.31 g C13H18O2
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Molar Mass as a Conversion Factor, continued
Sample Problem I Solution, continued
b. moles molecules
6.022 1023 molecules
mol C13H18O2 molecules C13H18O2
mol
c. moles C13H18O2 moles C grams C
13 mol C 12.01 g C
mol C13H18O2 gC
mol C13H18O2 mol C
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Molar Mass as a Conversion Factor, continued
Sample Problem I Solution, continued 0.16 mol
1 mol C13H18O2
33 a. C13H18O2
g 0.16 mol C13H18O2
206.31 g C13H18O2
b. 0.16mol C H O 6.022 1023 molecules
13 18 2
mol
9.6 1022 molecules C13H18O2
13 mol C 12.01 g C
c. 0.16 mol C H O 25 g C
13 18 2
mol C13H18O2 mol C
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Section 7.2
Models of Reactions
The following drawings 1. Synthesis means “putting
represent reactants and something together.”
products of three different
chemical reactions.
Reaction B is a synthesis
reaction, because the
two reactants come
together to make one
product.
Go to section Click the mouse button to display the answer.
Section 7.2
Models of Reactions
The following drawings 2. Decomposition means
represent reactants and “taking something apart.”
products of three different
chemical reactions.
Reaction A is a
decomposition reaction,
because the reactant is
taken apart to make two
products.
Go to section Click the mouse button to display the answer.
Section 7.2
Models of Reactions
The following drawings 3. Replacement means
represent reactants and “something taking the
products of three different place of another.”
chemical reactions.
Reaction C is a
replacement reaction,
because one reactant
takes the place of part
of another.
Go to section Click the mouse button to display the answer.
Section 7.2
Reading Strategy
Previewing
a. Decomposition
b. Double Replacement
c. AB → A + B
d. A + BC → B + AC
e. AB + CD → AD + CB
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Section 7.3
Interest Grabber
Exothermic and Endothermic Processes
Energy is either absorbed or released during chemical and
physical changes. During an endothermic change, energy is
absorbed from the surroundings. During an exothermic
change, energy is released to the surroundings. Two
different physical changes are described below. Read each
description and decide if the change is exothermic or
endothermic.
1. A plastic bottle of water is placed in a freezer. After
several hours, the water has frozen solid.
2. A chef places a stick of solid butter in a saucepan, and
heats the saucepan over low heat on a stove. After
several minutes, the stick of butter has melted.
Go to section
Section 7.3
Reading Strategy
Comparing and Contrasting
a. releases energy to the surroundings
b. absorbs energy from surroundings
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Figure 17
Combustion of Propane
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Figure 17
Combustion of Propane
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Figure 17
Combustion of Propane
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Figure 17
Combustion of Propane
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Reaction Energy Diagram for Figure 18A
an Exothermic Reaction
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Reaction Energy Diagram for Figure 18A
an Exothermic Reaction
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Reaction Energy Diagram for Figure 18B
an Endothermic Reaction
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Reaction Energy Diagram for Figure 18B
an Endothermic Reaction
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Section 7.4
Interest Grabber
Changing Reaction Rate
1. Place one effervescent antacid tablet into a plastic cup
filled with hot tap water and a second tablet into a plastic
cup filled with cold water at the same time. In which cup
did the fizzing last longer?
2. On a small piece of paper, crush a third tablet into a
powder using a plastic spoon. Place the crushed tablet
into a cup of cold water and a fourth (whole) tablet into
another cup of cold water at the same time. In which cup
did the fizzing last longer?
3. How does temperature affect the rate of fizzing?
4. How does crushing the reactant into a powder affect the
rate of fizzing?
Go to section
Section 7.4
Reading Strategy
Building Vocabulary
a. Temperature d. Concentration
b. Surface area e. Catalysts
c. Stirring
Go to section
Reaction Energy Diagram for a Figure 23
Catalyzed and an Uncatalyzed
Reaction
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Section 7.5
Interest Grabber
Opposing Changes
Imagine that you are emptying a swimming pool using a
pump that removes 5 gallons of water each minute.
Meanwhile, your friend turns on a hose that adds 5 gallons of
water to the pool each minute.
1. What happens to the water level of the
swimming pool?
2. What would happen to the water level of the swimming
pool if you increased the rate that the pump removed
water to 7 gallons of water each minute?
Go to section
Section 7.5
Reading Strategy
Outlining
1. Physical equilibrium B. Factors affecting chemical equilibrium
2. Chemical equilibrium 3. Concentration
Go to section
Figure 25
Physical Equilibrium
Go to section
Chapter 7
Pretest Answers
1. Which of the following is an example of a physical change?
a. Wood burns and becomes ash.
b. A steel nail rusts over time.
c. Ice melts and becomes water.
d. Milk curdles when acid is added to it.
2. Which of the following characteristics can you determine about a
substance based on its chemical formula?
a. the number and types of atoms that make up the substance
b. the mass of an unknown sample of the substance
c. the melting point of the substance
d. the density and state of the substance at room temperature
Click the mouse button to display the answers.
Chapter 7
Pretest Answers
(continued)
3. How do you find the atomic mass of an element?
The atomic mass of each element is listed on the periodic table.
4. Which conversion factor would you multiply 0.020 m by in
order to express the quantity in centimeters?
a. 1000 m/1 km
b. 1 km/1000 m
c. 1 m/100 cm
d. 100 cm/1 m
5. Which is the correct chemical formula for potassium
hydroxide?
a. POH b. KOH
c. P5OH d. K2OH
Click the mouse button to display the answers.
Section 7.1
Interest Grabber
Answers
1. Your boss asks you to make five skateboards. How many
trucks do you need?
10
2. The following diagram shows the “recipe” for one skateboard.
What do you notice about the relative amounts of each part
on either side of the arrow?
There are the same numbers of each part on either side of
the arrow.
Section 7.3
Interest Grabber
Answers
1. A plastic bottle of water is placed in a freezer. After
several hours, the water has frozen solid.
Exothermic. The liquid water releases energy as it
changes into ice.
2. A chef places a stick of solid butter in a saucepan, and
heats the saucepan over low heat on a stove. After
several minutes, the stick of butter has melted.
Endothermic. The solid butter absorbs energy as it
changes into liquid.
Section 7.4
Interest Grabber
Answers
1. Place one effervescent antacid tablet into a plastic cup filled with hot
tap water and a second tablet into a plastic cup filled with cold water
at the same time. In which cup did the fizzing last longer?
The fizzing lasts longer for the tablet placed in cold water.
2. On a small piece of paper, crush a third tablet into a powder using a
plastic spoon. Place the crushed tablet into a cup of cold water and
a fourth (whole) tablet into another cup of cold water at the same
time. In which cup did the fizzing last longer?
The fizzing lasts longer for the whole, uncrushed tablet.
3. How does temperature affect the rate of fizzing?
Increased temperature caused the fizzing to happen at a faster rate.
4. How does crushing the reactant into a powder affect the rate of
fizzing?
Crushing the reactant into a powder caused the fizzing to happen at
a faster rate.
Section 7.5
Interest Grabber
Answers
1. What happens to the water level of the swimming pool?
The water level stays the same.
2. What would happen to the water level of the swimming
pool if you increased the rate that the pump removed
water to 7 gallons of water each minute?
The water level would go down.
Chapter 7
Go Online
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