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Chapter 5 Chemical Reactions and Quantities

5.1 Chemical Changes

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Physical Change

In a physical change, §  The identity and

composition of the substance do not change.

§  The state can change, or the material can be torn into smaller pieces.

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Chemical Change

In a chemical change, n  Reacting substances

form new substances with different compositions and properties.

n  A chemical reaction takes place.

Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

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Some Examples of Chemical and Physical Changes

TABLE 5.1

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Classify each of the following as a 1) physical change or 2) chemical change.

A. ____Burning a candle. B. ____Ice melting on the street.

C. ____Toasting a marshmallow. D. ____Cutting a pizza. E. ____Polishing a silver bowl.

Learning Check

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Classify each of the following as a 1) physical change or 2) chemical change. A. 2 Burning a candle. B. 1 Ice melting on the street. C. 2 Toasting a marshmallow. D. 1 Cutting a pizza. E. 2 Polishing a silver bowl.

Solution

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Chemical Reaction

In a chemical reaction, §  A chemical change produces

one or more new substances. §  There is a change in the

composition of one or more substances.

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Chemical Reaction

In a chemical reaction, §  Old bonds are broken, and

new bonds are formed. §  Atoms in the reactants are

rearranged to form one or more different substances.

§  Fe and O2 form rust (Fe2O3).

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Learning Check

Identify the visible evidence of a chemical reaction in each of the following: A. Methane gas in an outdoor heater burns with a blue

flame. B. Bleach removes stains from a shirt. C. Bubbles of CO2 are released when baking soda is

mixed with vinegar.

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Solution

Identify the visible evidence of a chemical reaction in each of the following: A. Methane gas in an outdoor heater burns with a blue flame.

Heat and a flame B. Bleach removes stains from a shirt. Color change (color removed) C. Bubbles of CO2 are released when baking soda is mixed with vinegar.

Formation of gas (bubbles)

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Chapter 5 Chemical Reactions and Quantities

5.2 Chemical Equations

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Chemical Equations

A chemical equation §  Gives the chemical formulas of the reactants on the left of the

arrow and the products on the right.

Reactants Product

C(s)

O2 (g) CO2 (g)

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Symbols Used in Equations

Symbols are used in chemical equations to show §  The states of the reactants. §  The states of the products.

§  The reaction conditions.

TABLE 5.2

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Chemical Equations are Balanced

In a balanced chemical reaction, §  Atoms are not gained

or lost. §  The number of atoms

in the reactants is equal to the number of atoms in the products.

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A Balanced Chemical Equation

In a balanced chemical equation, §  There must be the same number of each type of atom on the

reactant side and on the product side. §  Numbers called coefficients are used in front of one or more

formulas. Al + S Al2S3 Not Balanced 2Al + 3S Al2S3 Balanced

2Al = 2Al 3S = 3S

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Learning Check

State the number of atoms of each element on the reactant side and the product side for each of the following balanced equations: A. P4(s) + 6Br2(l) → 4 PBr3(g)

B. 2Al(s) + Fe2O3(s) → 2Fe(s) + Al2O3(s)

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Solution

A. P4(s) + 6Br2(l) → 4PBr3(g) 4 P 4 P

12 Br 12 Br B. 2Al(s) + Fe2O3(s) → 2Fe(s) + Al2O3(s)

2 Al 2 Al 2 Fe 2 Fe

3 O 3 O

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Learning Check

Determine if each equation is balanced or not. A. Na(s) + N2(g) → Na3N(s) B. C2H4(g) + H2O(l) → C2H5OH(l)

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Solution

Determine if each equation is balanced or not. A. Na(s) + N2(g) → Na3N(s)

No. 2N on reactant side, 1N on product side. 1Na on reactant side, 3Na on product side.

Correctly balanced: 6Na(s) +N2(g) → 2Na3N(s) B. C2H4(g) + H2O(l) → C2H5OH(l)

Yes. 2C = 2C 6H = 6H 1O = 1O

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Equation for A Chemical Reaction

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Checking a Balanced Equation

Reactants Products 1 C atom = 1 C atom 4 H atoms = 4 H atoms 4 O atoms = 4 O atoms

Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

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Guide to Balancing Equations

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STEP 1 Write the equation with the correct formulas. NH3(g) + O2(g) NO(g) + H2O(g)

STEP 2 Determine if the equation is balanced. No, not all atoms are balanced.

STEP 3 Balance with coefficients in front of formulas. 4NH3 + 5O2 4NO + 6H2O

STEP 4 Check that atoms of each element are equal in reactants and products. 4 N (4 x 1N) = 4 N (4 x 1N) 12 H (4 x 3H) = 12 H (6 x 2H) 10 O (5 x 2O) = 10 O (4O + 6O)

Balancing Chemical Equations

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Check the balance of atoms in the following: Fe3O4(s) + 4H2(g) 3Fe(s) + 4H2O(l)

A. Number of H atoms in products. 1) 2 2) 4 3) 8

B. Number of O atoms in reactants. 1) 2 2) 4 3) 8 C. Number of Fe atoms in reactants. 1) 1 2) 3 3) 4

Learning Check

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Fe3O4(s) + 4H2(g) 3Fe(s) + 4H2O(l)

A. Number of H atoms in products. 3) 8 (4H2O)

B. Number of O atoms in reactants. 2) 4 (Fe3O4)

C. Number of Fe atoms in reactants. 2) 3 (Fe3O4)

Solution

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Balance each equation and list the coefficients in the balanced equation going from reactants to products: A. __Mg(s) + _N2(g) __Mg3N2(s)

1) 1, 3, 2 2) 3, 1, 2 3) 3, 1, 1

B. __Al(s) + __Cl2(g) __AlCl3(s)

1) 3, 3, 2 2) 1, 3, 1 3) 2, 3, 2

Learning Check

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A. 3) 3, 1, 1

3Mg(s) + 1N2(g) 1Mg3N2(s)

B. 3) 2, 3, 2

2Al(s) + 3Cl2(g) 2AlCl3(s)

Solution

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Equations with Polyatomic Ions

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Balancing with Polyatomic Ions

Na3PO4(aq) + MgCl2(aq) NaCl(aq) + Mg3(PO4)2(s) Balance PO4

3- as a unit 2Na3PO4(aq) Mg3(PO4)2(s)

2PO43- = 2PO4

3- Balance Mg 3MgCl2(aq) Mg3(PO4)2(s)

3Mg2+ = 3Mg2+ Balance Na and Cl to complete balanced equation 3MgCl2(aq) + 2Na3PO4(aq) 6NaCl(aq) + Mg3(PO4)2(s)

6Na+ = 6Na+ 6Cl- = 6Cl-

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Balance and list the coefficients from reactants to products: A. __Fe2O3(s) + __C(s) __Fe(s) + __CO2(g)

1) 2, 3, 2,3 2) 2, 3, 4, 3 3) 1, 1, 2, 3 B. __Al(s) + __FeO(s) __Fe(s) + __Al2O3(s)

1) 2, 3, 3, 1 2) 2, 1, 1, 1 3) 3, 3, 3, 1 C. __Al(s) + __H2SO4(aq) __Al2(SO4)3(aq) + __H2(g)

1) 3, 2, 1, 2 2) 2, 3, 1, 3 3) 2, 3, 2, 3

Learning Check

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A. 2) 2, 3, 4, 3 2Fe2O3(s) + 3C(s) 4Fe(s) + 3CO2(g) B. 1) 2, 3, 3, 1 2Al(s) + 3FeO(s) 3Fe(s) + 1Al2O3(s)

C. 2) 2, 3, 1, 3 2Al(s) + 3H2SO4(aq) 1Al2(SO4)3(aq) + 3H2(g)

Solution