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Chapter 5 Chemical Quantities and Reactions

5.4 Chemical Equations

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Chemical Equations

A chemical equation gives• the formulas of the reactants on the left of the arrow.• the formulas of the products on the right of the arrow.

Reactants Product

C(s)

O2 (g)CO2 (g)

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Symbols Used in Equations

Symbols in chemical equations show

• the states of the reactants.

• the states of the products.

• the reaction conditions.

TABLE

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Chemical Equations are Balanced

In a balancedchemical reaction• no atoms are

lost or gained.

• the number of reacting atoms is equal to the number of product atoms.

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A Balanced Chemical Equation

In a balanced chemical equation,• the number of each type of atom on the reactant side is

equal to the number of each type of atom on the product side.

• numbers called coefficients are used in front of one or more formulas to balance the number of atoms.

Al + S Al2S3 Not Balanced

2Al + 3S Al2S3 Balanced using coefficients

2 Al = 2 Al Equal number of Al atoms 3 S = 3 S Equal number of S atoms

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A Study Tip: Using Coefficients

When balancing a chemical equation, • use one or more coefficients to balance atoms.• never change the subscripts of any formula.

N2 + O2 NO Not Balanced

N2 + O2 N2O2 Incorrect formula

N2 + O2 2NO Correctly balanced

using coefficients

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Learning Check

State the number of atoms of each element on thereactant side and the product side for each of thefollowing balanced equations.

A. P4(s) + 6Br2(l) 4PBr3(g)

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

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Solution

A. P4(s) + 6Br2(l) 4PBr3(g)

4 P atoms = 4 P atoms

12 Br atoms = 12 Br atoms

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

2 Al atoms = 2 Al atoms

2 Fe atoms = 2 Fe atoms

3 O atoms = 3 O atoms

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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) C2H6O(l)

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Solution

Determine if each equation is balanced or not.

A. Na(s) + N2(g) Na3N(s) No. 2 N atoms on reactant side; only 1 N atom

on the product side. 1 Na atom on reactant side; 3 Na atoms on the product side.

B. C2H4(g) + H2O(l) C2H6O(l) Yes. 2 C atoms = 2 C atoms 6 H atoms = 6 H atoms

1 O atom = 1 O atom

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Guide to Balancing a Chemical Equation

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To balance the following equation, Fe3O4(s) + H2(g) Fe(s) + H2O(l)

• work on one element at a time.• use only coefficients in front of formulas.• do not change any subscripts.Fe: Fe3O4(s) + H2(g) 3Fe(s) + H2O(l)

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

H: Fe3O4(s) + 4H2(g) 3Fe(s) +

4H2O(l)

Steps in Balancing an Equation

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

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

3. Balance with coefficients in front of formulas. 2NH3(g) + O2(g) 2NO(g) + 3H2O(g)

Double the coefficients to give even number of O atoms.4NH3(g) + 7O2(g) 4NO(g) + 6H2O(g)

Balancing Chemical Equations

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4. Check that atoms of each element are equal in reactants and products.

4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)

4 N (4 x 1 N) = 4 N (4 x 1 N) 12 H (4 x 3 H) = 12 H (6 x 2 H) 10 O (5 x 2 O) = 10 O (4 O + 6 O)

Balancing Chemical Equations

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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

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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

2Na3PO4(aq) + 3MgCl2 (aq) Mg3 ( PO4(s) + 6NaCl(aq)

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Balancing with Polyatomic Ions

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

Balance PO43- as a unit

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

3- = 2 PO43-

Check Na + balance 6 Na+ = 6 Na+

Balance Mg and Cl2Na3PO4(aq) + 3MgCl2(aq) Mg3(PO4)2(s) + 6NaCl(aq)

3 Mg2+ = 3 Mg2+

6 Cl- = 6 Cl-

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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