Balancing Chemical Reactions Honors chemistry – Semester 1

Balancing Equations

• All chemical equations must be balanced • Law of conservation of mass

• Same number of atoms of each type on each side

• Add coefficients to increase number of atoms as needed • NEVER, EVER change subscripts in a formula!!!

• Reduce coefficients to smallest possible • E.g., 2H2 + O2 2H2O NOT 6H2 + 3O2 6H2O

Balancing Equations

• 1) Identify reactants and products

• 2) Count number of atoms per element for reactants and products separately. Compare.

• 3) If the number of atoms is unequal for any of the elements, insert coefficients to balance the equation.

• 4) Verify correct balance

• Example: hydrogen mixes with oxygen to produce water

• H2 + O2 H2O Unbalanced Equation

• Reactants: H = 2, O = 2

• Products: H = 2, O = 1

• 2H2 + O2 2H2O Balanced Equation

• Reactants: H = 4, O = 2

• Products: H = 4, O = 2

Balancing Equations- Practice

Balance the equation for reaction of iron (III) oxide with hydrogen to form iron and water.

• 1) Identify reactants and products

• Fe2O3 + H2 Fe + H2O

• 2) Count atoms

• Reactants: Fe = 2, O = 3, H = 2

• Products: Fe = 1, O = 1, H = 2

• NOT balanced!

Balancing Equations- Practice

• 3) Insert coefficients to balance equation • Start with atoms in compounds

• Leave pure elements until last

• Example: Fe2O3 + H2 Fe + H2O

• Start with Fe: Fe2O3 + H2 2Fe + H2O

• Then balance O: Fe2O3 + H2 2Fe + 3H2O

• Fix H last: Fe2O3 + 3H2 2Fe + 3H2O

• 4) Verify balance

• Reactants: Fe = 2, O = 3, H = 6

• Products: Fe = 2, O = 3, H = 6

• Balanced!

Example

Silicon reacts with carbon dioxide to form silicon carbide, SiC, and silicon dioxide.

• Identify reactants and products • Si + CO2 SiC + SiO2

• Count atoms

• Reactants Si = 1, C = 1, O = 2

• Products Si = 2, C = 1, O = 2

• Balance with coefficients as needed

• 2 Si + CO2 SiC + SiO2

Practice

Write balanced equations for:

• P4 + O2 P2O5

• P4 + 5 O2 2 P2O5

• Octane (C3H8) plus oxygen combust to form carbon dioxide and water

• C3H8 + 5 O2 3 CO2 + 4 H2O

• Ca2Si + Cl2 CaCl2 + SiCl4

• Ca2Si + 4 Cl2 2 CaCl2 + SiCl4

Odd – Even Technique

• After first try at coefficients may have odd number on one side and even on other

• Often with diatomic molecule on one side and same element in compound on the other

• Multiply both sides of reaction by 2

• Gives even (but still unequal) numbers of atoms on both sides

• Complete balancing as needed

Sample Exercise – Odd-Even Technique The reaction of ammonia with oxygen produces nitrogen monoxide and water vapor. Write a balanced equation for this reaction.

Solution:

• Identify reactants and products:

• NH3 + O2 NO + H2O

• Count atoms:

Atoms Reactants Products

N 1 1

H 3 2

O 2 2

Sample Exercise – Odd-Even Technique • Fix coefficient for H, adjust others as required

• 2NH3 + O2 2NO + 3H2O

• Multiplying by 2 ALWAYS results in an even number!

• 4NH3 + 2O2 4NO + 6H2O

• Balance out oxygen on reactant side

• 4NH3 + 5O2 4NO + 6H2O

Atoms Reactants Products

N 4 4

H 12 12

O 10 10

Atoms Reactants Products

N 2 2

H 6 6

O 2 5

Atoms Reactants Products

N 4 4

H 12 12

O 4 10

Practice

Write a balanced chemical equation for the following:

• C2H2 + O2 CO2 + H2O • 2 C2H2 + 5 O2 4 CO2 + 2 H2O

• Fe(OH)2 + H2O2 Fe(OH)3

• 2 Fe(OH)2 + H2O2 2 Fe(OH)3

• FeS2 + Cl2 FeCl3 + S2Cl2

• 2 FeS2 + 5 Cl2 2 FeCl3 + 2 S2Cl2

Sample – Polyatomic Ions

Balance polyatomic ions as a unit

Example: Aluminum reacts with arsenic acid, HAsO3, to form hydrogen and aluminum arsenate, Al(AsO3)3. Write a balanced equation.

Solution:

• Al + HAsO3 H2 + Al(AsO3)3 (Unbalanced)

• Count atoms / ions: Atoms / ions Reactants Products

Al 1 1

H 1 2

AsO3 1 3

Sample – Polyatomic Ions

• Balance arsenate ion

• Al + 3 HAsO3 H2 + Al(AsO3)3

• Double all coefficients:

• 2 Al + 6 HAsO3 2 H2 + 2 Al(AsO3)3

• Balance hydrogen atoms

• 2 Al + 6 HAsO3 3 H2 + 2 Al(AsO3)3

Atoms / ions Reactants Products

Al 2 2

H 6 6

AsO3 6 6

Atoms / ions Reactants Products

Al 1 1

H 3 2

AsO3 3 3

Atoms / ions Reactants Products

Al 2 2

H 6 4

AsO3 6 6

Practice

Write balanced equations for:

• HgCl2 + AgNO3 Hg(NO3)2 + AgCl

• HgCl2 + 2 AgNO3 Hg(NO3)2 + 2 AgCl

• Al + Hg(CH3COO)2 Al(CH3COO)3 + Hg

• 2 Al + 3 Hg(CH3COO)2 2 Al(CH3COO)3 + 3 Hg

• Calcium phosphate and water are produced when calcium hydroxide reacts with phosphoric acid, H3PO4

• 3 Ca(OH)2 + 2 H3PO4 Ca3(PO4)2 + 6 H2O