Balancing Chemical Reactions Honors chemistry – Semester 1
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