Section 2.7—Balancing Equations We need to finish writing those equations we started!

Law of Conservation of Matter/Mass Law of Conservation of Matter – Matter cannot be created nor destroyed during chemical or physical changes Also called the Law of Conservation of Mass (since all matter has mass)

How Does the Law Lead to Balancing? Law of Conservation of Matter Therefore… So we must… Matter cannot be created nor destroyed during a chemical or physical change The matter on the reactants side and the matter one the products side must be the same Ensure the numbers of each type of atom are the same on both sides of the equation…by balancing!

How do we Balance Equations? Number of compounds in the reaction Coefficients 2 H2 + O2  2 H2O Subscripts # of atoms in a compound Subscripts balance charges within a compound. Coefficients balance atoms in an equation

What do Coefficients Really Mean? CH4 + 2 O2  CO2 + 2 H2O Total: 1 C 4 H 4 O Total: 1 C 4 H 4 O The equation is balanced.

How to Balance Chemical Equations

How to Balance By Inspection: 1 Make a table of elements _____ C H4 + _____ O2  _____ H2 O + _____ C O2 Reactants Products C H O

How to Balance By Inspection: 2 Count the number of each element or ion on the reactants and products side. Don’t forget to add all the atoms of the same element together—even if it appears in more than one compound! _____ C H4 + _____ O2  _____ H2 O + _____ C O2 Reactants Products C 1 1 H 4 2 O 2 3

How to Balance By Inspection: 3 Add coefficients to balance the numbers Each time you add a coefficient, update your table with the new quantities of each atom. _____ C H4 + _____ 2 O2  _____ 2 H2 O + _____ C O2 Reactants Products C 1 1 H 4 2 4 O 2 4 3 4

How to Balance By Inspection: 4 Place a “1” in any empty coefficient location Filling each coefficient location lets you and the grader know that you finished the problem rather than you left some blank because you weren’t done! _____ 1 C H4 + _____ 2 O2  _____ 2 H2 O + _____ 1 C O2 Reactants Products C 1 1 H 4 2 4 O 2 4 3 4

Choosing the Order of Balancing How do you know what order to balance in? Start Elements that appear only 1 time per side Save for later Elements that are uncombined Pb + PbO2 + H+  Pb2+ + H2O Start Elements in most complicated molecules Save for later Elements that appear more than 1 time per side To balance this equation, use the order: O, H, Pb

How is Balancing Affected by Order? What happens if we balance in the order determined in the last slide? O, H, Pb _____ 1 Pb + _____ 1 Pb O2 + _____ 4 H+  _____ 2 H2 O + _____ 2 Pb2+ Reactants Products O 2 1 2 H 1 4 2 4 Pb 2 1 2

What about a different order? How is it different if we balance in a different order? H, O, Pb 4 _____ 1 Pb + _____ 1 Pb O2 + _____ 2 H+  _____ 2 H2 O + _____ 2 Pb2+ Reactants Products  H 1 2 4 2 4 O 2 1 2 Pb 2 1 2 You’ll still get to the correct answer, but it will take longer and be more complicated!

Polyatomic Ions Polyatomic ion – Group of atoms that together has a net charge e.g. Nitrate NO31- Carbonate CO32- A polyatomic ion is a group of atoms that together have a charge.

Balancing with Polyatomic Ions: 1 Make a table of elements You may leave polyatomic ions together—IF they appear intact on both sides of the reaction. OH is a polyatomic ion that is sometimes “hidden” in H2O. Re-write H2O as HOH to “see” the OH polyatomic ion. H OH _____ H3 PO4 + _____ Ca (OH)2  _____ Ca3 (PO4)2 + _____ H2 O Reactants Products H PO4 Ca OH

Balancing with Polyatomic Ions: Count the number of each element or ion on the reactants and products side. 2 H OH _____ H3 PO4 + _____ Ca (OH)2  _____ Ca3 (PO4)2 + _____ H2 O Reactants Products H 3 1 PO4 1 2 Ca 1 3 OH 2 1

Balancing with Polyatomic Ions: 3 Add coefficients to balance the numbers H OH _____ 2 H3 PO4 + _____ 3 Ca (OH)2  _____ Ca3 (PO4)2 + _____ 6 H2 O Reactants Products H 3 6 1 6 PO4 1 2 2 Ca 1 3 3 OH 2 6 1 6

Balancing with Polyatomic Ions: 5 Place a “1” in any empty coefficient location H OH _____ 2 H3 PO4 + _____ 3 Ca (OH)2  _____ 1 Ca3 (PO4)2 + _____ 6 H2 O Reactants Products H 3 6 1 6 PO4 1 2 2 Ca 1 3 3 OH 2 6 1 6

Balance the following equation Let’s Practice #1 Example: Balance the following equation __ HCl + __ Ca(OH)2  __ CaCl2 + __ H2O

Balance the following equation Let’s Practice #1 Example: Balance the following equation Did you see the “OH” polyatomic ion & change H2O to HOH? 2 1 1 2 __ HCl + __ Ca(OH)2  __ CaCl2 + __ H2O HOH

Balance the following equation Let’s Practice #2 Example: Balance the following equation __ H2 + __ O2  __ H2O

Balance the following equation Let’s Practice #2 Example: Balance the following equation 2 1 2 __ H2 + __ O2  __ H2O

Balance the following equation Let’s Practice #3 Example: Balance the following equation __ Fe + __ O2  ___ Fe2O3

Balance the following equation Let’s Practice #3 Example: Balance the following equation 4 3 2 __ Fe + __ O2  ___ Fe2O3