Section 2-7: Balancing Chemical Equations

Section 2.7—Balancing Equations :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/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? :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? :How do we Balance Equations? 2 H2 + O2  2 H2O Subscripts Coefficients # of atoms in a compound Number of compounds in the reaction Subscripts balance charges within a compound. Coefficients balance atoms in an equation


What do Coefficients Really Mean? :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 Chemical Equations


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


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


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


How to Balance By Inspection: :4 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! Place a “1” in any empty coefficient location _____ _____ _____ 2 How to Balance By Inspection: Reactants Products H O 4 2 3 2 C 1 1 2 4 4 4 1 1 C H4 + O2  _____ H2 O C O2 +


Another Example :_____ Another Example Reactants Products H Pb 1 2 1 2 O 2 1 4 2 _____ _____ _____ 1 2 1 2 Pb Pb + O2  H2 O Pb2+ + H+ _____ 4 + 2 4


Polyatomic Ions :Polyatomic Ions Polyatomic ion – Group of atoms that together has a net charge e.g. Nitrate NO31- Carbonate CO32-


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


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


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


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


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


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


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


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


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


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