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

2. 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)

3. 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 on 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!

4. How do we Balance Equations? 2 H 2 + O 2  2 H 2 O 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

5. What do Coefficients Really Mean? CH 4 + 2 O 2  CO 2 + 2 H 2 O Total: 1 C 4 H 4 O Total: 1 C 4 H 4 O The equation is balanced.

6. How to Balance Chemical Equations

7. Make a table of elements _____ How to Balance By Inspection: 1 Reactants Products H O C CH4H4 + O2O2  H2H2 OCO2O2 +_____

8. 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 23 2 C11 CH4H4 + O2O2  H2H2 OCO2O2 +_____ Don’t forget to add all the atoms of the same element together— even if it appears in more than one compound!

9. 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 23 2 C11 2 4 44 CH4H4 + O2O2  _____H2H2 OCO2O2 +

10. 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 23 2 C11 2 4 44 11CH4H4 + O2O2  _____H2H2 OCO2O2 +

11. Choosing the Order of Balancing Save for later Elements that are uncombined Save for later Elements that appear more than 1 time per side Start Elements that appear only 1 time per side Start Elements in most complicated molecules How do you know what order to balance in? Pb + PbO 2 + H +  Pb 2+ + H 2 O To balance this equation, use the order: O, H, Pb

12. _____ How is Balancing Affected by Order? What happens if we balance in the order determined in the last slide? Reactants Products H Pb 1 21 2 O21 4 2 _____ 1212Pb +O2O2  H2H2 OPb 2+ + H+H+ _____4 + 2 4 O, H, Pb

13. 2 _____ What about a different order? How is it different if we balance in a different order? Reactants Products O Pb 2 21 1 H1 2 2 _____ 1212Pb +O2O2  H2H2 OPb 2+ + H+H+ _____2 + 24 4 4  You’ll still get to the correct answer, but it will take longer and be more complicated! H, O, Pb

14. Polyatomic Ions Polyatomic ion – Group of atoms that together has a net charge e.g. Nitrate NO 3 1- Carbonate CO 3 2-

15. OH is a polyatomic ion that is sometimes “hidden” in H 2 O. Re-write H 2 O 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 PO 4 Ca H (OH) 2 + H3H3  Ca 3 (PO 4 ) 2 H2H2 O + PO 4 OH H 1

16. 2 Count the number of each element or ion on the reactants and products side. _____ Balancing with Polyatomic Ions: Reactants Products PO 4 Ca 1 13 2 H31 (OH) 2 + H3H3  Ca 3 (PO 4 ) 2 H2H2 O + PO 4 OH21 H

17. 3 Add coefficients to balance the numbers _____ 2 Balancing with Polyatomic Ions: Reactants Products PO 4 Ca 1 13 2 H31 6 3 36 (OH) 2 + H3H3  Ca 3 (PO 4 ) 2 H2H2 O + PO 4 OH21 H 6 2 6 6

18. Let’s Practice #1 Example: Balance the following equation __ HCl + __ Ca(OH) 2  __ CaCl 2 + __ H 2 O

19. Let’s Practice #1 Example: Balance the following equation __ HCl + __ Ca(OH) 2  __ CaCl 2 + __ H 2 O 2112 HOH Did you see the “OH” polyatomic ion & change H 2 O to HOH?

20. Let’s Practice #2 Example: Balance the following equation __ H 2 + __ O 2  __ H 2 O

21. Let’s Practice #2 Example: Balance the following equation __ H 2 + __ O 2  __ H 2 O 212

22. Let’s Practice #3 Example: Balance the following equation __ Fe + __ O 2  ___ Fe 2 O 3

23. Let’s Practice #3 Example: Balance the following equation __ Fe + __ O 2  ___ Fe 2 O 3 432