1. Balancing Equations

2. Chemical rxns occur when bonds (between electrons of atoms) are formed or broken Chemical rxns occur when bonds (between electrons of atoms) are formed or broken Chemical rxns involve: Chemical rxns involve: changes in the chemical composition of matter changes in the chemical composition of matter making new materials with new properties making new materials with new properties energy changes: energy changes: Bond breaking absorbs Energy (endothermic) Bond breaking absorbs Energy (endothermic) Bond making releases Energy (exothermic) Bond making releases Energy (exothermic)Note: The overall change in energy for certain rxns can be found on Ref. Table I Chemical Reactions

3. Depict the kind of reactants and products and their relative amounts in a reaction. 4 Al (s) + 3 O 2 (g) → 2 Al 2 O 3 (s) The numbers in the front are called stoichiometric coefficients Chemical Equations

4. Symbols Used in Equations Solid (s), Liquid (l), Gas (g) Solid (s), Liquid (l), Gas (g) Aqueous solution (aq) (dissolved in water) Aqueous solution (aq) (dissolved in water) Catalyst H 2 SO 4 or Pt Catalyst H 2 SO 4 or Pt Escaping gas (  ) Escaping gas (  ) Change of heat energy Change of heat energy (  or + 3kJ or – 3kJ) (  or + 3kJ or – 3kJ) *there is no subtraction…a negative sign means released/exothermic *there is no subtraction…a negative sign means released/exothermic

5. Express a chemical equation as follows: Express a chemical equation as follows: Reactants  Products The arrow is equivalent to an “=“ math. When describing the equation use the word “yields” or “produces” instead of equals The arrow is equivalent to an “=“ math. When describing the equation use the word “yields” or “produces” instead of equals Ex:C + O 2  CO 2 This reads “carbon plus oxygen react to yield carbon dioxide”

6. Because of the principle of the conservation of matter an equation must be balanced. Because of the principle of the conservation of matter an equation must be balanced. matter can’t be created or destroyed matter can’t be created or destroyed Must have the same number of each type of atom on both sides. Must have the same number of each type of atom on both sides. Must have same # grams of matter on both sides!! Must have same # grams of matter on both sides!! Lavoisier, 1788

7. Coefficients not Subscripts When balancing add coefficients in front of compounds to balance the atoms in the reaction When balancing add coefficients in front of compounds to balance the atoms in the reaction DO NOT change the subscripts of the formulas. DO NOT change the subscripts of the formulas. Changing subscripts changes the compound. Changing subscripts changes the compound.

8. Subscripts: tell you how many atoms of a particular element are in a compound. Subscripts: tell you how many atoms of a particular element are in a compound. Coefficient: tells you about the quantity, or number, of molecules of the compound. Coefficient: tells you about the quantity, or number, of molecules of the compound.

9. Balancing a chemical equation: DO NOT CHANGE THE FORMULAS! Find number of atoms for an element on left side. Find number of atoms for an element on left side. Compare those against number of the atoms of the same element on right side. Compare those against number of the atoms of the same element on right side. Place coefficients in front of formulas so that left side has the same number of atoms as the right side for EACH element Place coefficients in front of formulas so that left side has the same number of atoms as the right side for EACH element Check your answer to see if: Check your answer to see if: The numbers of atoms on both sides of the equation balanced. The numbers of atoms on both sides of the equation balanced. The coefficients are in the lowest possible whole number ratios. The coefficients are in the lowest possible whole number ratios.

10. Helpful hints for balancing equations: Take one element at a time, working left to right. Take one element at a time, working left to right. Save H for next to last, and O until last. Save H for next to last, and O until last. IF everything balances except for O, and there is no way to balance O with a whole number, double all the coefficients and try again. IF everything balances except for O, and there is no way to balance O with a whole number, double all the coefficients and try again. Useful in combustion reactions! Useful in combustion reactions! (Shortcut) Polyatomic ions that appear on both sides of the equation should be balanced as independent units (Shortcut) Polyatomic ions that appear on both sides of the equation should be balanced as independent units Just circle them and think of them as one thing Just circle them and think of them as one thing

11. Balancing Equations ___ H 2 (g) + ___ O 2 (g) → ___ H 2 O(l) What happened to the other Oxygen atom????? This equation is not balanced!

12. Balance Equation Na + Cl 2 NaCl Na + Cl 2 NaCl Na = 1Na = 1 Na = 1Na = 1 Cl = 2Cl = 1 Cl = 2Cl = 1 The number of sodium atoms balance but the chlorine does not. Use coefficients in order to balance this equation.

13. Inserting Coefficients Na + Cl 2 2 NaCl Na = 1Na = 2 Na = 1Na = 2 Cl = 2Cl = 2 Cl = 2Cl = 2 Now chlorine balances but the sodium does not! So we go back and balance the sodium.

14. Finally balanced! 2Na + Cl 2 2 NaCl 2Na + Cl 2 2 NaCl Na = 2Na = 2 Na = 2Na = 2 Cl = 2Cl = 2 Cl = 2Cl = 2 Since the number of each element on the reactant side and the product side of the equation are equal, the equation is balanced.

15. Balance Practice (Page 2 in Chemical Equations Packet) ___Al 2 O 3 + ___H 2 → ___H 2 O + ___Al ___MgI 2 + ___K → ___KI + ___Mg ___AlF 3 → ___Al + ___F 2

16. Balance Practice (Page 2 in Chemical Equations Packet) ___Na 2 CO 3 + ___Mg → ___MgCO 3 + ___Na ___ZnO → ___Zn + ___O 2 ___Fe + ___O 2 → ___Fe 2 O 3

17. Balancing Practice For more help go to: http://richardbowles.tripod.com/chemistry/ balance.htm#part0 For more help go to: http://richardbowles.tripod.com/chemistry/ balance.htm#part0 http://richardbowles.tripod.com/chemistry/ balance.htm#part0 http://richardbowles.tripod.com/chemistry/ balance.htm#part0 For some fun balancing equations go to: For some fun balancing equations go to: http://www.mpcfaculty.net/mark_bishop/b alancing_equations_tutorial.htm http://www.mpcfaculty.net/mark_bishop/b alancing_equations_tutorial.htm http://www.mpcfaculty.net/mark_bishop/b alancing_equations_tutorial.htm http://www.mpcfaculty.net/mark_bishop/b alancing_equations_tutorial.htm