Homer Helps with Bart’s Chemistry Homework Play This as a Slide Show to see the Balancing in Action!

 To Obey the LAW of Conservation of Matter!!! ◦ We must have the same #atoms of each type at the start & end of a chemical reaction ◦ # Reactant atoms = # Product atoms ◦ Otherwise we have created or destroyed Matter! ◦ Ex: Na + CaF2  NaF + CaNa=1Ca=1 F=2F=1  We broke the LAW  Matter was DESTROYED!!

 Subscripts ◦ These are the numbers below the symbol ◦ They tell how many atoms are bonded together in one molecule ◦ For example C 2 H 6 has 2C atoms bonded to 6H atoms ◦ The subscripts are determined by the valence electrons ◦ WE CAN’T CHANGE THE SUBSCRIPT!! ◦ We find the subscripts by using:  Bohr Model  Lewis Dots  Criss-cross /oxidation # HH CCH HH H

 Coefficients ◦ These are the numbers in front of the symbol ◦ They tell how many atoms or molecules we have ◦ It multiplies the number of atoms or molecules (including the subscript) ◦ Ex: 3Na = 3Na atoms 2 C 2 H 6 = 2x2 = 4C atoms =2x6 = 12H atoms ◦ IT IS THE ONLY NUMBER THAT WE CAN CHANGE!!

Polyatomic Ions ◦ Are many atoms bonded together to form an ion ◦ Examples  NO 3 = nitrate; ion charge = -1  SO 4 = sulphate; ion charge = -2  NH 4 = ammonium ion; ion charge = +1  CO 3 = carbonate; ion charge = -2  PO 4 = sulphate; ion charge = -3 ◦ Count how many polyatomic ions you have when balancing; not the individual atoms Ex:2HNO 3 H=2 and NO 3 =2Not H=2 & N=2 and O=6

Na + MgF2  NaF + MgNa=1Mg=1 F=2F=1  2 22 D’Oh! H’mm Yum

Cl 2 + NaBr  NaCl + Br 2Na=1 Br=1Br=2 Cl=2Cl =1  2 22 D’Oh! H’mm  2 Now I get it!

1. Na + MgF 2  NaF + Mg 2. Mg + HCl  MgCl 2 + H 2 3. Cl 2 + KI  KCl + I 2 4. NaCl  Na + Cl 2 5. Na + O 2  Na 2 O 6. Na + HCl  H 2 + NaCl 7. K + Cl 2  KCl 8. N 2 + H 2  NH 3 9. CH 4 + O 2  CO 2 +H 2 O 10. Al + Fe 2 O 3  Al 2 O 3 +Fe

◦ Called Reaction Kinetics ◦ Reactions happen when the reactant atoms collide with enough energy to form new product molecules  Factors that influence the rates of chemical reactions:  Concentration  More particles present in a given volume increases the probability of collisions  Temperature  Increases the vibrational movement of the atoms which increases the probability of collisions  Also increases the energy of the collisions  Surface Area  Increases the number of atoms exposed to collisions  Ex: grinding up a reactant into a powder  Catalysts  Increase reaction rates, but are not consumed in the reaction  Collect the reactant atoms and bring them close together so they can react

 Reactions either absorb (endothermic) or release heat (exothermic)  This has to do with the bond energy ◦ It takes energy to break bonds ◦ Energy is released when bonds form because an atom has lower energy when it gains an electron

 Exo means “out” & thermic means heat (“Heat Out”)  The products have more energy than the reactants  When the products form, they release more energy than it took to break the reactant bonds  HEAT IS RELEASED

 Endo means “in” & thermic means heat (“Heat In”)  The products have less energy than the reactants  When the products form, they release less energy than it took to break the reactant bonds  HEAT IS ABSORBED