Balancing Chemical Equations, Reaction Kinetics, & Heats of Reactions

1. Balancing Chemical Equations, Reaction Kinetics, & Heats of Reactions Homer Helps with Bart’s Chemistry Homework Play This as a Slide Show to see the Balancing in Action!

2. Why Do we Need to Balance Equations? • 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 + Ca Na=1 Na=1 Ca=1 Ca=1 F=2 F=1 • We broke the LAW • Matter was DESTROYED!!

3. Balancing Tips • Subscripts • These are the numbers below the symbol • They tell how many atoms are bonded together in one molecule • For example C2H6 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 # H H H C C H H H

4. Balancing tips • 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 C2H6 = 2x2 = 4C atoms =2x6 = 12H atoms • IT IS THE ONLY NUMBER THAT WE CAN CHANGE!!

5. Balancing tips Polyatomic Ions • Are many atoms bonded together to form an ion • Examples • NO3 = nitrate; ion charge = -1 • SO4 = sulphate; ion charge = -2 • NH4 = ammonium ion; ion charge = +1 • CO3 = carbonate; ion charge = -2 • PO4 = sulphate; ion charge = -3 • Count how many polyatomic ions you have when balancing; not the individual atoms Ex:2HNO3 H=2 and NO3 =2 Not H=2 & N=2 and O=6

6. Reaction #1 Na + MgF2  NaF+ Mg Na=1 Na=1 Mg=1 Mg=1 F=2 F=1 2 2  2  2  2 D’Oh! H’mm Yum

7. Reaction #2 Cl2+ NaBr NaCl+ Br2 Na=1 Na=1 Br=1 Br=2 Cl=2 Cl=1 2 2  2  2  2  2 Now I get it! D’Oh! H’mm

8. Practice Problems 1. Na + MgF2NaF + Mg 2. Mg + HCl MgCl2 + H2 3. Cl2 + KI KCl + I2 4. NaCl Na + Cl2 5. Na + O2Na2O 6. Na + HClH2 + NaCl 7. K + Cl2KCl 8. N2 + H2NH3 9. CH4 + O2CO2 +H2O 10. Al + Fe2O3Al2O3 +Fe

9. Rates of Chemical Reactions • 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

10. Exothermic & Endothermic Reactions • 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

11. Exothermic reactions release energy • 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

12. Endothermic reactions absorb energy • 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