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Join Homer in helping Bart with chemistry homework by learning how to balance chemical equations and understand reaction kinetics. Explore the importance of obeying the Law of Conservation of Matter, balancing tips, polyatomic ions, and exothermic/endothermic reactions. Practice balancing equations with helpful examples.
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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!
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!!
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
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!!
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
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
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
Practice Problems 1. Na + MgF2NaF + Mg 2. Mg + HCl MgCl2 + H2 3. Cl2 + KI KCl + I2 4. NaCl Na + Cl2 5. Na + O2Na2O 6. Na + HClH2 + NaCl 7. K + Cl2KCl 8. N2 + H2NH3 9. CH4 + O2CO2 +H2O 10. Al + Fe2O3Al2O3 +Fe
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
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
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
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