Chemical Reactions Chapter 17 Chemical Equations C O2 CO2 C s O2 g CO2 g Reactants on left products on right Each are balanced because same number of atoms of reactants as products Some equations show the phase of the substances also solid liquid gas Balancing Chemical Equations Molecules of reactants and products shown Cannot change the molecule Can change how many of them Cannot add or delete reactants or products Balanced equal number of same atoms on each side Water balancing Balancing Tips Never change the molecular formula of reactants or products Count atoms in reactant and products Always add whole molecules not parts Start by balancing atoms in compounds Save element reactants or products for last to make up any imbalance Rusting of Iron Fe O2 Fe2O3 not balanced start with oxygen Fe 3 O2 2 Fe2O3 next do iron 4 Fe 3 O2 2 Fe2O3 Problem 1 in class Balance the following equation on the worksheet provided Put your name on the back of the sheet please N2 H2 NH3 Combustion of propane C3H8 O2 CO2 H2O fix hydrogen first it s in two compounds C3H8 O2 CO2 4 H2O need more carbon product C3H8 O2 3 CO2 4 H2O Do oxygen last because it is single C3H8 5 O2 3 CO2 4 H2O Problem 2 in class Balance the following chemical equation the combustion of methane CH4 O2 CO2 H2O Methane oxygen carbon dioxide water Volume Relationships Equal volumes of gases at the same temperature and pressure have the same number of molecules Gases react in small whole number quantities Avogadro s hypothesis chemicals react in consistent small whole number ratios Avogadro s hypothesis Combustion of propane C3H8 5 O2 3 CO2 4 H2O What volume of oxygen is needed to burn 0 5 L of propane Ratio of Oxygen molecules to propane molecules is 5 1 2 5 L Oxygen 0 5 L x 5 L O2 1 L Propane Problem 3 in class Combustion of propane C3H8 5 O2 3 CO2 4 H2O Calculate how much CO2 is produced when 2 L of propane is burned Atomic vs Molecular Weight Atomic weight on periodic table is average of natural abundance of isotopes Atomic mass is the number of nucleons in a particular atom specified by isotope Molecular mass is the mass of one mole of molecules One atomic mass number of grams 6 0221367x1023 molecules Calculate Molecular Mass O atomic weight 15 9996 round to 16 for this class O 16 atomic mass 16 u Molecular oxygen O2 atomic mass 32 u Molecular O2 molecular mass 32 g mole CO2 molecular mass C 12 g mole O2 32 g mole CO2 12 32 44 g mole Problem 4 in class Calculate the molar mass of propane C3H8 Round atomic weight of C to 12 Round atomic weight of H to 1 g mole g mole Grams calculated from Moles Can find the molar mass of substance g Na 23 mole 1 4 mole of Na Multiply molar mass times moles g 23 mole x 0 250 moles 5 75 g Moles calculated from Grams 176 g of CO2 Number of moles g Molar mass of CO2 44 mole If you multiply g mole 176 g x 44 results in units of you get a unit mess g2 mole UNITS alert you that you made an error KEEP UNITS WITH NUMBERS Moles calculated from Grams 176 g of CO2 Number of moles g Molar mass of CO2 44 mole g Divide 176 g by 44 mole Same as multiply by reciprocal 176 g x 1 mole 44 g Now the grams cancel 4 moles CO2 Problem 5 in class 132 g of propane is how many moles Mass to moles relationship To convert between grams and moles Make sure equation is balanced So you know the molar ratios of them Find molar mass of them Set up proportions for moles and grams Proportions Mathematical device to compare ratios Cross multiply to solve Correctly organized Be sure you keep same same in columns and rows Proportions to find grams and moles Correctly organized Example moles of a g of a moles of b g of b or moles of a moles of b g of a g of b cross multiply to solve Proportions Correctly organized It doesn t matter how you write the first ratio as long as you label the numbers Try to put unknown on the top easier to solve The second ratio needs to match the first Problem NO O2 NO2 64 grams O2 How many grams NO2 produced First Balance Equation 2 NO O2 2 NO2 Problem 2 NO O2 2 NO2 64 grams O2 How many grams NO2 produced Balance Equation Determine molar ratios of them 1 2 Find molar mass of each component NO2 46 g O2 32 g NO 30 g Problem 2 NO O2 2 NO2 64 grams O2 How many grams NO2 produced Molar mass of each O2 32 g NO2 46 g How many moles is 64 grams O2 One mole Problem 2 NO O2 2 NO2 64 grams O2 How many grams NO2 produced Molar mass of each O2 32 g NO2 46 g Molar ratios O2 NO2 is 1 2 Two mole of O2 So four moles of NO2 is produced How many grams is that 4 mol x 46 g mol 184 grams Problem H 2 S O2 SO2 H 2O 32 grams SO2 How many grams O2 used Problem 32 grams SO2 How many grams O2 used H 2 S O2 SO2 H 2O Balance first 2 H 2 S 3O2 2SO2 2 H 2O Then determine molar ratios 2 SO2 to 3 O2 Problem 32 grams SO2 How many grams O2 used 2 H 2 S 3O2 2SO2 2 H 2O Find molar masses SO2 32 32 O2 2x1 16 H2O 2x1 16 H2S 2x1 32 64 g mol SO2 32 g mol O2 18 g mol H2O 34 g mol H2S Problem 32 grams SO2 How many grams O2 used 2 H 2 S 3O2 2SO2 2 H 2O 32 g SO2 needs how many grams O2 How many moles is 32 g SO2 1 mole 32 g SO 2 0 5 moles SO 2 64 g Problem 32 grams SO2 How many grams O2 used 2 H 2 S 3O2 2SO2 2 H 2O How many moles O2 is needed 0 5 moles SO2 in 2 3 ratio with O2 0 75 moles O2 1 mole 32 g SO 2 0 5 moles SO 2 64 g 32 grams SO2 How many grams O2 used Set up proportion with the unknown on top the O2 g O2 Problem 6 C O2 CO2 Is it balanced Molar ratio 1 1 1 4 grams oxygen Grams carbon consumed Grams carbon dioxide produced Reaction Speed Collision of molecules required for it to occur Increase concentration Increase temperature Catalyst can facilitate reaction Chlorine catalyst Energy of reactions Release energy EXOTHERMIC Methane combustion Consume energy ENDOTHERMIC Formation of water
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