ES105 I 2006 Jan 24 Chemistry Chemical Equations A C O2 CO2 1 2 3 4 Carbon reacts with oxygen to become carbon dioxide Reactants and products Each is shown as MOLECULES Balanced because same number of reactant atoms as product atoms 5 Sometimes physical state indicated in subscript parenthesis a C s O2 g CO2 g b l liquid aq dissolved in water B Learn to balance equations 1 Reactants and products shown as MOLECULES a Cannot change the proportions of atoms within the molecules b Cannot add or delete reactants or products from equation must use only those indicated in the equation 2 numbers within the equation a Subscripts apply only to element or group in parentheses it is touching b Coefficient the number out front full size applies to entire molecule it is in front of and this number CAN be changed in balancing the reaction 1 can change the number of each molecule coefficients 2 cannot change subscripts of elements the molecule 3 The same number of atoms of reactants and products will exist 4 Examples a hydrogen gas and oxygen gas react to become water H2 O2 H2O not balanced Cannot simply go to H2 O2 H2O O Because that is adding a product not originally present in equation So let s double the water H2 O2 2 H2O still not balanced too much hydrogen on product side so lets double H2 on reactant side 2 H2 O2 2 H2O four hydrogen two oxygen balanced b Iron reacting with oxygen to become rust Fe O2 Fe2O3 4 Fe 3 O2 2 Fe2O3 looks good c In class activity You do this one N2 H2 NH3 d Combustion of methane CH4 O2 CO2 H2O not balanced CH4 2 O2 CO2 2 H2O How s this e In class activity Now you do propane C3H8 O2 CO2 H2O f Here s another example where SO4 is a polyatomic group H2SO4 NaCN HCN Na2SO4 not balanced fix sodium H2SO4 2 NaCN HCN Na2SO4 now do C and N H2SO4 2 NaCN 2 HCN Na2SO4 that fixes H SO4 is fine g PO4 is a polyatomic group in this example as is OH hydroxyl H3PO4 Ca OH 2 notice hydroxyl is doubled with subscript H3PO4 Ca OH 2 Ca3 PO4 2 H2O See the PO4 in parentheses in products needs to be shown to accomplish the doubling with the subscript don t need in reactant side We will add the parentheses in the reactants also for clarity H3 PO4 Ca OH 2 Ca3 PO4 2 H2O start with Ca H3 PO4 3 Ca OH 2 Ca3 PO4 2 H2O now do PO4 2 H3 PO4 3 Ca OH 2 Ca3 PO4 2 H2O 12 H reactants 2 H3 PO4 3 Ca OH 2 Ca3 PO4 2 6 H2O check it II Volume relationships in chemical equations A Equal volumes of gases at the same temperature and pressure have the same number of molecules 1 Determined by reacting gases and found that combine in small whole number ratios 2 2 volume hydrogen 1 volume oxygen 2 volume steam 3 3 hydrogen H2 1 nitrogen N2 2 ammonia NH3 4 Can use to calculate volume changes in reactions B Avogadro s hypothesis accepted 6 years after his death C Example combustion of propane C3H8 5 O2 3 CO2 4 H2O Start with 0 556 L of propane How much oxygen will be needed to burn the propane Oxygen Propane in ratio of 5 1 0 556 L C3H8 X 5 L O2 2 87 L O2 D You calculate how much CO2 is produced when 2 L of C3H8 is burned on your worksheet E Avogadro s number 6 02 x 1023 is one mole of molecules III Molecular mass Formula mass Molar mass A Calculate the molecular weight from molecular formula 1 Oxygen example atomic weight 16 u O2 molecular weight is 32 u 32 grams mole of molecular oxygen 2 carbon dioxide example CO2 molecular weight C 12 O2 32 44 g mole 3 You calculate the molecular weight of propane Use C 12 H 1 B Ionic compounds not correctly referred to with molecular mass but called formula mass Either can be referred to as molar mass the atomic mass of one mole of the substance C Can determine number of moles of a substance from molar mass and gram quantity and vice versa D Examples 1 0 250 moles of Na is how many grams First determine molar mass of sodium 23 g mole Multiply 0 250 moles x 23 g 5 75 g Na Mole 2 Number of moles in 176 g of CO2 First find molar mass of CO2 C 12 O2 32 44 g mole Now divide quantity by molar mass 176 g 4 moles 44 g mole Keep units with numbers so you can use dimensional analysis to be sure you multiply or divide Units just like numbers cancel when paired above and below division bar fraction bar 3 Your task How many moles of Si 28 are there in 140 grams of Si 28 3920 g2 what kind of units 140 g Si 28 x 28 g Mole mole Stupid units This alerts you that you made an error Try again 140 g 5 moles of Si 28 28 g mole IV STP is 0o C and 1 atm Pressure 760 mm Hg 29 92 in Hg A One mole of gas at standard temperature and pressure STP occupies 22 4 L volume about 0 8 cu Ft B One mole of gas has a mass of its molar mass in grams
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