ES105 2006 Jan 24 Chemistry I. Chemical Equations A. C+O2 ÆCO2 1. Carbon reacts with oxygen to become carbon dioxide 2. Reactants and products 3. Each is shown as MOLECULES 4. Balanced, because same number of reactant atoms as product atoms 5. Sometimes physical state indicated in subscript parenthesis a. C(s) +O2 (g) ÆCO2 (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 Æ H2O not balanced Cannot simply go to H2 + O2 Æ H2O + O Because that is adding a product not originally present in equation So…let’s double the water H2 + O2 Æ 2 H2O still not balanced, too much hydrogen on product side, so lets double H2 on reactant side 2 H2 + O2 Æ 2 H2O four hydrogen, two oxygen, balanced!b. Iron reacting with oxygen to become rust Fe+O2 Æ Fe2O34 Fe+3 O2 Æ 2 Fe2O3 looks good! c. In-class activity--You do this one: N2 + H2 Æ NH3 d. Combustion of methane CH4 + O2 Æ CO2 + H2O not balanced, CH4 + 2 O2 Æ CO2 + 2 H2O How’s this? e. In-class activity--Now you do propane C3H8 + O2 Æ CO2 + H2O f. Here’s another example, where SO4 is a polyatomic group H2SO4 + NaCN Æ HCN + Na2SO4 not balanced, fix sodium H2SO4 + 2 NaCN Æ HCN + Na2SO4 now do C and N H2SO4 + 2 NaCN Æ 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 Æ 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 Æ Ca3(PO4)2 + H2O start with Ca? H3(PO4) + 3 Ca(OH)2 Æ Ca3(PO4)2 + H2O now do PO4 2 H3(PO4) + 3 Ca(OH)2 Æ Ca3(PO4)2 + H2O 12 H reactants 2 H3(PO4) + 3 Ca(OH)2 Æ 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 Æ 2 volume steam 3. 3 hydrogen H2 + 1 nitrogen N2 Æ 2 ammonia NH3 4. Can use to calculate volume changes in reactions B. Avogadro’s hypothesis accepted 6 years after his death C. Example C3H8 + 5 O2 Æ 3 CO2 + 4 H2O combustion of propane 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 O2D. 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, Æ 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, Æ 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? 140 g Si-28 x 28 g = 3920 g2 what kind of units? 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
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