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This 4 page Class Notes was uploaded by Andrew Notetaker on Wednesday September 14, 2016. The Class Notes belongs to CHM 113 at Arizona State University taught by Cabirac in Fall 2016. Since its upload, it has received 8 views. For similar materials see General chemistry 1 in Science at Arizona State University.
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Date Created: 09/14/16
Chapter 3: 9-7, 9-12 & 9-14 Wednesday, September 7, 2016 3:10 PM Chemical Reactions and Reaction Stoichiometry 3.1 Chemical Equations Chemical reactions are represented by writing chemical equations Conservation of mass= reactant and product sides must have same number of atoms Subscripts tell the number of atoms in a molecule Coefficients tell the number of molecules General Guidelines for balancing equations 1. Write correct formulas for reactants and products 2. Can only adjust coefficients, not subscripts 3. Start with more complex molecules 4. Balance polyatomic ions as a single unit 5. Check to verify coefficients Notes from reading: Changing subscript changes identity and properties Changing coefficient changes amount CH 4 O -2> CO +2H O 2Unbalanced) CH 4 O -2> CO +22H O 2balanced) A chemical equation needs to be completely accurate, it must include the state of matter for all reactants and products. (s)=solid (g)=gas (l)=liquid (aq)= aqueous (dissolved in water) Simple Patterns of Chemical Reactivity 1. Combination reactions 2. Decomposition reactions 3. Combustion reactions Combination reactions CHM 113 Lecture Page 1 Combination reactions A+B=C 2Mg (s) + O 2g) -> 2MgO (s) Decomposition reaction one or more substance breaks down into two or more substances C -> A + B 2NaN (3) -> 2Na (s) + 3N 2g) Combustion reactions most often involve hydrocarbons or carbohydrates reacting with oxygen. Complete combustion leads to production of CO an2 H O. 2 Formula Weights The quantitative significance of chemical formulas A formula weight is the sum of atomic weights for all atoms in a chemical formula. H 2 fw = 18 amu or g/mol A molecular weight is the sum of atomic weights of the atoms in a molecule C2H 5H mw = 46.07 amu or g/mol Percent Composition One can find the mass percentage of each element in a compound by using the equation: Avogadro's number and the mole 1 gram = 6.02 x 10 amu The mass of one C-12 atom is exactly 12 amu 1 mole = 6.022 x 10 C-12 atoms 1 mole = 6.022 x 10 of anything CHM 113 Lecture Page 2 1 mole = 6.022 x 10 of anything Molar mass is the mass of 1 mol of a substance g/mol Molar mass can be used as a conversion factor Mass in grams of 0.2500 mole of O 2 Grams -> Molar mass <- Moles -> Avogadros # <- Molecules Empirical Formulas from Analysis Assume 100g of compound 30.45% Nitrogen and 69.56% O -30.45g N -60.56g O Convert to moles Divide each by the lowest number of mol (2.173 mol) 2.173 mol N/2.173 = 1 mol 4.348 mol O/ 2.173 = 2 mol Molecular formula can be determined only if molecular weight is known. If needed, multiple to get whole numbers if empirical formula gives decimal numbers How to interpret a balanced chemical equation N 2 3H (2) -> 3NH 3 1:3:2 1 molecule: 3 molecule: 2 molecules Mole ratio for moles of N 2hat will react with 2.4 mol of H 2 2.4 mol H 2 (1 mol N )2= 0.8 mol N 2 (2 mol H ) 2 General scheme for calculating grams of a product if grams of a substance is known Limiting Reactants 2H 2 O -2 2H O 2 CHM 113 Lecture Page 3 2H 2 O -2 2H O 2 What is the limiting reactant? Hydrogen gas. How many moles of H O wi2l form when the following are mixed? 4 mol H 2nd 2 mol O 4 m2l H O 2 2 mol H 2nd 2 mol O 2 m2l H O 2 10 mol H a2d 4 mol O 8 m2l H O 2 How to determine limiting reactants: Method 1: Compare moles of each reactant required to consume all of other reactant. Use limiting reactant to calculate the amount of the product Method 2: 1. Calculate moles or grams of desired product from both (or all) starting reactants. Whichever produces less product is the limiting reagent. Easily calculate limiting reactant by calculating moles of each reactant and divide by the balanced equation coefficient. The smaller number is the limiting reactant. Limiting reactant-> moles of limiting reactant-> moles of product using mole ratio -> theoretical yield of product Theoretical Yields Theoretical yield is what's calculated from the limiting reactant. Actual yield is what's produced when the reaction is run. Percent yield= Actual/Theoretical x 100% CHM 113 Lecture Page 4
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