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CHAPTER 2: Chemical Equations and Reaction Yields •Chemical Formulas •Percent Composition and Elemental Analysis •Balanced Equations and Stoichiometry •Combustion Reactions •Volume Relationships of Gases •Limiting Reagents •Percent Yield CHEM 1310 A/B Fall 2006 Chemical Formulas • Molecular formula: Gives exact # of atoms for each element in one molecule of the substance. Assumes molecule made of well-defined molecules. • Empirical formula: Gives smallest whole numbers consistent with the ratios of the #’s of each type of element. Used when substance is not made of distinct molecules. CHEM 1310 A/B Fall 2006 Percent Composition and Elemental Analysis • Compute the fraction of the total mass due to silver atoms in AgNO3 • A 100g sample of a compound is found to contain 85.63g of C and 14.37g of H atoms. What’s the empirical formula? CHEM 1310 A/B Fall 2006 Balanced Chemical Equations • “Sodium drop” __ Na + __ H2O → __ NaOH + __ H2 • Balanced chemical equation needs to reflect: (a) atoms not created/destroyed, (b) mass is conserved • Algebraic approach to balancing equations CHEM 1310 A/B Fall 2006 Stoichiometry • The balancing of chemical equations or the study of the relationships between chemical species in a balanced chemical equation • Lots of possible stoichiometry problems; frequently convert to moles, use relationships between moles of different species, then convert back to grams • E.g., How many g of H2 are produced if 10g of Na is dropped into a large beaker of H2O? • E.g., How many g of products are produced from the complete combustion of 10g of acetylene? CHEM 1310 A/B Fall 2006 Volume relationships of gases • Recall the law of combining volumes says the volumes of gaseous reactants and products stand in ratios of simple integers (assuming those volumes measured at same temperature & pressure) • 2 C2H2 + 5 O2 → 4 CO2 + 2H2O, for every 2L of C2H2, 4L of CO2 result, etc. • Why do volumes behave just like moles? CHEM 1310 A/B Fall 2006 Limiting Reagents Reaction cannot go to completion because one reagent is limiting… CHEM 1310 A/B Fall 2006 Identifying the Limiting Reactant • • Procedure: 1. Compute # of moles of each reactant 2. Divide each by the coefficient in the balanced chemical equation 3. The smallest value corresponds to the limiting reagent Example: 10g of C2H2 reacts with 20g of O2 to undergo complete combustion. Which reactant is limiting, and how many g of CO2 are produced? CHEM 1310 A/B Fall 2006 Percent Yield • Amount of product may be less than computed by stoichiometry: 1. reaction not run to completion 2. competing side reactions 3. product lost in separation/purification • If we expected 22g of CO2 in the combustion reaction and only got 20g, what’s the % yield? • Cumulative effect of low yields in multi-step reactions CHEM 1310 A/B Fall 2006