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Chapter 4 Quantities of Reactants and Products Antoine Lavoisier 1743-1794. “Father of modern chemistry.” Recognized true elements. Used quantitative measurements in chemical reactions. 1 Chemical Equations • Lavoisier: mass is conserved in a chemical reaction. • Chemical equations: descriptions of chemical reactions. • Two parts to an equation: reactants and products: 2H2 + O2 2H2O Reactants Product 2 H2 O2 H2O 3 • Stoichiometric coefficients: numbers in front of the chemical formulas give numbers of molecules or atoms reacting (and numbers being produced). 4 Law of Conservation of Mass: All reactions must be balanced CH4 + O2 CO2 + H2O is not balanced. (Why?) Count atoms: Reactants: 1C 4H 2O Products: 1C 2H 3O 5 Balance reactions only by changing coefficients, not by altering chemical formula 6 Combustion is the burning of a substance in oxygen: C3H8(g) + 5O2(g) 3CO2 (g) + 4H2O(l) 7 Which is correct? (Blue=A; Red=B) a) A2 + B A2B b) A2 + 4B 2 AB2 c) 2A + B4 2 AB2 d) A + B2 AB2 8 Atomic and Molecular Weights Percentage Composition from Formulas Percent composition is the atomic weight for each element divided by the formula weight of the compound multiplied by 100: Atoms of Element AW % Element 100 FW of Compound 9 Percentage Composition from Formulas What is % O in H2SO4 (by mass)? FW= (2x1) + (1x32)+ (4x16)= 98 amu FW of O in H2SO4 = 4 x 16 = 64 amu %O = 64 x 100 = 65.3% 98 10 The Mole* *MSJ Ch 3 pp 100-104 •The “amu” is an “atomic mass unit.” •O has a mass of 16 amu – but we can’t weigh out anything in amu •If we want to keep the number “16” for the mass of oxygen in some real units (like grams) then we are dealing with a whole bunch of atoms (in 16 g of oxygen). •That bunch of atoms is called a mole. •Experimentally, 1 mole = 6.02 x 1023 things (atoms) •This number is called Avogadro’s number. 11 The mole is defined so that one mole of a substance has a mass equal to its AW or MW in grams Basically, you are replacing amu with grams, e.g. The mass of a P atom is 31 amu. The molar mass of P is 31 grams. The mass of a Ca atom is 40 amu The molar mass of Ca is 40 grams This amount of P or Ca each contains Avogadro’s number (6.02 x 1023) of atoms of P and Ca. 12 The Mole Experimentally, 1 mole of 12C has a mass of exactly 12 g. (recall from Ch. 2) Molar Mass Molar mass: mass in grams of 1 mole of substance Units: g/mol or g.mol-1. Mass of 1 mole of 12C = 12 g exactly 13 14 This photograph shows one mole of : solid NaCl (58.5 g), liquid H2O (18 g), and gaseous N2 (28 g). 15 The Mole Molar Mass Molar mass: sum of the molar masses of the atoms: What is Molar Mass of H2SO4? 16 The Mole Interconverting Masses, Moles, and Numbers of Particles 17 Example 1: (a) contains (b) contains 5.00 g of P mol of P atoms of P Example 2: 5.00 x 1024 atoms of C (a) equals mol of C (b) has mass equal to grams. 18 Example 3: 3.5 mol CO2: (a) has what mass? (b) contains how many molecules of CO2? (c) contains how many atoms of O? 19 Empirical Formulas from Analyses Start with mass % of elements (i.e. empirical data) and calculate a formula. 20 Empirical Formulas from Analyses* Example: compound of N and O *MSJ Ch 3 pp 104-108 Given analysis: N: 25.9%; O: 74.1% Assume 100g; N: 25.9 g; O: 74.1 g Change to mol: N: 25.9 g x mol = 1.85 mol 14 g O: 74.1 g x mol = 4.63 mol 16 g Preliminary emp. Formula: N1.85O4.63 Clean it up: divide both by 1.85: Get N1O2.5; get rid of fractions, multiply both by 2: Get N2O5 which is the empirical (simplest) formula What are some possible molecular formulas? 21 Empirical Formulas from Analyses Molecular Formula from Empirical Formula Once we know the empirical formula, we need the MW to find the molecular formula. Subscripts in the molecular formula are always wholenumber multiples of subscripts in the empirical formula. Example: suppose compound of C and H has empirical formula of C3H8 and a MW = 176 g/mol. What is the molecular formula? 22 Quantitative Information from Balanced Equations Balanced chemical equation gives number of molecules (or moles) that react to form products. Interpretation: balanced equation gives us the ratio of number of moles of reactant to product (or v.v.) .These ratios are called stoichiometric ratios. Example: 2 H2 + O2 Molecules: 2 1 2 1 Moles: Ratio of O2:H2O = 1:2 2 H 2O 2 2 (either molecules or moles) 23 Quantitative Information from Balanced Equations The ratio of grams of reactant cannot be directly related to the grams of product. 24 Stoichiometry Problem: aluminum sulfide + water aluminum hydroxide + hydrogen sulfide (a) Write balanced reaction: (b) How many g aluminum hydroxide obtained from 10.5 g of aluminum sulfide? 25 Stoichiometry Problem: 2 NaN3(s) 2Na(s) + 3 N2(g) (a) how many mol N2 produced from 2.50 mol NaN3? (b) how many g NaN3 needed to form 6.00 g N2 (c) how many g NaN3 needed to produce 10.0 ft3 of N2? (1.00 ft3 = 28.3 L; density of N2 = 1.25 g/L) 26 Limiting Reactants If the reactants are not present in stoichiometric amounts, at end of reaction some reactants are still present (in excess). Limiting Reactant: one reactant that is consumed. O2 H2 O2 INXS 27 RECIPE: 3 cups flour + 4 eggs + 2 cups sugar cake -------------------------------------------------------On Hand: cups flour 20 eggs 20 cups sugar 20 How many cakes can be made from these amounts? What is the “limiting reactant” (LR) What’s left over and how much of it is left over? 28 RECIPE: 3 cups flour + 4 eggs + 2 cups sugar cake -------------------------------------------------------On Hand: cups flour 24 eggs 28 cups sugar 10 How many cakes can be made from these amounts? What is the “limiting reactant” (LR) What’s left over and how much of it is left over? 29 Limiting Reagent (a)Assume a reactant (any one) is LR. Calculate stoichiometric amount of product (any product) formed. (b) Pick another reactant and make it the LR. Calculate the stoichiometric amount of same product formed. (c) Whichever reactant gives the smaller amount of product is the Limiting Reagent 30 Limiting Reagent The LR is always used up in a chemical reaction. Everything else is in excess (INXS). 31 Limiting Reagent Problem: 4 NH3 + 5 O2 4 NO + 6 H2O 2.25 g NH3 mixed with 3.75 g O2 and allowed to react (a) which is LR? (b) how many grams NO formed? (c) how much of excess reactant remains? 32 Limiting Reactants Theoretical Yields The amount of product predicted from stoichiometry taking into account limiting reagents is called the theoretical yield. The percent yield relates the actual yield (amount of material recovered in the laboratory) to the theoretical yield: Actual yield % Yield 100 Theoretica l yield 33 Theoretical Yield Problem: C6H6 + Br2 C6H5Br + HBr (a) theoretical yield of C6H5Br when 30.0 g of C6H6 reacts with 65.0 g of Br2? (b) if actual yield of C6H5Br is 56.7 g, calculate %yield 34