Copyright 1999, PRENTICE HALLChapter 31 Stoichiometry: Calculations with Chemical Formulas and Equations Chapter 3 David P. White University of North Carolina, Wilmington
Copyright 1999, PRENTICE HALLChapter 32 Lavoisier: mass is conserved in a chemical reaction. Chemical equations: descriptions of chemical reactions. Two parts to an equation: reactants and products: 2H 2 + O 2 2H 2 O Chemical Equations
Copyright 1999, PRENTICE HALLChapter 33 Stoichiometric coefficients: numbers in front of the chemical formulas; give ratio of reactants and products. Chemical Equations
Copyright 1999, PRENTICE HALLChapter 34 Law of conservation of mass: matter cannot be lost in any chemical reactions. Chemical Equations
Copyright 1999, PRENTICE HALLChapter 35 Using the Periodic Table Properties of compounds vary systematically because of good ordering in the periodic table. 2K(s) + 2H 2 O(l) 2KOH(aq) + H 2 (g) 2M(s) + 2H 2 O(l) 2MOH(aq) + H 2 (g) Combustion in Air Combustion is the burning of a substance in oxygen from air: C 3 H 8 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2 O(l) Patterns of Chemical Reactivity
Copyright 1999, PRENTICE HALLChapter 36 Patterns of Chemical Reactivity
Copyright 1999, PRENTICE HALLChapter 37 Combination and Decomposition Reactions 2Mg(s) + O 2 (g) 2MgO(s) There are fewer products than reactants; the Mg has combined with O 2 to form MgO. 2NaN 3 (s) 2Na(s) + 3N 2 (g) (the reaction that occurs in an air bag) There are more products than reactants; the sodium azide has decomposed into Na and nitrogen gas. Patterns of Chemical Reactivity
Copyright 1999, PRENTICE HALLChapter 38 Combination and Decomposition Reactions Combination reactions: fewer reactants than products. Decomposition reactions: more products than reactants. Patterns of Chemical Reactivity
Copyright 1999, PRENTICE HALLChapter 39 The Atomic Mass Scale 1 H weighs x g and 16 O x g. We define: mass of 12 C = exactly 12 amu. Using atomic mass units: 1 amu = x g 1 g = x amu Atomic and Molecular Weights
Copyright 1999, PRENTICE HALLChapter 310 Average Atomic Mass Relative atomic mass: average masses of isotopes: Naturally occurring C: % 12 C % 13 C. Average mass of C: ( )(12 amu) + (0.0108)( ) = amu. Atomic weight (AW) is also known as average atomic mass (atomic weight). Atomic weights are listed on the periodic table. Atomic and Molecular Weights
Copyright 1999, PRENTICE HALLChapter 311 Formula and Molecular Weights Formula weights (FW): sum of AW for atoms in formula. FW (H 2 SO 4 ) = 2AW(H) + AW(S) + 4AW(O) = 2(1.0 amu) + (32.0 amu) + 4(16.0) = 98.0 amu Molecular weight (MW) is the weight of the molecular formula. MW(C 6 H 12 O 6 ) = 6(12.0 amu) + 12(1.0 amu) + 6(16.0 amu) Atomic and Molecular Weights
Copyright 1999, PRENTICE HALLChapter 312 Percentage Composition from Formulas Percent composition is the atomic weight for each element divided by the formula weight of the compound multiplied by 100: Atomic and Molecular Weights
Copyright 1999, PRENTICE HALLChapter 313 Mole: convenient measure chemical quantities. 1 mole of something = x of that thing. Experimentally, 1 mole of 12 C has a mass of 12 g. Molar Mass Molar mass: mass in grams of 1 mole of substance (units g/mol, g.mol -1 ). Mass of 1 mole of 12 C = 12 g. The Mole
Copyright 1999, PRENTICE HALLChapter 314 Molar Mass Molar mass: sum of the molar masses of the atoms: molar mass of N 2 = 2 x (molar mass of N). Molar masses for elements are found on the periodic table. Formula weights are numerically equal to the molar mass. The Mole
Copyright 1999, PRENTICE HALLChapter 315 Interconverting Masses, Moles, and Numbers of Particles The Mole
Copyright 1999, PRENTICE HALLChapter 316 Start with mass % of elements (i.e. empirical data) and calculate a formula, or Start with the formula and calculate the mass % elements. Empirical Formulas from Analyses
Copyright 1999, PRENTICE HALLChapter 317 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 whole- number multiples of subscripts in the empirical formula. Empirical Formulas from Analyses
Copyright 1999, PRENTICE HALLChapter 318 Combustion Analysis Empirical formulas are determined by combustion analysis: Empirical Formulas from Analyses
Copyright 1999, PRENTICE HALLChapter 319 Balanced chemical equation gives number of molecules that react to form products. Interpretation: ratio of number of moles of reactant required to give the ratio of number of moles of product. These ratios are called stoichiometric ratios. NB: Stoichiometric ratios are ideal proportions Real ratios of reactants and products in the laboratory need to be measured (in grams and converted to moles). Quantitative Information from Balanced Equations
Copyright 1999, PRENTICE HALLChapter 320 The ratio of grams of reactant cannot be directly related to the grams of product. Quantitative Information from Balanced Equations
Copyright 1999, PRENTICE HALLChapter 321 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. Limiting Reactants
Copyright 1999, PRENTICE HALLChapter 322 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: Limiting Reactants
Copyright 1999, PRENTICE HALLChapter 323 Stoichiometry: Calculations with Chemical Formulas and Equations End of Chapter 3