UNIT FOUR UNIT FOUR Stoichiometry: Calculations with Chemical Formulas and Equations CHEMISTRY The Central Science 9th Edition

UNIT FOUR 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

UNIT FOUR The chemical equation for the formation of water can be visualized as two hydrogen molecules reacting with one oxygen molecule to form two water molecules: 2H 2 + O 2  2H 2 O Chemical Equations

UNIT FOUR 2Na + 2H 2 O  2NaOH + H 2 2K + 2H 2 O  2KOH + H 2 Chemical Equations

UNIT FOUR Stoichiometric coefficients: numbers in front of the chemical formulas; give ratio of reactants and products. Chemical Equations

UNIT FOUR Chemical Equations

UNIT FOUR Law of conservation of mass: matter cannot be lost in any chemical reactions. Chemical Equations

UNIT FOUR Combination and Decomposition Reactions Synthesis reactions have fewer products than reactants: 2Mg(s) + O 2 (g)  2MgO(s) The Mg has combined with O 2 to form MgO. Decomposition reactions have fewer reactants than products: 2NaN 3 (s)  2Na(s) + 3N 2 (g) (the reaction that occurs in an air bag) The NaN 3 has decomposed into Na and N 2 gas. Some Simple Patterns of Chemical Reactivity

UNIT FOUR Combination and Decomposition Reactions Some Simple Patterns of Chemical Reactivity

UNIT FOUR Synthesis and Decomposition Reactions Some Simple Patterns of Chemical Reactivity

UNIT FOUR Combustion in Air Some Simple Patterns of Chemical Reactivity 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)

UNIT FOUR 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) Formula Weights

UNIT FOUR Percentage Composition from Formulas Percent composition is the atomic weight for each element divided by the formula weight of the compound multiplied by 100: Formula Weights

UNIT FOUR Mole: convenient measure chemical quantities. 1 mole of something =  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

UNIT FOUR ARE YOU PAYING ATTENTION? \ Your mother wears army boots

UNIT FOUR The Mole

UNIT FOUR The Mole This photograph shows one mole of solid (NaCl), liquid (H 2 O), and gas (N 2 ).

UNIT FOUR Interconverting Masses, Moles, and Number of Particles Molar mass: sum of the molar masses of the atoms: molar mass of N 2 = 2  (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

UNIT FOUR 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

UNIT FOUR 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

UNIT FOUR Combustion Analysis Empirical formulas are determined by combustion analysis: Empirical Formulas from Analyses

UNIT FOUR 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. 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

UNIT FOUR 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

UNIT FOUR Limiting Reactants

UNIT FOUR 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

UNIT FOUR Stoichiometry: Calculations with Chemical Formulas and Equations