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UNIT FOUR Stoichiometry: Calculations with Chemical Formulas and Equations

CHEMISTRY The Central Science 9th Edition. UNIT FOUR Stoichiometry: Calculations with Chemical Formulas and Equations. Chemical Equations. Lavoisier: mass is conserved in a chemical reaction. Chemical equations: descriptions of chemical reactions.

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UNIT FOUR Stoichiometry: Calculations with Chemical Formulas and Equations

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  1. CHEMISTRYThe Central Science 9th Edition UNIT FOURStoichiometry: Calculations with Chemical Formulas and Equations UNIT FOUR

  2. 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 UNIT FOUR

  3. Chemical Equations • 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: • 2H2+ O22H2O UNIT FOUR

  4. Chemical Equations 2Na + 2H2O  2NaOH + H2 2K + 2H2O  2KOH + H2 UNIT FOUR

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

  6. Chemical Equations UNIT FOUR

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

  8. Some Simple Patterns of Chemical Reactivity Combination and Decomposition Reactions • Synthesis reactions have fewer products than reactants: 2Mg(s) + O2(g)  2MgO(s) • The Mg has combined with O2 to form MgO. • Decomposition reactions have fewer reactants than products: 2NaN3(s)  2Na(s) + 3N2(g) (the reaction that occurs in an air bag) • The NaN3 has decomposed into Na and N2 gas. UNIT FOUR

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

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

  11. Some Simple Patterns of Chemical Reactivity Combustion in Air Combustion is the burning of a substance in oxygen from air: C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l) UNIT FOUR

  12. Formula Weights Formula and Molecular Weights • Formula weights (FW): sum of AW for atoms in formula. FW (H2SO4) = 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(C6H12O6) = 6(12.0 amu) + 12(1.0 amu) + 6(16.0 amu) UNIT FOUR

  13. Formula 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: UNIT FOUR

  14. The Mole • Mole: convenient measure chemical quantities. • 1 mole of something = 6.0221367  1023 of that thing. • Experimentally, 1 mole of 12C 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 12C = 12 g. UNIT FOUR

  15. The Mole

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

  17. The Mole UNIT FOUR

  18. The Mole This photograph shows one mole of solid (NaCl), liquid (H2O), and gas (N2). UNIT FOUR

  19. The Mole • Interconverting Masses, Moles, and Number of Particles • Molar mass: sum of the molar masses of the atoms: • molar mass of N2 = 2 (molar mass of N). • Molar masses for elements are found on the periodic table. • Formula weights are numerically equal to the molar mass. UNIT FOUR

  20. Empirical Formulas from Analyses • Start with mass % of elements (i.e. empirical data) and calculate a formula, or • Start with the formula and calculate the mass % elements. UNIT FOUR

  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 whole-number multiples of subscripts in the empirical formula UNIT FOUR

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

  23. Quantitative Information from Balanced Equations • 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). UNIT FOUR

  24. 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 UNIT FOUR

  25. Limiting Reactants UNIT FOUR

  26. 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: UNIT FOUR

  27. Stoichiometry: Calculations with Chemical Formulas and Equations UNIT FOUR

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