unit four unit four stoichiometry: calculations with chemical formulas and equations chemistry the...
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UNIT FOUR
UNIT FOURUNIT FOURStoichiometry: Calculations with Stoichiometry: Calculations with
Chemical Formulas and EquationsChemical 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:
2H2 + O2 2H2O
Chemical EquationsChemical Equations
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• 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
Chemical EquationsChemical Equations
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2Na + 2H2O 2NaOH + H2
2K + 2H2O 2KOH + H2
Chemical EquationsChemical Equations
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• Stoichiometric coefficients: numbers in front of the chemical formulas; give ratio of reactants and products.
Chemical EquationsChemical Equations
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Chemical EquationsChemical Equations
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• Law of conservation of mass: matter cannot be lost in any chemical reactions.
Chemical EquationsChemical Equations
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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.
Some Simple Patterns of Some Simple Patterns of Chemical ReactivityChemical Reactivity
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Combination and Decomposition Reactions
Some Simple Patterns of Some Simple Patterns of Chemical ReactivityChemical Reactivity
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Synthesis and Decomposition Reactions
Some Simple Patterns of Some Simple Patterns of Chemical ReactivityChemical Reactivity
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Combustion in Air
Some Simple Patterns of Some Simple Patterns of Chemical ReactivityChemical Reactivity
Combustion is the burning of a substance in oxygen from air:
C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(l)
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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)
Formula WeightsFormula Weights
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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 WeightsFormula Weights
100
Compound ofFW AWElement of Atoms
Element %
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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.
The MoleThe Mole
The MoleThe Mole
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ARE YOU PAYING ARE YOU PAYING ATTENTION?ATTENTION?
• \• Your mother wears army boots
UNIT FOUR
The MoleThe Mole
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The MoleThe Mole
This photograph shows one mole of solid (NaCl), liquid (H2O), and gas (N2).
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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.
The MoleThe Mole
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• 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 Empirical Formulas from AnalysesAnalyses
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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 Empirical Formulas from AnalysesAnalyses
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Combustion Analysis• Empirical formulas are determined by combustion
analysis:
Empirical Formulas from Empirical Formulas from AnalysesAnalyses
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• 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 Quantitative Information from Balanced Equationsfrom Balanced Equations
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• 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 ReactantsLimiting Reactants
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Limiting ReactantsLimiting Reactants
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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 ReactantsLimiting Reactants
100yield lTheoretica
yield ActualYield %
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Stoichiometry: Calculations with Stoichiometry: Calculations with Chemical Formulas and Chemical Formulas and
EquationsEquations