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
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:
2H2 + O2 2H2O
Chemical EquationsChemical Equations
UNIT FOUR
• Stoichiometric coefficients: numbers in front of the chemical formulas; give ratio of reactants and products.
Chemical EquationsChemical Equations
UNIT FOUR
• Law of conservation of mass: matter cannot be lost in any chemical reactions.
Chemical EquationsChemical Equations
UNIT FOUR
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
UNIT FOUR
Combination and Decomposition Reactions
Some Simple Patterns of Some Simple Patterns of Chemical ReactivityChemical Reactivity
UNIT FOUR
Synthesis and Decomposition Reactions
Some Simple Patterns of Some Simple Patterns of Chemical ReactivityChemical Reactivity
UNIT FOUR
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)
UNIT FOUR
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
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 WeightsFormula Weights
100
Compound ofFW AWElement of Atoms
Element %
UNIT FOUR
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
UNIT FOUR
The MoleThe Mole
This photograph shows one mole of solid (NaCl), liquid (H2O), and gas (N2).
UNIT FOUR
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
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 Empirical Formulas from AnalysesAnalyses
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 Empirical Formulas from AnalysesAnalyses
UNIT FOUR
Combustion Analysis• Empirical formulas are determined by combustion
analysis:
Empirical Formulas from Empirical Formulas from AnalysesAnalyses
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 Quantitative Information from Balanced Equationsfrom 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 ReactantsLimiting 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 ReactantsLimiting Reactants
100yield lTheoretica
yield ActualYield %
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