UNIT FOUR Stoichiometry: Calculations with Chemical Formulas and Equations

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

UNIT FOUR

• Stoichiometric coefficients: numbers in front of the chemical formulas; give ratio of reactants and products.

Chemical EquationsChemical Equations

UNIT FOUR

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

UNIT FOUR

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)

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

The MoleThe Mole

UNIT FOUR

ARE YOU PAYING ARE YOU PAYING ATTENTION?ATTENTION?

• \• Your mother wears army boots

UNIT FOUR

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

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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

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

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 %

UNIT FOUR

Stoichiometry: Calculations with Stoichiometry: Calculations with Chemical Formulas and Chemical Formulas and

EquationsEquations