copyright 1999, prentice hallchapter 31 stoichiometry: calculations with chemical formulas and...

Copyright 1999, PRENTICE HALL Chapter 3 1

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

and Equationsand Equations

Chapter 3Chapter 3

David P. WhiteDavid P. White

University of North Carolina, WilmingtonUniversity of North Carolina, Wilmington


• 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


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



• Law of conservation of mass: matter cannot be lost in any chemical reactions.



Using the Periodic TableUsing the Periodic Table• Properties of compounds vary systematically because

of good ordering in the periodic table.

2K(s) + 2H2O(l) 2KOH(aq) + H2(g)

2M(s) + 2H2O(l) 2MOH(aq) + H2(g)

Combustion in AirCombustion in Air• Combustion is the burning of a substance in oxygen

from air:

C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(l)

Patterns of Chemical ReactivityPatterns of Chemical Reactivity


Combination and Decomposition ReactionsCombination and Decomposition Reactions2Mg(s) + O2(g) 2MgO(s)

There are fewer products than reactants; the Mg has combined with O2 to form MgO.

2NaN3(s) 2Na(s) + 3N2(g) (the reaction that occurs in an air bag)

There are more products than reactants; the sodium azide has decomposed into Na and nitrogen gas.



Combination and Decomposition ReactionsCombination and Decomposition ReactionsCombination reactions: fewer reactants than products.Decomposition reactions: more products than reactants.



The Atomic Mass ScaleThe Atomic Mass Scale1H weighs 1.6735 x 10-24 g and 16O 2.6560 x 10-23 g.We define: mass of 12C = exactly 12 amu.Using atomic mass units:

1 amu = 1.66054 x 10-24 g1 g = 6.02214 x 1023 amu

Atomic and Molecular WeightsAtomic and Molecular Weights


Average Atomic MassAverage Atomic MassRelative atomic mass: average masses of isotopes:

Naturally occurring C: 98.892 % 12C + 1.108 % 13C.Average mass of C:

(0.98892)(12 amu) + (0.0108)(13.00335) = 12.011 amu.

Atomic weight (AW) is also known as average atomic mass (atomic weight).Atomic weights are listed on the periodic table.



Formula and Molecular WeightsFormula and Molecular WeightsFormula 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 amuMolecular weight (MW) is the weight of the molecular formula.

MW(C6H12O6) = 6(12.0 amu) + 12(1.0 amu) + 6(16.0 amu)



Percentage Composition from FormulasPercentage Composition from FormulasPercent composition is the atomic weight for each element divided by the formula weight of the compound multiplied by 100:


100

Compound of FWAWElement of Atoms

Element %


Mole: convenient measure chemical quantities.1 mole of something = 6.0221367 x 1023 of that thing.Experimentally, 1 mole of 12C has a mass of 12 g.

Molar MassMolar MassMolar 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


Molar MassMolar MassMolar mass: sum of the molar masses of the atoms:

molar mass of N2 = 2 x (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


Interconverting Masses, Moles, and Numbers of Interconverting Masses, Moles, and Numbers of ParticlesParticles

The MoleThe Mole


Start with mass % of elements (i.e. empirical data) and calculate a formula, orStart with the formula and calculate the mass % elements.

Empirical Formulas from AnalysesEmpirical Formulas from Analyses


Molecular Formula from Empirical FormulaMolecular Formula from Empirical FormulaOnce 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.



Combustion AnalysisCombustion AnalysisEmpirical formulas are determined by combustion analysis:



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.

NB: Stoichiometric ratios are ideal proportionsReal ratios of reactants and products in the laboratory need to be measured (in grams and converted to moles).

Quantitative Information from Quantitative Information from Balanced EquationsBalanced Equations


The ratio of grams of reactant cannot be directly related to the grams of product.

Quantitative Information from Quantitative Information from Balanced EquationsBalanced Equations


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


Theoretical YieldsTheoretical YieldsThe 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 %


Stoichiometry: Calculations Stoichiometry: Calculations with Chemical Formulas with Chemical Formulas and Equationsand Equations

End of Chapter 3End of Chapter 3

copyright 1999, prentice hallchapter 31 stoichiometry: calculations with chemical formulas and...

Documents

atomic mass units

amu atomic

atomic mass scale1h

average mass of c

molar massmolar mass

molar mass of n2

chemical formulas

atomic weights