Chemical Stoichiometry Reacting Quantities and Material Balance

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Chemical StoichiometryReacting Quantities
andMaterial Balance

• Edward A. Mottel
• Department of Chemistry
• Rose-Hulman Institute of Technology

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Resources Available

• Fine, Beall Stuehr Chapter 3.5
• A balanced chemical equation is used to predict
  the maximum amount of product which can be formed
  in a chemical reaction.
• Chemical nomenclature, molecular weight, mass and
  moles of each reactant present and product formed
  are concepts involved in these calculations.

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Chemical Equations

• Stoichiometry (atom balance)
• Charge (charge balance)
• Format
• Phase annotation (s, l, g, aq)
• Arrow notations

Write an annotated chemical equation for
the decomposition of a 30 solution of hydrogen
peroxide to give water and oxygen.
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Hydrogen PeroxideDecomposition
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• Concentrated hydrogen peroxide contains 30
  hydrogen peroxide by weight and the balance
  water. The density of 30 hydrogen peroxide is
  1.110 gmL-1.
• Hydrogen peroxide decomposes upon standing, over
  a period of several months, to form water and
  oxygen.
• Assume an empty 500 mL plastic bottle used to
  hold hydrogen peroxide has a mass of 100.0 grams.

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• What would be the difference in mass and the
  percentage difference in mass between 500 mL of
  fresh 30 hydrogen peroxide in the 500 mL bottle
  and a bottle of hydrogen peroxide which is
  decomposed?
• Assume any oxygen gas formed escapes.
• The density of water at 25.0 C is 0.9970gcm-3.
• The density of pure hydrogen peroxide is 1.442
  gcm-3.

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mass of 30 H2O2 in solution
mass of H2O2 in solution
volume of 30 H2O2
30 H2O by mass
36/68
32/68
balanced equation
initial mass of bottle solution
mass of H2O in original solution
mass of O2 produced
mass of H2O produced
mass is conserved
mass is conserved
mass of empty bottle
volume of H2O at end
final mass of bottle solution
total mass of H2O at end
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Problem Solving Format

• Define the problem
• Outline the solution to the problem
• include relevant diagrams equations
• Solve the problem

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Stoichiometry

• The study of heat, energy and material balances
  of a chemical system.
• The amount of reactants and products in a
  reaction are proportional to each other.
• The amount of heat energy released or required by
  a reaction is proportional to the amount of
  reactants and products involved in the reaction.

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19th Century Chemistry
3 grams of carbon
4 grams of oxygen
Analyzed reactions using weights and volumes
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Stoichiometry Rules

• Law of Conservation of Mass
• Law of Constant Composition
• Law of Multiple Proportions

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Law of Conservation of Mass
In a chemical reaction, mass is neither created
nor destroyed.
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Law of Constant CompositionLaw of Definite
Proportions
Any sample of a pure compound always consists of
the same elements combined in the same
proportions by mass.
C(s) O2(g) ? CO2(g)
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Law of Multiple Proportions
If two elements combine to form more than one
stable compound, the masses of one element that
combines with a fixed mass of the second
element will be in the ratio of small whole
numbers.
C(s) 1/2 O2(g) ? CO (g)
12 g C and 16 g O2
C(s) O2(g) ? CO2(g)
12 g C and 32 g O2
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Chemical Equations and Stoichiometry

• Moles of reactants and moles of product
• The balanced chemical equation deals with
  molecules or moles, not grams
• Limiting reagent
• Every reaction that is complete (100 reaction)
  has at least one limiting reactant

If equal masses of carbon and oxygen are reacted
to give carbon dioxide, which is the limiting
reactant?
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Chemical Equations and Stoichiometry

• Theoretical yield
• The maximum number of moles or mass of a product
  that could be formed from the given amounts of
  reactants

What is the theoretical yield when 120 grams of
carbon reacts with 120 grams of oxygen to give
carbon dioxide?
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Chemical Equations and Stoichiometry

• Percentage yield
• The amount of product formed divided by the
  theoretical yield times 100

What is the percentage yield if 120 grams of
carbon reacts with 120 grams of oxygen to give
200 grams of carbon monoxide?
Show how your results support the Law of
Conservation of Mass.
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Reaction of Methane and Carbon Dioxide

• 50. grams of methane reacts with 50. grams of
  carbon dioxide to give carbon monoxide and water
  vapor.
• If the reaction is 100 complete determine
• the limiting reagent.
• the mass of each reactant product at the end of
  the reaction.

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Reaction of Methane and Carbon Dioxide

• Excel spreadsheet

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Challenge

• Design a spreadsheet to determine the maximum
  amount of carbon dioxide that could be formed
  when any amount of sucrose (C12H22O11) reacts
  with any amount of oxygen gas. The other product
  is water.

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Limestone

• Limestone is a naturally occurring material
  consisting principally of calcium carbonate.
• Upon heating (roasting) at gt1000 C, it
  decomposes into calcium oxide and carbon dioxide.
• You are the chief engineer. How could you
  determine when this roasting process is
  completed?

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