Chemical StoichiometryReacting Quantities and

Material Balance

Edward A. Mottel

Department of Chemistry

Rose-Hulman Institute of Technology

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 ofhydrogen peroxide to give

water and oxygen.

Hydrogen PeroxideDecomposition

30%

H2O2

decomposesH2O

waterfrom decompositionand original water

2 H2O2(aq) 2 H2O(l) + O2(g)

70% water

Concentrated hydrogen peroxide contains 30% hydrogen peroxide by weight and the balance water. The density of 30% hydrogen peroxide is 1.110 g•mL-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.

30%H2O2

decomposesH2O

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

0.9970•g•cm-3. The density of pure hydrogen peroxide is

1.442 g•cm-3.

30%H2O2

decomposesH2O

volume of30% H2O2

density of30% H2O2

mass of30% H2O2

in solution

30% H2O2

by massmass of

H2O2

in solution

mass ofO2

produced

32/68balancedequation

mass ofH2O

produced

36/68

mass ofH2O

in originalsolution

30% H2Oby mass

total massof H2Oat end

mass isconserved

initial massof bottle

& solution

final massof bottle

& solution

mass ofemptybottle

volumeof H2Oat end

densityof H2O

mass isconserved

Problem Solving Format

Define the problem Outline the solution to the problem

• include relevant diagrams & equations Solve the problem

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.

19th Century Chemistry

3 grams of carbon

4 grams of oxygen

400grams

300 grams

Analyzed reactions using

weights and volumes

Stoichiometry Rules

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

Law of Conservation of Mass

C(s) + O2(g) CO2(g)

12 g 32 g 44 g

In a chemical reaction, mass is neithercreated nor destroyed.

Law of Constant CompositionLaw of Definite Proportions

C(s) + O2(g) CO2(g)

12 g 32 g 44 g

6 g 16 g 22 g

3 g 8 g 11 g

Any sample of a pure compound alwaysconsists of the same elements combined in

the same proportions by mass.

Law of Multiple Proportions

C(s) + O2(g) CO2(g)

C(s) + 1/2 O2(g) CO (g) 12 g C and 16 g O2

12 g C and 32 g O2

If two elements combine to form more thanone stable compound,

the masses of one element that combineswith a fixed mass of the second element

will be in the ratio of small whole numbers.

16:32 is 1 to 2

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 oxygenare reacted to give carbon dioxide,

which is the limiting reactant?

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 when120 grams of carbon reacts with

120 grams of oxygen to give carbon dioxide?

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 carbonreacts with 120 grams of oxygen

to give 200 grams of carbon monoxide?

Show how your results support theLaw of Conservation of Mass.

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.

Excel spreadsheet

Reaction of Methane and Carbon Dioxide

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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