The Energy Debate - Stoichiometry

Written for PULSE by Christopher Martin
Edited By: Stephanie Nardei

Time: 30 minutes


Materials: --

Students find information about the different fractions of crude oil; its use, formulae and write a balanced equation for the reaction with oxygen.

Students will be able to:

  1. Understand crude oil can be separated into useful fractions by a process of fractional distillation.
  2. Write a balanced equation for the reaction between a hydrocarbon and oxygen.
  3. Understand the relationship between mole, mass and mole ratio

National Science Education Standard
Content Standard E: The Physical Setting

Energy Transformations
Different energy levels are associated with different configurations of atoms and molecules. Some changes of configuration require an input of energy whereas others release energy.

Strand 5: Physical Science
Concept 4: Chemical Reactions
Investigate relationships between reactants and products in chemical reactions.
PO 6. 

Explain the energy transfers within chemical reactions using the law of conservation of energy.

Concept 3: Conservation of energy and increase in disorder
PO.6 Distinguish between heat and temperature.

Teacher Background
Crude oil can be separated into useful fractions in a fractionating column. Each fraction boils off at a different temperature. The fractions with increasing numbers of carbon atoms have a higher boiling point. These can be “cracked” to make smaller molecules such as ethane [ethylene]1. Many of these fractions can be used as fuels; reacting with oxygen to form carbon dioxide, water and energy.

Stoichiometry rests upon the law of conservation of mass, the law of definite proportions (i.e., the law of constant composition) and the law of multiple proportions. In general, chemical reactions will combine in definite ratios of chemicals. Since matter cannot be created or destroyed, the amount of each element must be the same throughout the overall reaction. For example, the amount of element X on the reactant side must equal the amount of element X on the product side.

Stoichiometry is often used to balance chemical equations. For example, the two diatomic gases, hydrogen and oxygen, can combine to form liquid, water, in an exothermic reaction, as described by the following equation:
2H_2 + O_2 \rightarrow 2H_2O

The term stoichiometry is also often used for the molar proportions of elements in stoichiometric compounds. For example, the stoichiometry of hydrogen and oxygen in H2O is 2:1. In stoichiometric compounds, the molar proportions are whole numbers (that is what the law of multiple proportions is about).

Related and Resource Websites
EPA Climate Change http://www.epa.gov/climatechange/index.html
Fossil Fuels http://library.thinkquest.org/17531/fossil.html
Oil & Its Useful Products http://www.wpbschoolhouse.btinternet.co.uk/page04/OilProducts.htm#properties
Stoichiometry Table of Contents http://dbhs.wvusd.k12.ca.us/webdocs/Stoichiometry/Stoichiometry.html
Stoichiometry on Wikipedia http://en.wikipedia.org/wiki/Stoichiometry
International Union of Pure and Applied Chemistry (IUPAC) http://www.iupac.org/dhtml_home.html
Shodor Education Foundation on Stoichiometry http://www.shodor.org/UNChem/basic/stoic/index.html




  1. Ask the students where crude oil is found. Explain that crude oil is a sticky black liquid. How do we get gasoline from crude oil?

  2. Explain that crude oil is separated into useful fractions such as gasoline and diesel fuel by fractional distillation. The crude oil is heated and the different fractions condense at different temperatures. An illustration of the fractional distillation process can be found at:


  1. Students should use the website to find information about the fractions of crude oil and complete the table:





Mass of CO2 produced when 1Kg of fraction is burnt.













Diesel Oil




Lubricating oils and waxes







3138 g

All the fractions are alkanes and have the general formulae CnH2n+2

  1. Students should use stoichoimetry to calculate the mass of CO2 produced when 1Kg of fraction reacts with oxygen. An example is given below:


Balanced equation                     C101H204 + 152 O2 à 101 CO2 + 102 H2O

Molar Mass of C101H204 of = (101 x12) + (204 X 1) = 1416

Mass of C101H204  = 1 Kg = 1000 grams

# moles of C101H204 = 1000 / 1416 = 0.71

# moles CO2 = 0.71 x 101 = 72

Molar Mass of CO2 = 12 + 16 + 16 = 44

Mass of CO2 = 72 x 44 = 3168 grams or 3.2 Kg

One mole of gas occupies a volume of 22.4 L at standard temperature and pressure STP (zero degrees Celsius and 1 atmosphere pressure)

Volume of CO2 = # moles x 22.4

                        = 72 x 22.4 = 1613 liters

Embedded Assessment
Understanding shall be demonstrated by successful completion of the worksheet.




PULSE is a project of the Community Outreach and Education Program of the Southwest Environmental Health Sciences Center and is funded by:

NIH/NCRR award #16260-01A1
The Community Outreach and Education Program is part of the Southwest Environmental Health Sciences Center: an NIEHS Award


Supported by NIEHS grant # ES06694

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Last update: March 7, 2007
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