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Writing_Chemical_Equations - WRITING CHEMICAL EQUATIONS...

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Writing_Chemical_Equations.wpd 8/28/08 WRITING CHEMICAL EQUATIONS Chemical equations are representations of reactions in terms of the formulas of the elements and/or compounds involved. Usually, the reactants (starting materials) appear on the left-hand side and the products appear on the right-hand side. An arrow, in between and always pointing from the reactants to the products, may be considered as an abbreviation for "yields." Chemical equations report the results of experimentation and therefore great care must be taken so that the equation is in accord with the results of the experiment. A great deal of information is provided by the chemical equation; the exact nature of all reactants and products as well as the overall stoichiometry (ratio by which the species react) of the reaction is provided in this form of chemical shorthand. For any particular reaction the stoichiometry will always be unique. In order for the information to be understood universally, certain rules and conventions must be followed. The same reactants under different reaction conditions can produce different products (or a different ratio of products). When this happens, two separate equations must be written. For example, methane (natural gas) burns in air to yield water and either pure carbon monoxide, pure carbon dioxide, or a mixture of the two. The unique equations for the formation of CO and CO 2 are: 2 CH 4 + 3 O 2 xxv 4 H 2 O + 2 CO , and CH 4 + 2 O 2 xxv 2 H 2 O + CO 2 . However, if you attempted to write an equation which produced both CO and CO 2 , you could write an infinite number of such equations, each with a different ratio of CO to CO 2 . Such equations could also be obtained by combining the above two equations in different ratios. But, you would not want to do this because the equations would not be unique. Therefore, the question we need to answer is: What does a chemical equation mean? Choosing the first equation as an example, it means: IF methane reacts with oxygen to yield water and carbon monoxide, then for every two moles of methane that reacts, three moles of oxygen react with it, to yield four moles of water and two moles of carbon monoxide. Since the species in the given equation are molecular, "molecules" can be substituted for "moles" in this statement.
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2 RULES AND CONVENTIONS FOR WRITING CHEMICAL EQUATIONS . THE EQUATION MUST BE IN ACCORD WITH THE CHEMICAL FACTS . The first step in writing a chemical equation is to ascertain all of the reactants and products of the reaction in question including their physical states. Knowing what phase the reactant/product is in will determine how to write the species appropriately. (Also, you would want as much quantitative information about the species as possible.) This entire process is referred to as "inventorying" the system. Then, the following conventions can be applied: Ionic symbols are used whenever separated, individual ions are present.
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Writing_Chemical_Equations - WRITING CHEMICAL EQUATIONS...

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