Stoichiometry

The science of manipulating reactants and products

What is stoichiometry?

Stoichiometry is the science of measuring and calculating the quantity of reactants and products that would be produced from a given chemical equation. To help understand how stoichiometry works, we will use the analogy of cooking to explore the relationships between different "ingredients" in chemical equations.
The example we will be using to demonstrate stoichiometry is, as known by their IUPAC names, octane and oxygen. In this equation- as is the case with every equation in which a diatomic molecule of oxygen is a reactant- then its reaction type is combustion. Octane and oxygen yield to (by their IUPAC nomenclature) carbon dioxide and water. The balanced chemical equation is the following (numbers in bold indicate coefficients):


2C8H18+25O2→16CO2+18H2O

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Finding the Molar Mass

Molar mass is defined as the average mass of a given substance, as derived from the amount of given substance that is known to exist. In order to calculate the total molar mass of a given reactant or product, one must look at the molar mass of an individual element in the compound (as shown on the periodic table) and multiply the value by the element's subscript. Then, simply add every element's total molar mass to find the total molar mass of the given reactant or product. Therefore, the molar mass for the "ingredients" methane and oxygen, is 54.21(g/mol) and 32.00(g/mol), respectively. The molar mass for carbon dioxide is 44.01(g/mol), and the molar mass for water is 18.02(g/mol).

Mole to Mole Conversions

A mole is a unit of chemistry that is unique to every element, similar to how eggs are sold and measured by the dozen. Therefore, if you were to take the number of moles for the quantity of a given substance and applied it to another substance, the quantity would not be the same. In that case, it would be necessary to perform mole to mole conversion.


Let's say that the octane represents strawberries, and the oxygen represents bananas, and we are trying to produce a smoothie with an equal blend of both. In this example, we would use dimensional analysis to take 3.21 moles of the strawberries (A value) and multiply it by the coefficient 25 of the bananas (B value). Finally, we would divide it by the coefficient 2 of the strawberries (A value). The result is 40.1 mol, meaning that 3.21 moles of octane is equivalent to 40.1 moles of oxygen.

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Mass to Mass Conversions

Mass to mass conversions concern how an amount of a reactant could yield to a certain amount of product, or vice versa. In this case, assuming that the carbon dioxide represents the "blended fruit" and the water represents "fruit chunks", we will be calculating the amount of blended fruit that 12.1 g of strawberries (octane) can create.
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In this case, 12.1 g of strawberries (octane) would be able to produce 44.01 g of carbon dioxide (blended fruit).

Limiting and Excess Reactant

The limiting reactant is, out of all the reactants in a given chemical equation, the one to produce the least quantity of product. From the mass to mass conversions, we learned that 12.1 g of octane would yield to 78.6 g of carbon dioxide. If we applied the same value of 12.1 g to the other reactant, oxygen, we would discover that it yields 17.7 g of carbon dioxide. Since the reactant of oxygen would yield far less carbon dioxide than the octane, then the oxygen is the limiting reactant. If one were to conduct a chemical experiment, the amount of product produced would be dependent on the quantity of oxygen available. The excess reactant, which produces "leftovers", is the octane.

Theoretical Yield and Percent Yield

Theoretical yield is the amount of product expected to be gained in a given chemical reaction, and is dependent on the limiting reactant. In this case, the theoretical yield is 17.7 grams.

The percent yield, on the other hand, is calculated by placing the quantity of product gained in the actual experiment (having been altered by external factors during the experiment) over your calculated theoretical yield. You would then multiply that value by 100 in order to convert your answer into a percent.

Real World Application

For our purposes, we have been referring to the chemical reaction in terms of food. However, in the real world, the reaction of octane and oxygen produces gasoline. This is a valuable agent withing internal combustion engines for transportation vehicles, such as cars, motorcycles, and boats. Gasoline, having been derived from crude oil, is the primary resource of transportation fuel in the United States, constituting 65% of all the energy used for transportation in America in the year 2014. The aforementioned vehicles consume 90% of the total gasoline in the United States.