Stoichiometry is the study of the quantitative relationships between reactants and products in chemical reactions. A balanced chemical equation provides the precise, theoretical ratio in which reactants should combine. However, in laboratory and industrial processes, chemists rarely mix reactants in these perfect proportions. Instead, they often intentionally introduce a quantity of one reactant that is greater than what the stoichiometry demands for a complete reaction.
Defining Reactants in Stoichiometry
When reactants are not mixed in their exact stoichiometric ratios, one substance will always be fully consumed, stopping the reaction. This substance is known as the limiting reactant because its quantity dictates the maximum amount of product that can be formed. The remaining substances left over after the reaction are collectively called the excess reactant.
The excess reactant is defined as the substance present in a greater amount than is required to react completely with the limiting reactant. Since the limiting reactant is completely used up, it determines the final amount of product created, regardless of the quantity of the excess material. The primary characteristic of an excess reactant is that some portion of it will remain unreacted in the reaction vessel once the process is complete.
The Practical Application of Excess Reactants
Chemists deliberately use an excess reactant for several practical and economic reasons. One major reason is to maximize the conversion of the limiting reactant into the desired product. By flooding the reaction with a surplus of one reactant, it guarantees that all of the limiting material, often the more expensive or harder-to-obtain component, is consumed.
In equilibrium reactions, an excess reactant can be used to drive the reaction forward. According to Le Chatelier’s principle, increasing the concentration of a reactant pushes the equilibrium toward the formation of more product, increasing the final yield. Using a surplus also helps to overcome issues like impurities or side reactions that might otherwise waste the limiting material. If one reactant is inexpensive and the other is costly, using an excess of the cheap one ensures that none of the expensive one is wasted.
Identifying the Excess Reactant
Determining the excess reactant requires a systematic, step-by-step comparison based on the mole ratios from the balanced chemical equation.
Converting Mass to Moles
The first step is to convert the given masses of all reactants into moles using their respective molar masses. This conversion is necessary because the balanced equation’s coefficients represent the ratio of moles, not mass.
Identifying the Limiting Reactant
The available moles of each reactant must be compared to the required amounts based on stoichiometry. This is often done by calculating how much product could be formed from the initial amount of each reactant. The reactant that would produce the smaller amount of product is identified as the limiting reactant. Consequently, the other substance is the excess reactant.
Calculating Leftover Material
The final step involves calculating the amount of material that remains unconsumed after the reaction is finished. This is accomplished by using the moles of the limiting reactant to calculate the exact amount of the excess reactant that was used up in the process. Subtracting the consumed amount from the initial amount provides the leftover quantity, which can then be converted back to a mass measurement if needed. This final calculation is vital for process control, waste management, and determining the overall efficiency of the chemical synthesis.