Calculating excess reactants is essential in the practical application of chemistry. It allows us to assess the efficiency of a chemical reaction, and make predictions about the final products formed during such processes. In this article, we’ll provide a step-by-step guide on how to calculate excess reactants for a given chemical equation.

Step 1: Balance the chemical equation

Before determining the excess reactant, ensure that the given chemical equation is balanced. A balanced equation has equal numbers of atoms for each element on both sides of the equation.

Step 2: Identify the limiting and excess reactants

In a chemical reaction, one or more reactants are often added in an excess amount to ensure complete reaction of another reactant, which is present in limited quantities. Compare the stoichiometric ratios to identify which reactant will be used up first (the limiting reactant) and which will remain after the reaction (the excess reactant).

Step 3: Calculate moles of each reactant

Use the given mass of each reactant to calculate the number of moles using their molar masses. To do this, divide the mass by its molar mass (found on a periodic table or through calculations).

Moles = mass / molar mass

Step 4: Determine stoichiometric ratios

Find the ratio of moles for each reactant in the balanced equation. This involves dividing each mole value by their respective coefficients in the balanced equation.

Stoichiometric ratio = moles / coefficient

Step 5: Compare stoichiometric ratios

Compare the stoichiometric ratios calculated in Step 4 with those obtained from Step 2. The smallest ratio will indicate which substance is the limiting reactant, while chemicals with greater ratios are excess reactants.

Step 6: Calculate mass of excess reactant remaining

To determine the amount of excess reactant remaining after the reaction is complete, find the moles of excess reactant consumed during the reaction. First, calculate the moles of limiting reactant that would react with 1 mole of the excess reactant using balanced stoichiometric coefficients. Next, multiply the moles of limiting reactant by that ratio, then subtract this amount from the original moles of excess reactant.

Remaining moles = initial moles – (moles of limiting reactant × stoichiometric ratio)

Finally, convert the remaining moles to mass by multiplying by their respective molar mass.

Mass remaining = remaining moles × molar mass

Conclusion:

Following these steps will allow you to calculate the excess reactant remaining after a chemical reaction is completed. Understanding how to determine excess reactants is important for industrial applications and laboratory experimentation, as it provides insight into the efficiency of chemical processes and minimizes waste.