Brief Summary
This video explains the concepts of limiting and excess reactants using real-life examples and chemical equations. It provides a simple trick to identify these reactants and demonstrates how to solve numerical problems involving them, including calculating the amount of product formed and the amount of excess reactant leftover.
- Limiting reactant is the reactant that is completely used up in a chemical reaction.
- Excess reactant is the reactant that is left over after the reaction stops.
- The reactant with the smallest mole-to-coefficient ratio is the limiting reactant.
Daily Life Example of Limiting and Excess Reactants
The concept of limiting and excess reactants is illustrated using a marriage analogy. With 5 men and 4 women, only 4 couples can form, leaving one man unmarried. The 4 women are the limiting reactants because they are all "used up," while the 5 men are the excess reactants because one is "leftover." The limiting reactant dictates when the reaction (marriage) stops, even if excess reactant (the single man) is still available.
Definition of Limiting and Excess Reactants
The limiting reactant is the first reactant to be completely consumed in a chemical reaction, thereby halting the formation of additional products. Conversely, the excess reactant is the reactant that remains after the chemical reaction has ceased, representing the unused portion. It's important to note that the limiting reactant isn't necessarily the one present in the smallest amount.
Easy Trick to Identify Limiting and Excess Reactants
To identify the limiting reactant, calculate the simplest molar ratio for each reactant by dividing the number of moles by its coefficient in the balanced chemical equation. The reactant with the smaller ratio is the limiting reactant, while the other is the excess reactant. For example, in the reaction of 5 moles of carbon with 2 moles of oxygen, the balanced equation shows a 1:1 ratio. Dividing the moles by the coefficients gives ratios of 5 for carbon and 2 for oxygen. Since 2 is smaller, oxygen is the limiting reactant.
Numerical Question: Finding Amount of H2O
The video demonstrates how to solve a numerical problem: determining the amount of H2O produced from 5 moles of H2 and 2 moles of O2, and identifying the limiting and excess reactants. First, balance the chemical equation. Then, calculate the mole-to-coefficient ratio for each reactant to identify the limiting reactant (O2 in this case). Next, use the ratio of limiting reactant to excess reactant from the balanced equation to find out how much of the excess reactant (H2) is used. Finally, calculate the moles of H2O formed using the ratio of limiting reactant to product.
Numerical Question: Finding Mass of Carbon Dioxide Gas
The video explains how to calculate the mass of carbon dioxide gas (CO2) formed from the reaction of 20 grams of glucose with 50 grams of oxygen. First, convert the given masses of reactants into moles by dividing by their respective molar masses. Then, identify the limiting reactant by comparing the mole-to-coefficient ratios. Use the limiting reactant to determine the moles of CO2 produced based on the balanced chemical equation. Finally, convert the moles of CO2 to grams by multiplying by its molar mass.
