4.2
Limiting Reactant
The relative amounts of reactants and products represented in a balanced chemical equation are often referred to as stoichiometric amounts. However, i…
In a chemical reaction, reactants interact with each other to form products. The reactant that is completely consumed is the limiting reactant, and the reactant present in a quantity greater than necessary to react completely with the limiting reactant is the excess reactant.
A recipe analogy is helpful to understand these concepts. Here, one cup of flour, 2 eggs, and 3 tablespoons of sugar make five waffles. If there are 3 cups of flour, 4 eggs, and 8 tablespoons of sugar, how many waffles can be made?
There is enough flour to make 15 waffles and enough sugar to make 13 1/3 waffles. However, there are only enough eggs for 10 waffles. Here, egg is the limiting reactant because it makes the least amount of waffles, while the flour and sugar are in excess.
Now, consider the combustion reaction between methane and oxygen to produce carbon dioxide and water. Recall that the coefficients of a balanced equation represent the stoichiometric amounts of the reactants and products.
Therefore, the stoichiometric mole ratio of methane to carbon dioxide is one to one, and that of oxygen to carbon dioxide is two to one.
Suppose there are 80 grams of methane and 128 grams of oxygen. Which is the limiting reactant, and how much carbon dioxide will form? First, the masses of the reactants must be converted to moles using their molar masses.
Stoichiometrically, 5 moles of methane produce 5 moles of carbon dioxide, while 4 moles of oxygen produce only 2 moles of carbon dioxide. Since oxygen produces the least amount of carbon dioxide, it is the limiting reactant, while methane is the excess reactant.
Knowing the limiting reactant, the number of product moles can be converted to grams. Therefore, 88 grams of carbon dioxide can be produced.
But how much methane is unreacted? The mole ratio of methane to oxygen indicates that four moles of oxygen would completely react with 2 moles of methane. So, 3 moles of unreacted methane are in excess.
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Q1: What is a limiting reactant in a chemical reaction?
The limiting reactant is the substance that is completely consumed during a chemical reaction and determines the maximum amount of product formed. Once the limiting reactant is used up, the reaction stops, even if other reactants remain. In contrast, the excess reactant is present in a quantity greater than necessary to react completely with the limiting reactant.
Q2: How do you identify the limiting reactant using mole ratios?
To identify the limiting reactant, convert the masses of all reactants to moles using their molar masses. Then compare the mole ratio of reactants provided to the stoichiometric ratio from the balanced equation. The reactant with a ratio lower than the stoichiometric ratio is the limiting reactant. This approach relies on understanding reaction stoichiometry stoichiometric coefficient and ratio relationships.
Q3: What is an alternative method for finding the limiting reactant?
An alternative approach involves calculating the amount of product formed in moles from each reactant separately, using the balanced equation's stoichiometry. Compare the product amounts calculated for each reactant. The reactant that produces the least amount of product is the limiting reactant, while others are in excess.
Q4: How much product forms when the limiting reactant is identified?
Once you identify the limiting reactant, use its mole amount and the stoichiometric ratio from the balanced equation to calculate product moles. Convert the product moles to grams using the product's molar mass. This gives the theoretical maximum amount of product that can form from the available limiting reactant.
Q5: Why is excess reactant left over after a reaction completes?
Excess reactant remains because it is present in a quantity greater than the stoichiometric amount needed to react completely with the limiting reactant. The reaction stops when the limiting reactant is entirely consumed, leaving unreacted excess reactant behind. The amount of unreacted excess can be calculated by subtracting the moles consumed from the initial moles available.
Q6: Can you use a recipe analogy to understand limiting reactants?
Yes. In a recipe, if one cup of flour, two eggs, and three tablespoons of sugar make five waffles, and you have three cups of flour, four eggs, and eight tablespoons of sugar, eggs become the limiting ingredient. Flour and sugar are in excess because eggs run out first, limiting waffle production to ten instead of the maximum possible amount.
Q7: How do stoichiometric ratios help predict which reactant is limiting?
Stoichiometric ratios from a balanced equation show the proportions in which reactants should combine. If the actual mole ratio of reactants differs from the stoichiometric ratio, one reactant will be limiting. For example, if hydrogen and oxygen should combine in a 2:1 ratio but are provided in a 2.5:1 ratio, oxygen becomes the limiting reactant.