In this case, the headlights are in excess. Because the number of cars formed by 20 tires is less than number of cars produced by 14 headlights, the tires are the limiting reagent they limit the full completion of the reaction, in which all of the reactants are used up. This scenario is illustrated below:. The initial condition is that there must be 4 tires to 2 headlights.
The reactants must thus occur in that ratio; otherwise, one will limit the reaction. There are 20 tires and 14 headlights, so there are two ways of looking at this problem. For 20 tires, 10 headlights are required, whereas for 14 headlights, 28 tires are required.
Because there are not enough tires 20 tires is less than the 28 required , tires are the limiting "reactant. The limiting reagent is the reactant that is completely used up in a reaction, and thus determines when the reaction stops.
From the reaction stoichiometry , the exact amount of reactant needed to react with another element can be calculated. If the reactants are not mixed in the correct stoichiometric proportions as indicated by the balanced chemical equation , then one of the reactants will be entirely consumed while another will be left over.
The limiting reagent is the one that is totally consumed; it limits the reaction from continuing because there is none left to react with the in-excess reactant. There are two ways to determine the limiting reagent. One method is to find and compare the mole ratio of the reactants used in the reaction approach 1. Another way is to calculate the grams of products produced from the given quantities of reactants; the reactant that produces the smallest amount of product is the limiting reagent approach 2.
Find the limiting reagent by calculating and comparing the amount of product each reactant will produce. What mass of carbon dioxide forms in the reaction of 25 grams of glucose with 40 grams of oxygen?
Step 1: Determine the balanced chemical equation for the chemical reaction. Step 2: Convert all given information into moles most likely, through the use of molar mass as a conversion factor. Step 3: Calculate the mole ratio from the given information. Compare the calculated ratio to the actual ratio. If all of the 1. There is only 0. If more than 6 moles of O 2 are available per mole of C 6 H 12 O 6 , the oxygen is in excess and glucose is the limiting reactant.
If less than 6 moles of oxygen are available per mole of glucose, oxygen is the limiting reactant. This gives a 4. Calculate the mass of limiting reactant needed to react with the leftover excess reactant. It takes 0. Here is another example How do you determine how much of the excess reactant is left over?
Also, how do you determine how much MORE of the limiting reagent would you need to use up the excess? Chemistry Stoichiometry Limiting Reagent. Ernest Z. Mar 7, The general problem Given the chemical equation and the masses of reactants, determine the mass of excess reactant and the mass of the limiting reactant required to use up the excess. A specific problem A 2.
Strategy Write the chemical equation. Calculate the moles of product from the first reactant. Mass of excess reactant calculated using the limiting reactant :. Mass of total excess reactant given — mass of excess reactant consumed in the reaction:. Since the amount of product in grams is not required, only the molar mass of the reactants is needed. Because the question only asks for the limiting reactant, we can perform two mass-mole calculations and determine which amount is less.
Given: 5. Because the question asks what mass of magnesium is formed, we can perform two mass-mass calculations and determine which amount is less.
The 0. To determine how much of the other reactant is left, we have to do one more mass-mass calculation to determine what mass of MgCl 2 reacted with the 5. Given the initial amounts listed, what is the limiting reactant, and what is the mass of the leftover reactant? This page was constructed from content via the following contributor s and edited topically or extensively by the LibreTexts development team to meet platform style, presentation, and quality:.
Learning Objectives Identify the limiting reactant limiting reagent in a given chemical reaction. Calculate how much product will be produced from the limiting reactant. Calculate how much reactant s remains when the reaction is complete. PhET Simulation: Reactants, Products and Leftovers View this interactive simulation illustrating the concepts of limiting and excess reactants.
Determine the balanced chemical equation for the chemical reaction. Convert all given information into moles most likely, through the use of molar mass as a conversion factor. Calculate the mole ratio from the given information. Use the amount of limiting reactant to calculate the amount of product produced. If necessary, calculate how much is left in excess of the non-limiting excess reactant. Balance the chemical equation for the chemical reaction. Convert the given information into moles.
Use stoichiometry for each individual reactant to find the mass of product produced. The reactant that produces a lesser amount of product is the limiting reactant. The reactant that produces a larger amount of product is the excess reactant. To find the amount of remaining excess reactant, subtract the mass of excess reactant consumed from the total mass of excess reactant given.
Solution Using Approach 1: Step 1: Balance the chemical equation. Using Approach 2: Step 1: Balance the chemical equation. Solution Following Approach 1: Step 1: Balance the chemical equation. Balance the equation. Cancel units and calculate. Identify the "given" information and what the problem is asking you to "find. Find mass Mg formed based on mass of Rb Find mass of Mg formed based on mass of MgCl 2 Use limiting reactant to determine amount of excess reactant consumed Cancel units and calculate.
It usually is not possible to determine the limiting reactant using just the initial masses, as the reagents have different molar masses and coefficients.
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