What is a Limiting Reactant?
In chemical reactions, usually one chemical gets used up before the other and this is known as the limiting reactant. The limiting reactant in a chemical reaction limits the amount of products that can be formed. Additionally, the reaction will stop when all of the limiting reactant is consumed.
What is an Excess Reactant?
The excess reactant in a chemical reaction remains when a reaction stops and when the limiting reactant is completely consumed. The excess reactant remains because there is nothing with which it can react.
How to Find What is the Limiting Reactant and Excess Reactant in a Reaction?
Prior to solving a limiting reagent problem, you must:
- write the balanced chemical reaction
- determine the stoichiometry of the reaction, or the relative proportion of components in the equation
Example One:
90.0 g of FeCl3 reacts with 52.0 g of H2S. What is the limiting reactant? What is the mass of HCl produced? What mass of excess reactant remains after the reaction?
Balanced Chemical Reaction:
- 2FeCl3 + 3H2S ----> 1Fe2 S3 + 6HCl
Example Two:
3.50 g of Silver Nitrate reacts with 3.50 g of Copper (II) Chloride. What is the limiting reactant? What is the number of moles of excess reactant which remains after the reaction?
Balanced Chemical Reaction:
- 2AgNO3 + 1CuCl2 ----> 1AgCl2 S3 + 1Cu(NO3)2
Limited Reactant:
- 3.50 g of AgNO3 x (1 mol/ 169.9 g) x (1/2) x (134.5 g/1 mol) = 1.39 grams of CuCl2 (ENOUGH)
- 3.50 g of CuCl2 x (1 mol/ 134.5 g) x (2/1) x (169.5 g/1 mol) = 8.82 grams of AgNO3 (NOT ENOUGH, AgNO3 is the limiting reactant)
Excess Reactant:
CuCl2 is the excess reactant so
- 3.50 - 1.39 = 2.11 x (1 mol/134.5g) = 0.0157 mol of CuCl2
- 0.0157 mol of CuCl2 = 1.57 x 10^-2 excess mol of CuCl2
The following video should provide you with any additional or more thorough knowledge on the very important concept of limiting and excess reactants:
- Simon Sierra
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