# Section 3: Limiting Reactants

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Section 3: Limiting Reactants
Stoichiometry

Learning Goals: Identify the limiting reactant in a chemical equation.
Identify the excess reactant, and calculate the amount remaining after the reaction is complete. Calculate the mass of a product when the amounts of more than one reactant are given.

Why Do Reactions Stop? Reactions proceed until one of the reactants is used up and one is left in excess. The limiting reactant limits the extent of the reaction and, thereby, determines the amount of product formed. The excess reactants are all the leftover unused reactants.

Why Do Reactions Stop? Determining the limiting reactant is important because the amount of the product formed depends on this reactant.

Calculating Products Ex.) S8(l) + 4Cl2(g) → 4S2Cl2(l)
If 200.0g of sulfur reacts with 100.0g of chlorine, what mass of disulfur dichloride is produced?

Calculating Products Determine moles of reactants

Calculating Products Determine whether the two reactants are in the correct mole ratio, as given in the balanced chemical equation. Only mol of chlorine is available for every 1 mol sulfur, instead of the 4mol of chlorine required by the balanced chemical equation, thus chlorine is the limiting reactant.

Calculating Products Calculate the amount of product formed.

Calculating Products Now that you have determined the limiting reactant and the amount of product formed, what about the excess reactant, sulfur? How much of it reacted? You need to make a mole-to-mass calculation to determine the mass of sulfur needed to react completely with mol of chlorine.

Calculating Products Next, obtain the mass of sulfur needed:

Calculating Products Knowing that 200.0g of sulfur is available and only g is needed, you can calculate the amount of sulfur left unreacted when the reaction ends.

Calculating Products Using an excess reactant can speed up the reaction. Using an excess reactant can drive a reaction to completion.

Practice The reaction between solid white phosphorus (P4) and oxygen produces solid tetraphosphorus decoxide. Determine the mass of P4O10 formed if 25.0 g of P4 and 50.0 g of oxygen are combined. How much of the excess reactant remains after the reaction stops?

Section 4: Percent Yield
Stoichiometry

Learning Goals Calculate the theoretical yield of a chemical reaction from data. Determine the percent yield for a chemical reaction.

How Much Product? Laboratory reactions do not always produce the calculated amount of products. Reactants stick to containers. Competing reactions form other products.

How Much Product?

How Much Product? The theoretical yield is the maximum amount of product that can be produced from a given amount of reactant. The actual yield is the amount of product actually produced when the chemical reaction is carried out in an experiment.

How Much Product? The percent yield of a product is the ratio of the actual yield expressed as a percent.

Practice Solid silver chromate (Ag2CrO4) forms when potassium chromate is added to a solution containing 0.500g of silver nitrate. Determine the theoretical yield of silver chromate. Calculate the percent yield if the reaction produces g of silver chromate.