Section 3: Limiting

ReactantsStoichiometry

Learning Goals:0 Identify the limiting reactant in a chemical

equation.

0 Identify the excess reactant, and calculate the amount remaining after the reaction is complete.

0 Calculate the mass of a product when the amounts of more than one reactant are given.

Why Do Reactions Stop?0 Reactions proceed until one of the reactants

is used up and one is left in excess.

0 The limiting reactant limits the extent of the reaction and, thereby, determines the amount of product formed.

0 The excess reactants are all the leftover unused reactants.

Why Do Reactions Stop?0Determining the limiting reactant is

important because the amount of the product formed depends on this reactant.

Calculating Products0Ex.) S8(l) + 4Cl2(g) → 4S2Cl2(l)

0 If 200.0g of sulfur reacts with 100.0g of chlorine, what mass of disulfur dichloride is produced?

Calculating Products1. Determine moles of reactants

Calculating Products2. Determine whether the two reactants are in

the correct mole ratio, as given in the balanced chemical equation.

Only 1.808 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 Products3. Calculate the amount of product formed.

Calculating Products0 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?

1.You need to make a mole-to-mass calculation to determine the mass of sulfur needed to react completely with 1.410 mol of chlorine.

Calculating Products2. Next, obtain the mass of sulfur

needed:

Calculating Products3. Knowing that 200.0g of sulfur is available

and only 90.42g is needed, you can calculate the amount of sulfur left unreacted when the reaction ends.

Calculating Products0Using an excess reactant can speed

up the reaction.

0Using an excess reactant can drive a reaction to completion.

Practice0 The reaction between solid white phosphorus

(P4) and oxygen produces solid tetraphosphorus decoxide.

0 Determine the mass of P4O10 formed if 25.0 g of P4 and 50.0 g of oxygen are combined.

0 How much of the excess reactant remains after the reaction stops?

Section 4: Percent YieldStoichiometry

Learning Goals0Calculate the theoretical yield of a

chemical reaction from data.

0Determine the percent yield for a chemical reaction.

How Much Product?0Laboratory reactions do not always

produce the calculated amount of products.

0Reactants stick to containers.

0Competing reactions form other products.

How Much Product?

How Much Product?0 The theoretical yield is the maximum

amount of product that can be produced from a given amount of reactant.

0 The actual yield is the amount of product actually produced when the chemical reaction is carried out in an experiment.

How Much Product?0The percent yield of a product is the

ratio of the actual yield expressed as a percent.

Practice0 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 0.455 g of silver chromate.