The Limiting Reactant Quantities in Chemical Reactions SCH 3U0

The Limiting Reactant Up until now, we have calculated our hearts out under the following assumptions:  Chemicals react in perfect amounts. In other words, all of the reactants are used up in a chemical reaction.  All of the reactants are converted into products. In other words, the maximum amount of product that can form, will form.

Excess and Limiting Reactants In any chemical reaction, one reactant is always present to EXCESS - in other words:  the reactants are mixed in imperfect proportions, the excess reactant is the one which has more than what is needed to react in the reaction.  when the reaction has stopped reacting, some of the excess reactant will be left over, unreacted.

Excess and Limiting Reactants The other reactant is LIMITING - in other words:  the reactant is entirely used up in the reaction, when the reaction has stopped reacting, the limiting reactant will be completely gone.  this reactant limits how much of the other reactant(s) react and it limits how much of the product(s) will form when the reaction is done.

Excess and Limiting Reactants Example: g of Iron reacts with 10.81g of Oxygen gas. How much iron (III) oxide forms? How do you recognize a limiting/excess question?  You are given a mass (or other quantity) for more than one reactant.

Solving Excess and Limiting Reactants Write out the balanced reaction equation BRE: ___Fe (s) + ___O 2(g) → ___Fe 2 O 3(s)

Solving Excess and Limiting Reactants Calculate the moles you HAVE for each reactant - MC1: Moles of Fe Moles of O 2

Solving Excess and Limiting Reactants Determine the moles of product that form for the amount of EACH reactant given - MR: Moles of Fe 2 O 3

Solving Excess and Limiting Reactants Compare the moles of product formed from each amount of reactant:  The reactant that gives the greatest amount of product is said to be in excess. This amount of product will not form because not all of this reactant reacts, some will be left unreacted.  The reactant that gives the least amount of product is said to be limiting. This amount of product is the theoretical yield. All of the limiting reactant is used up in the reaction.

Solving Excess and Limiting Reactants Important Note:  You ALWAYS use the LIMITING MOLES to calculate the amount of product formed.  That’s why it’s called the LIMITING reactant - it LIMITS the amount of product formed.

Solving Excess and Limiting Reactants Calculate the theoretical yield of the product in moles, then convert it to grams: