The Limiting Reactant We will use what we know about stoichiometry to solve problems where there may be one reactant in limited amount, which thus affects how much product can be produced.

Stoichiometry Review Write a balanced chemical equation If you are given the mass or number of particles of a substance, convert it to the number of moles Calculate the number of moles of the required substance based on the number of moles of the given substance using the appropriate mole ratio Convert the number of moles of the required substance to mass or number of particles, as directed by the question

A balanced chemical equation shows the mole ratios of the reactants and products. The coefficients in a reaction are sometimes referred to as the stoichiometric coefficients. When the amount of reactants is exactly enough to produce the amount of product called for by the mole ratio, then reactants are said to be in stoichiometric amounts. This means that once the reaction is complete, there are no reactants left. In the lab, however, there are often reactants left.

Given the smores analogy  To make one smore you need… 2 graham crackers + 1 marshmallow + 10chocolate chips  1 smore Now lets say you have: 6 crackers + 10 marshmallows + 10 chocolate chips How many smores can you make?  1 What is in excess? Marshmallows , crackers What is limiting the number of smores that you can produce? The number of chocolate chips

Determining the Limiting Reactant The reactant that is completely used up in a chemical reaction is called the limiting reactant . It determines how much product is produced. Why? When the limiting reactant is used up, the reaction stops. A reactant that is left over after a reaction is over is called the excess reactant. Once the limiting reactant is used up, no more product is made regardless of how much excess reactant remains. When you are given amounts of two or more reactants to solve a stoichiometric problem, you need to identify the limiting reactant. To do this, you must determine how much product each amount of reactant would produce if the other reactant was in excess. The reactant that would produce the least amount of product is the limiting reactant.

Problem Lithium nitride reacts with water to form ammonia and lithium hydroxide according the to following balanced equation: Li3N(s) + 3H2O(l)  NH3(g) + 3LiOH(aq)  If 4.87g of lithum nitride reacts with 5.80g of water, find the limiting reactant.

How can we Use this Idea Now you can use the information about limiting reactants in stoichiometric problems. Here are the steps you need to take: (note change from handout) write a balanced chemical equation identify the limiting reactant by finding the number of moles of one of the products each reactant can make; the one the can make fewer is limiting convert the amount of the required substance to mass or number of particles, as directed by the question.

  Problem White phosphorus consists of a molecule made up of four phosphorus atoms. It burns in pure oxygen to produce tetraphosphorus decaoxide. P4 + 5O2  P4O10 A 1.00g piece of phosphorus is burned in a flask filled with 2.60 x1023 molecules of oxygen gas. What mass of tetraphosphorus decaoxide is produced?

  Practice Problems # 1 – 3 on pg. 334 # 1, 3, 7 -10 on pg. 335