Chemistry – April 23, 2018 P3 Challenge – Consider the combustion reaction of 2 C3H8O + 9 O2  6 CO2 + 8 H2O. How many grams of CO2 will be formed from the combustion of C3H8O with 15.0 L of oxygen at STP? Today’s Objective – Limiting Reactants Agenda Percent yield S’mores and Hotdogs Limiting Reactant Problems Assignment: Limiting Reactant Worksheet Pick up if you don’t have it yet

Theoretical and Percent yield The mass of product you can calculate from a given mass of reactant is called the theoretical yield. (The result of a g-mol-mol-g calculation) When you actually perform the reaction with the given masses of reactants, you will typically recover some lesser amount of product. This is called the actual yield. Percent yield is the percent of theoretical yield you are actually able to recover experimentally: Ex: If 14.2 g of ZnCl2 are recovered from the reaction of 0.200 mol of zinc with excess hydrochloric acid, what is the percent yield of this reaction?

Picnic Limiting Reactant situations You purchase 3 packages of buns (10 in a pack) and 4 packages of hotdogs (8 in a pack). How many hotdogs can you serve if it takes one hotdog and one bun to make a serving? Which item (hotdogs or buns) limits the number of servings? Which item is in excess? How many of the excess item remain?

Picnic Limiting Reactant situations You purchase a box of graham crackers (1 box contains 48 cracker squares), 6 chocolate bars (each bar contains 8 squares of chocolate) and 2 bags of jumbo marshmallows (one back contains 30 marshmallows). If a s’mores recipe requires two crackers, 3 squares of chocolate and two marshmallows, how many s’mores can be made with these supplies? What is the limiting ingredient? Which ingredients are in excess and how much of each remains after all the s’mores have been made?

Limiting reactant Most of the time when chemical reactions occur, the exactly correct relative masses needed to react with one another are not present. As the reaction occurs, one of the reactants will “run out” before the other. This is the limiting reactant. It limits how much product can form. For a limiting reactant problem, you are told the masses of all reactants and asked a) which is the limiting reactant and b) how much product will form.

Limiting Reactant Problem Solving Calculate the theoretical yield starting with each of the given masses. (2 stoichiometry calculations) Whichever result is the smallest is the answer is how much product will form. The reactant you started with to get the smallest result is the limiting reactant. The other reactant is present in excess. Amount of excess reactant left over can be found by finding how much reacted and subtracting this from how much you started out with.

Limiting Reactant Sample Problem If 7.5 g of Zn are added to a solution containing 8.0 g of HCl, a) which is the limiting reactant and b) how much ZnCl2 will form? c) If 10.2 g ZnCl2 are recovered, what is the percent yield? d) Which reactant is present in excess? What mass remains after the reaction is done?

Limiting Reactant Problem If 15.8 g of carbon are allowed to react with 125 g of copper (I) oxide carbon monoxide and elemental copper are formed. a) Which is the limiting reactant and b) how much Cu will form? c) If 93.2 g Cu are recovered, what is the percent yield? d) Which reactant is present in excess? e) What mass remains after the reaction is done?

Exit Slip - Homework Exit Slip: Consider Zn + 2 HCl  ZnCl2 + H2 1) What is the limiting reactant if 3.5 moles of zinc are placed into a solution containing 4.6 moles of hydrochloric acid? 2) How many grams of ZnCl2 will form, theoretically? What’s Due? (Pending assignments to complete.) Limiting reactant worksheet What’s Next? (How to prepare for the next day) Read p100-102, p224-232, 241 p246-247