2 Limiting Reactants You have a lit candle You put a jar over the candle What will happen?It will go outWhy?It runs out of oxygen
3 Limiting Reactants Limiting reactant – A chemical reaction will stop when you run out of one of your productsLimiting reactant –limits the extent of the reaction.Determines the amount of product that is formed. It runs out firstExcess reactant – left over reactant
4 Limiting Reactant Problems A limiting reactant problem can be easily identified because you have TWO givensYou are going to do 2 stoichiometry problems
5 Steps Write and balance the chemical equation Start with given #1 and go to desired productStart with given #2 and go to the same productThe one that formed the least amount of grams is the limiting reactant
6 ExampleS8 + 4Cl2 4S2Cl2g of S8 and g of Cl2 are combined in a flask. How much S2Cl2 will you get?Start with given #1g S8 x 1 mol S8 x 4 mol S2Cl2 x g S2Cl2 = g S2Cl2256.5 g S mol S mol S2Cl2
7 Example Now do given #2 Now we have a decision to make. S8 + 4Cl2 4S2Cl2g of S8 and g of Cl2 are combined in a flask. How much S2Cl2 will you get?Now do given #2g Cl2 x 1 mol Cl2 x 4 mol S2Cl2 x g S2Cl2 = g S2Cl270.91 g Cl mol Cl mol S2Cl2Now we have a decision to make.Did we make g S2Cl2 or g S2Cl2The answer is g.After that amount, the reaction stopped
8 Other questions What was the limiting reactant? Cl2 What was the excess reactant?S8How much excess did we have left over after the reaction was completed?
9 Other questionsBefore we can determine how much excess was left over, we must figure out how much was used.There are several ways to do this.Using stoichiometry, start with how much product you formed ORthe total amount of the limiting reactant and go to the amount (g) of excess
10 Other questions This will tell you how much excess was used Now you subtract how much you used from how much you started with to give you the amount left over
11 Other questions In the previous example g Cl2 x 1 mol Cl2 x 1 mol S8 x g S8 = g S870.91 g Cl mol Cl mol S8This tells you how much S8 was used. We want to know how much was left overWe started with g S8 and we used g S8.Therefore, we must have g S8 left over
12 Another Example P4 + 5O2 P4O10 If we used 25.4 g P4 and 50.0 g O2 answer the following questions.What is the limiting reactant?What is the excess reactant?How much P4O10 will you get?How much excess did you use?How much excess will you have left over?
13 Another Example What is the limiting reactant? P4 What is the excess reactant?O2How much P4O10 will you get? g P4O10How much excess did you use? g O2 usedHow much excess will you have left over? g O2 left over
14 % Yield Remember % = (part / whole) x 100 When performing an experiment, things do not always go exactly perfectSome product may get spilled, some may get sneezed on, or the reaction may not have gone to completion
15 % YieldTheoretical yield –amount of product you should get if the experiment went perfectly. You get this number from stoichiometryActual Yield – this is what you actually got in the lab. You measure this on a balance% yield – how close you were to the correct answer% yield = (actual / theoretical) x 100
16 % Yield Example K2CrO4 + 2AgNO3 Ag2CrO4 + 2KNO3 What is the theoretical yield of Ag2CrO4 formed from g AgNO30.500 g AgNO3 x 1 mol AgNO3 x 1 mol Ag2CrO4 x g Ag2CrO4 = g169.9 g AgNO mol AgNO mol Ag2CrO Ag2CrO4What is the % yield if g is actually formed?% Yield = (actual / theoretical) x 100% Yield = (0.455 / 0.488) x 100 = 93.2%