Lecture 17. limiting reactant. Professor Hicks General Chemistry (CHE131) reactant that will run out first. causes reaction to STOP

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1 Lecture 17 Professor Hicks General Chemistry (CHE131) limiting reactant reactant that will run out first causes reaction to STOP LR is the reactant that is present in the smallest of also called limiting reagent compound moles of stoichiometric are used to calculate the limiting reactant 1

2 Ferrari factory are like sets assembly process parts on hand in factory sets on hand in factory 1 car body + 4 wheels + 2 seats 1 car 1000 car bodies = wheels 1900 seats = how many cars can be built? = cars that can be built = smallest # sets of parts when building the cars sets of seats will run out first limiting reagent / 1 car body + 4 wheels + 2 seats 1 car of car bodies of wheels of seats like of cars of N 2 (molecular nitrogen) of H 2 (molecular hydrogen) NH 3 (ammonia) N H 2 2 NH 3 of N 2 = of NH 3 = for this reaction! of H 2 = 3 moles H 2 Q. why didn t we worry about limiting reactant here? Example: How many moles of NH 3 will form if 0.78 moles of H 2 reacts in the reaction: N 2 (g) + 3H 2 (g) 2NH 3 (g) A. this means N 2 will not run out or limit the reaction H 2 is the LR 2 moles NH 0.78 moles H 2 x 3 = moles H 2 limiting reagent 3 moles H 2 3 moles H 2 conversion factor 3 mole H 2 3 mole H 2 amount of product (theoretical yield) and their one overs 2

3 calculating How many moles of AlCl 3 can be produced from 15.0 g HCl and 12.0 g Al(OH) 3? calculate amounts based on limiting reactant initial rxnts moles g/mol g/mol 3HCl (aq) + 1 Al(OH) 3 (aq) 3H 2 O (l) + AlCl 3 (aq) 15.0 g 12.0 g = = limiting reactant! smaller # = = moles of by stoichiometric moles HCl x 1 mole AlCl 3 3 moles HCl limiting reactant conversion factor from reaction = moles AlCl 3 theoretical yield compound moles of stoichiometric theoretical yield of a reaction amount of product obtained based on limiting reactant often expressed in grams does not account for experimental errors 3

4 calculating theoretical yield Example: Calculate the theoretical yield of CO 2 in grams If 10.0 grams lactic acid (C 3 H 6 O 3 ) undergoes combustion. 1) write combustion reaction 2) balance 3) calculate all es C 3 H 6 O 3 (s) + O 2 (g) CO 2 (g) + H 2 O (l) C 3 H 6 O 3 (s) + 3O 2 (g) 3CO 2 (g) + 3 H 2 O (l) 4) rewrite as equation 90.0 g C 3 H 6 O 3 (s) + 3x32.0 g O 2 (g) 3x44.0 g CO 2 (g) + 3x18.0 g H 2 O (l) 10.0 g C 3 H 6 O 3 x CO 2 C 1 x 12.0 O 2 x g/mol 3 x 44.0 g CO 2 10 x 3 x g C 3 H 6 O = = 14.7 g CO conversion factor from reaction etc. lactic acid C 3 H 6 O 3 C 3 x 12.0 H 6 x 1.0 O 3 x g/mol percent yield of a reaction = actual yield theoretical yield x 100% I finished the experiment early again! Theoretical Yield percent lost percent yield the part of the theoretical yield obtained reflects all experimental error usually 0 to 100% calculating percent yield Example: a) Calculate the theoretical yield of CO 2 in grams If 10.0 grams lactic acid (C 3 H 6 O 3 ) undergoes combustion. C 3 H 6 O 3 (s) + 3O 2 (g) 3CO 2 (g) + 3 H 2 O (l) 90.0 g C 3 H 6 O 3 (s) + 3x32.0 g O 2 (g) 3x44.0 g CO 2 (g) + 3x18.0 g H 2 O (l) 10.0 g C 3 H 6 O 3 x 3 x 44.0 g CO 2 10 x 3 x g C 3 H 6 O = = 14.7 g CO b) If 0.88 grams of CO 2 were obtained what was the percent yield of the reaction? actual yield 0.88 g percent yield = theoretical yield x 100% x 100% = 6.0 % 14.7 g 4

5 Heroin has the molecular formula C 21 H 23 NO 5. It is synthesized from morphine a natural product isolated from the poppy plant. Morphine has the molecular formula C 17 H 19 NO 3. Heroin is more potent than morphine because it enters the brain more efficiently, though once in the brain it is converted into morphine. Heroin has numerous street names including: horse, honk, junk, dope, H, and smack. 1 The reaction for synthesis of smack from morphine can be written, Mass (g) Moles Equivalents C 17 H 19 NO 3 (s) + 2 C 2 H 4 O 2 (l) C 21 H 23 NO 5 (s) + 2 H 2 O (l) a) What is the limiting reagent if 50.0 grams of morphine is reacted with 45.0 grams of C 2 H 4 O 2? b) Write the chemical reaction for the synthesis of smack from morphine using the interpretation. c) Calculate the theoretical yield of smack based on the limiting reagent from part a. d) Calculate the percent yield if g of smack was obtained. 1)

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