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Limiting & Excess Reactants How do you know which one is which?

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Presentation on theme: "Limiting & Excess Reactants How do you know which one is which?"— Presentation transcript:

1 Limiting & Excess Reactants How do you know which one is which?

2 What does limiting & excess mean?  Limiting Reactant - the reactant that runs out first in a chemical reaction, thus determining the amount of product produced  Excess Reactant - the reactant that there is a quantity of left over after a chemical reaction  *The excess reactant should be the cheaper reactant since we do not like to waste unused chemical  *The excess reactant should be easy to isolate from the product(s) once the reaction is complete since we do not want contamination  Limiting Reactant - the reactant that runs out first in a chemical reaction, thus determining the amount of product produced  Excess Reactant - the reactant that there is a quantity of left over after a chemical reaction  *The excess reactant should be the cheaper reactant since we do not like to waste unused chemical  *The excess reactant should be easy to isolate from the product(s) once the reaction is complete since we do not want contamination

3 Vinegar (5% by mass acetic acid) and Baking Soda Demo  NaHCO 3 (s) + HC 2 H 3 O 2 (aq)  NaC 2 H 3 O 2 (aq) + H 2 O (l) + CO 2 (g) Flask #1Flask #2Flask #3 Mass of Baking Soda 1.50 g3.50 g5.50 g Mass of Vinegar g (2.5 g HC 2 H 3 O 2 ) g (2.5 g HC 2 H 3 O 2 ) g (2.5 g HC 2 H 3 O 2 )

4 Outcome from Demo  Flask #1 showed the smallest amount of CO 2 produced in the balloon with a clear solution in the flask.  Flask #2 showed the same amount of CO 2 produced in the balloon as in flask #3, which was a larger amount compared to flask #1. The solution in the flask was still clear like in flask #1.  Flask #3 showed the same amount of CO 2 produced in the balloon as in flask #2. However, the solution in the flask was cloudy.  Flask #1 showed the smallest amount of CO 2 produced in the balloon with a clear solution in the flask.  Flask #2 showed the same amount of CO 2 produced in the balloon as in flask #3, which was a larger amount compared to flask #1. The solution in the flask was still clear like in flask #1.  Flask #3 showed the same amount of CO 2 produced in the balloon as in flask #2. However, the solution in the flask was cloudy.

5 Which reactant was limiting and which reactant was excess in each flask based off observations? Flask #1Flask #2Flask #3 Baking Sodalimitingtheoretical amount excess Vinegarexcesstheoretical amount limiting

6 The proof is in the stoichiometry 50.0 g vinegar 5 g HC 2 H 3 O 2 1 mol HC 2 H 3 O 2 1 mol NaHCO g NaHCO g NaHCO g vinegar g HC 2 H 3 O 2 1 mol HC 2 H 3 O 2 1 mol NaHCO 3 The theoretical amount of baking soda needed to react completely with the 50.0 grams of vinegar is 3.50 g.

7 Practice Problem 2NaI + Cl 2  2NaCl + I 2  1. You are given 22.1 g of NaI and 4.13 g of Cl 2. What is the limiting reactant?  Pick one of the values given for your reactants and through stoichiometry find out how much you need of the other reactant. 2NaI + Cl 2  2NaCl + I 2  1. You are given 22.1 g of NaI and 4.13 g of Cl 2. What is the limiting reactant?  Pick one of the values given for your reactants and through stoichiometry find out how much you need of the other reactant.

8 22.1 g NaI1 mol NaI1 mol Cl g Cl g Cl g NaI 2 mol NaI1 mol Cl 2 You have 4.13 g Cl 2 available You need 5.23 g to react completely with 22.1 g NaI Therefore, Cl 2 is the limiting reactant which will run out first in the reaction and determine the amount of product produced.

9 2.How much NaCl is produced? *Remember that you must always start with the limiting reactant quantity that you have because this is what determines the amount of product produced. 2.How much NaCl is produced? *Remember that you must always start with the limiting reactant quantity that you have because this is what determines the amount of product produced g Cl 2 1 mol Cl 2 2 mol NaCl58.44 g NaCl6.81 g NaCl g Cl 2 1 mol Cl 2 1 mol NaCl

10 3.How much excess reactant will be left over? *Remember that you must always start with the limiting reactant quantity that you have because this is what determines the amount of the excess reactant you will need. 3.How much excess reactant will be left over? *Remember that you must always start with the limiting reactant quantity that you have because this is what determines the amount of the excess reactant you will need g Cl 2 1 mol Cl 2 2 mol NaI g NaI 17.5 g NaI g Cl 2 1 mol Cl 2 1 mol NaI You have 22.1 g available You need 17.5 g 22.1 g NaI – 17.5 g NaI = 4.6 g NaI will be left over after the reaction is complete

11 % Yield  Chemists like to know how right we are  % yield = (actual/theoretical) x 100  Actual  lab results  Theoretical  mathematical prediction through stoichiometry % error = |theoretical – experimental|/theoretical x 100 % error + % yield = 100  Chemists like to know how right we are  % yield = (actual/theoretical) x 100  Actual  lab results  Theoretical  mathematical prediction through stoichiometry % error = |theoretical – experimental|/theoretical x 100 % error + % yield = 100


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