If we react 1g of Ca with 1g of Cl2, how many grams of CaCl2 will be formed as per the following equation?
Ca + Cl2 → CaCl2
Using the Law of Conservation of Mass we can say that the total weight of the reactants is 2g and hence the weight of the product (CaCl2) should also be 2g. But, only a little more than 1.5g of the product is formed. Why?
According to the Law of Conservation of Mass, the mass of the reactants should be equal to the mass of the products if all the reactants are consumed in the reaction. The catch is we will be able to us the Law of Conservation of Mass only if all the reactants take part in the reaction.
What if all the reactants are not consumed in the reaction? This brings in the concept of "Limiting Reagent".
Limiting Reagent in a reaction is that reactant that limits the amount of product that can be formed. The reaction will not proceed forward when this reactant is consumed completely. The other reactants may be left unreacted in the reaction mixture.
Let us take the example of an imaginary bicycle company to understand this. A bicycle has many parts. Some of them are the two wheels, the frame, the pedals, the tires, the handle bars, the brake shoes and the seat. This bicycle company has many independent units manufacturing these different parts that are needed to assemble one complete bicycle. Say, the wheel unit makes 1000 wheels in a day, the frame unit makes 1200 frames a day, the tire unit makes 1000 tires a day, the handle bar unit makes 2000 a day, the brake shoe unit makes 5000 a day and the seat unit makes 1 seat a day. How many complete bicycles can this company send out in a day? The answer is 1. Irrespective of the number of other parts manufactured, the cycle will not be complete without the seat, which is manufactured 1 a day.
Similarly, that reactant that brings less than required number of moles (or mass) for a reaction will be the limiting reagent.
Let us now get back to our question. According to the balanced equation, 1 mole of Ca will react with 1 mole of Cl2 and give 1 mole of CaCl2.
We have 1g of Ca which is (1⁄40) mole and 1g of Cl2 which is (1⁄71) mole.
(1⁄71) is less than (1⁄40) numerically.
This means that there is not enough Cl2 molecules to react with all the Ca atoms present. Hence, Cl2 is the "Limiting Reagent" in this reaction.
Now that we have identified the Limiting Reagent in this reaction, we will now base all our calculations on the weight or the number of moles of the Limiting Reagent.
According to the balanced equation, 1 mole of Cl2 gives 1 mole of CaCl2.
So, (1⁄71) mole of Cl2 will produce (1⁄71) mole of CaCl2.
The weight of CaCl2 produced thus will be (1⁄71 * 111) = 1.56g
Also, (1⁄71) mole of Cl2 will react only with (1⁄71) mole of Ca.
So, the mole of Ca left unreacted will be (1⁄40) - (1⁄71) = 0.011 mole.
Hence the weight the Ca left unreacted will be (0.011 * 40) = 0.44g
We will now be left with 1.56g of CaCl2 and 0.44g of unreacted Ca, the sum of which is the 2g of reactants taken.
So, if any of the reactant is left unreacted in a reaction, the total of the weight of the products formed and the weight of the reactant left unreacted will be equal to the weight of the reactants taken at the beginning for the reaction.
Ca + Cl2 → CaCl2
Using the Law of Conservation of Mass we can say that the total weight of the reactants is 2g and hence the weight of the product (CaCl2) should also be 2g. But, only a little more than 1.5g of the product is formed. Why?
According to the Law of Conservation of Mass, the mass of the reactants should be equal to the mass of the products if all the reactants are consumed in the reaction. The catch is we will be able to us the Law of Conservation of Mass only if all the reactants take part in the reaction.
What if all the reactants are not consumed in the reaction? This brings in the concept of "Limiting Reagent".
Limiting Reagent in a reaction is that reactant that limits the amount of product that can be formed. The reaction will not proceed forward when this reactant is consumed completely. The other reactants may be left unreacted in the reaction mixture.
Let us take the example of an imaginary bicycle company to understand this. A bicycle has many parts. Some of them are the two wheels, the frame, the pedals, the tires, the handle bars, the brake shoes and the seat. This bicycle company has many independent units manufacturing these different parts that are needed to assemble one complete bicycle. Say, the wheel unit makes 1000 wheels in a day, the frame unit makes 1200 frames a day, the tire unit makes 1000 tires a day, the handle bar unit makes 2000 a day, the brake shoe unit makes 5000 a day and the seat unit makes 1 seat a day. How many complete bicycles can this company send out in a day? The answer is 1. Irrespective of the number of other parts manufactured, the cycle will not be complete without the seat, which is manufactured 1 a day.
Similarly, that reactant that brings less than required number of moles (or mass) for a reaction will be the limiting reagent.
Let us now get back to our question. According to the balanced equation, 1 mole of Ca will react with 1 mole of Cl2 and give 1 mole of CaCl2.
We have 1g of Ca which is (1⁄40) mole and 1g of Cl2 which is (1⁄71) mole.
(1⁄71) is less than (1⁄40) numerically.
This means that there is not enough Cl2 molecules to react with all the Ca atoms present. Hence, Cl2 is the "Limiting Reagent" in this reaction.
Now that we have identified the Limiting Reagent in this reaction, we will now base all our calculations on the weight or the number of moles of the Limiting Reagent.
According to the balanced equation, 1 mole of Cl2 gives 1 mole of CaCl2.
So, (1⁄71) mole of Cl2 will produce (1⁄71) mole of CaCl2.
The weight of CaCl2 produced thus will be (1⁄71 * 111) = 1.56g
Also, (1⁄71) mole of Cl2 will react only with (1⁄71) mole of Ca.
So, the mole of Ca left unreacted will be (1⁄40) - (1⁄71) = 0.011 mole.
Hence the weight the Ca left unreacted will be (0.011 * 40) = 0.44g
We will now be left with 1.56g of CaCl2 and 0.44g of unreacted Ca, the sum of which is the 2g of reactants taken.
So, if any of the reactant is left unreacted in a reaction, the total of the weight of the products formed and the weight of the reactant left unreacted will be equal to the weight of the reactants taken at the beginning for the reaction.
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