Excess and Limiting Reactants

Today in class we learned the reactants in a chemical reaction is not always exact which means some reactant may have more amount than others.  Thus there will be excess of reactants during the reaction.  We also learned about the limiting reactant; a reactant that is not present in alarge enough quantity to fully react with another reactant.  The limiting reactant is significant because it helps us determine how much product can be formed.  This concludes what we learned today.

Examples





In this cass the car bodies are the litmiting reactant becaucause not matter how many tires there are only 8 car bodies are availible to make a car.  Thus only 8 cars can be formed and there is 16 tires excess.



A sample of 111.6g of Fe is mixed with 96.3g of  S.

a) Which reactant is in excess

b) Which is the limiting reactant?
c) When this reaction is carried out, what mass of FeS will actually be produced?

Solution
1) Write out the balance equation.
1Fe+1S----->1FeS

2)Convert Fe and S into moles
Mol Fe= 111.6g Fe *1molFe/55.8gFe =2.00 mol Fe
Mol S= 96.3g S * 1molS/32.1gS= 3.00mol S

3) Use mol ration to determined how much S is need to react with Fe
2.00mol Fe* 1molS/1molFe= 2.00mol S needed.  Thus S is the excess reactant because 3.00 moles of S is presented in the beginning. Since S is the excess than Fe will be the litming reactant.

4)Use the mass of the limiting reactant to find the mass of FeS that actually produced because we cannot start with the mass of the excess reactant S.  We need to use Fe the limiting reactant or we will get more mass of the product than we actually can.
111.6g Fe* 1molFe/55.8g Fe*1molFeS/1molFe*87.9gFeS/1molFeS=176gFeS