Excess and Limiting Reactants

Balanced equations tell us the ratios of the reactants used. However, in a real life situation, some reactions require more or less reactants...
reactants that are not completely used up in a reaction are excess reagents
''                     '' are completley used up ''                       '' limiting

Example

Finding Excess/Limiting Reagents

Find the limiting reagent and the reactant in excess when 0.5 moles of Zn react completely with 0.4 moles of HCl

 Zn + 2HCl -----> ZnCl₂ + H₂

 _{Compare the available moles of each reactant to the moles required for complete reaction using the mole ratio}

_{Zn->HCl           0.5 x 2= 1.0 mole HCl}

    There are only 0.4 moles of HCl available which is less than the required 1.0 moles.

HCl->Zn           0.4 x  1  = 0.2 mols Zn
                              2
    There are 0.5 moles of Zn available which is more than the required 0.2 moles.


    The limiting reagent is HCl, Excess reagent= Zn
    all of the 0.4 moles of HCl will be used up

    when the reaction has gone to completion there will be
    0.5 - 0.2 = 0.3 moles of Zn left over.

Example 2


Find the amount of H20 produced when 1.5g of CaCO₃ react completely with 0.73g of HCl.

CaCO₃ + 2HCl -----> CaCl₂ + CO₂ + H₂O


1.5g CaCO3 x 1  x 18gH20 = 0.27g H20
                 100g

0.73gHCl x 1  x   1molH20 x 18gH20 = 0.18gH20
              36.5g  2molHCl        


Since HCl is the limiting reactant, you can only produce 0.18g of H20.