Tuesday, September 13, 2011

Stoichiometry Note #5: Limiting Reactants

The Concept
Suppose that you need one cup of flour plus one egg to make a dozen cookies:
1 egg + 1 cup a flour→ 12 cookies

If you have 2 eggs and 2 cups of flour, then supposedly you would be able to make 24 cookies, right? But how many cookies will you be able to make if you have two eggs and ten cups of flour? Well, you can still only make 24 cookies.

How come? Well, in the second case, the egg was the limiting reactant. In other words, there was a shortage of eggs, and as a result even though there were still many cups of flour, no extra cookie could me made.

Similar to the above example, in chemical reactions there can also be limiting reactants. For example, if you have one mole of Na and ten moles of Cl in the following chemical reaction, then Na is the limiting reactant:
Na + Cl → NaCl
And in this case, only one mole of NaCl will be produced.

Finding the Limiting Reactant
Consider the following reaction:
3O2+ 4Al→ 2Al2O3

If you have 75 moles of O2, and 96 moles of Al, then which element is the limiting reactant? Well, there is a simple way to find out. You simply divide each of the numbers of moles by the corresponding coefficients, and the element with the smallest result is the limiting reactant:
  • O2: 75÷3= 25  
  • Al: 96÷4= 24 (limiting reactant!)

Applying The Concept of Limiting Reactants:
Whenever you want to calculate the amount of products, it is always important to only use the molar ratio between the limiting reactant and the product! For example, if we were to calculate the amount of  Al2Oproduced by the reaction described earlier, then this would be our molar ratio conversion:
  • Amount of Al2O= (amount of Al) × (coefficient of Al2O3) ÷ (coefficient of Al)
         Amount of Al2O3 = 96 × 2 ÷ 4 
                                    = 48 moles