Limiting Reactants

I. In chemical reactions, usually one chemical gets used up first.

                A. This is known as the limiting reactant

                B. Once the L.R. is used up, the reaction stops

                C. The L.R. determines the quantity of products formed

                D. To find the L.R. assume one reactant is used up. Determine how much of this reactant is used up

Ex.

2.5 mol of CuSO₄ reacts with 2.5 mol of NaNO₃. A) Determine the L.R. B)Calculate how many moles of NaNO₃ are formed.

CuSo₄ + 2NaNO₃ --> Cu(NO₃)₂ + Na₂SO₄

A) 2.5 mol x (2/1*) = 5.0 mol of NaNO₃

*What you need over what you have

Since you do not have 5.0 mol of NaNO₃ and you only have 2.5 mol of it, the L.R. is NaNo₃

B) 2.5 mol x (1/2) = 1.25 mol of NaNO₃

Ex.

Determine the L.R. when 1.4 mol of hydrogen reacts with 0.09 mol of oxygen.

2H₂ + O₂ --> 2H₂O

1.4 mol of hydrogen x (1/2) = 0.70 mol of O₂

Therefore the L.R. is H₂

Ex.

What is the L.R. when 125g of P₄ reacts with 320g of Cl₂ to form phosphorus trichloride?

P₄ + 6Cl₂ --> 4PCl₃

125g x (1 mol/ 124g*) x (6/1) x (71.0g/ 1 mol**) = 429g of Cl₂

* Molar mass of P

**Molar mass of Cl

Therefore Cl₂ is the L.R.

Ex. In the formation of water 50g of oxygen react with 60g of hydrogen. Determine the L.R. and theoretical yield.

2H₂ + O₂ --> 2H₂O

50g of O₂ x (1 mol/ 32g) x (2/1) x (2.0g/ 1 mol) = 6.25 of H₂

Therefore the L.R. is O₂