Limiting Reagents:
What are Limiting reactant?
Limiting Reactant or Limiting Reagents are those reactants which are completely used up or entirely consumed in chemical reaction. It limits or determines the amount of products formed in a reaction.
What are Excess reactant?
Excess Reactant are those reactants which are not completely used up or entirely reacted in chemical reaction. The excess amount of product is independent.
The amount of product which is left behind in chemical reaction is called residue and a constant chemical compound is formed.
Let consider reaction for the formation of sodium chloride.
2Na(s)+Cl2(g)→2NaCl(s)
There are two moles of sodium and one mole of chlorine gas forming two moles of sodium chloride.
But if there are two moles of chlorine as well so extra chlorine will be formed in reaction hence chlorine will be excess reactant and sodium will be limited.
Example of excess reactant
- Combustion of methane in air where there is a excess amount of oxygen hence the oxygen will be excess reactant.
- Combustion reaction of candle when burn in air with oxygen so here oxygen will be excess reactant.
Use of Limiting Reagents
- It is used in chemical industries in order to get maximum yield of product at minimum cost.
How to find out limiting reactants
Example 1:
The reaction between zinc and Sulphur produces large amount of heat and zinc sulfide.
Zn (s) + S (s) → ZnS (s)
When 13g of powdered zinc is heated with 8g of powder Sulphur, a vigorous reaction occurred and zinc sulfide was formed.
- Identify the limiting reactant in this reactant
- Calculate the mass of excess reactant
Solution
(1) Step 1: Find out molar mass of each reactant from equation.
In equation:
Molar mass of zinc is 65.38g
Molar mass of sulfur is
Molar mass of zinc sulfide is 97.4
In Question:
Mass of zinc is 13g
Mass of sulfur is 8g
(2) Step 2: Compare the ratio of reactants and product.
For zinc:
65.38 : 97.4 = 13 : x
65.38x = 13 x 97.4
65.38x = 1266.2
x = 1266.2 / 65.38
x = 19.36 g
It means 19.36 g of zinc sulfide formed by 13 g of zinc.
For Sulphur:
32 : 97.4 = 8 : x
32x = 8 x 97.4
32x = 779.2
x = 779.2 / 32
x = 24.35 g
It means 24.35 g of zinc sulfide formed by 8 g of zinc.
(3) Step 3: Analyze which reactant create less amount of product.
Hence zinc produces less amount of product so it is limiting reactant and Sulphur produces greater amount of product so it is a excess reactant.
