Meaning and mechanism
If the reactants are not mixed in the stoichiometric ratio, one substance will be consumed first, thereby limiting the reaction amount. Other reactants may remain at that point in time.
For N₂+3H₂→2NH₃, mixing 1 mol of N₂ with 2 mol of H₂ leaves hydrogen limiting: reacting all the nitrogen would require 3 mol of hydrogen.
Relationships and conditions
The minimum value of nᵢ/νᵢ among the reactants gives the possible extent of reaction ξ. The amount of product formed is ν_productξ, and the amount of reactant remaining is nᵢ−νᵢξ.
The yield obtained theoretically is based on the assumption that only the defined reaction proceeds completely. In reality, due to equilibrium, side reactions, and separation losses, the recovered amount may be smaller than the theoretical yield.
What to observe in the simulation
The limiting reactant determines the amount produced. After a reaction, both the products and remaining reactants must be included to preserve mass.
Assume that the three reactions are completed without any side reactions. Equilibrium, speed, and actual yield are excluded. Molar mass is calculated using educational atomic weights of H=1, C=12, N=14, and O=16 g/mol. The circle is a schematic diagram of a bundle of moles, not the actual number of molecules. The unit of ξ is mol.
Key relationships and symbols
ξ = min(nᵢ/νᵢ) · Remaining amount = nᵢ−νᵢξ · Amount produced = νproductξ
| Symbol | Meaning | Unit |
|---|---|---|
| ξ | Extent of reaction | mol |
| nᵢ | Initial amount of reactant | mol |
| νᵢ | Reaction coefficient | Dimensionless |
Worked example
What value is the limiting reactant set to?
We need to compare the number of moles divided by the coefficient ν in the reaction equation. H₂ is 3/2=1.5 mol, O₂ is 1/1=1 mol, so ξ=1 mol and 2 mol of water is produced.
Common misconceptions
- It is not possible to simply state that the smallest mass of a substance is immediately a limiting reactant.