Limiting Reactant in Chemical Reactions

Introduction to Limiting Reactant

• In chemical reactions, reactants are rarely present in precisely the correct amounts for both to react completely.
• One reactant will run out first, termed the limiting reactant (reagent).
• The reactant that remains is called the reactant in excess.

Analogy: Cheese Sandwich

• Reaction: 1 slice of cheese + 2 slices of bread = 1 cheese sandwich.
• Stoichiometric coefficients: 1:2:1.
• Example:
  • 28 slices of bread and 11 slices of cheese.
  • Potential sandwiches with bread: 14.
  • Potential sandwiches with cheese: 11.
  • Cheese is the limiting reagent (can only make 11 sandwiches).
  • Bread is in excess (6 slices left over).
• Note: Limiting reagent is not always the one in smaller quantity by count.
  • If 15 slices of cheese and 28 slices of bread, bread becomes the limiting reactant.

Real Chemistry Application

Reaction: Silicon Nitride Production

• Need stoichiometric ratios to determine the limiting reactant.
• Example:
  • 2 grams of silicon and 1.5 grams of nitrogen.
  • Grams are not useful directly; convert to moles using molar masses.
  • Calculate potential product moles for each reactant.
  • Silicon: Can generate 0.0237 moles of product.
  • Nitrogen: Can generate 0.0268 moles of product.
  • Silicon limits the product, hence it's the limiting reactant.

Conclusion

• Always convert to moles and use stoichiometric ratios to identify the limiting reagent in a reaction.
• Limiting reactant dictates the amount of product formed.