HS Chemistry - Working With Chemical Reactions

Limiting Reagents & Reaction Yields

Overview of The Page

This page will cover:

• What are limiting reagents and excess reagents?
• What are reaction yields?

A chemical reaction doesn't always completely turn all its reactants into products. Sometimes, we have too little of one reactant, and the reaction can't progress. In that case, it stops, and the portions of the other reactants that haven't been changed into product are just left over and transferred to the product side. The reactant we have too little of is called the limiting reagent - if we had more of this reactant, we could create more product, without needing to increase the amount of the other reactants we have. In the reaction:

5 H₂ + Cl₂ → 2HCl + 4H₂

Cl₂ is the limiting reagent. If we had more Cl₂, we could have produced more HCl without needing to add more H₂ to the reaction. Thus, the H₂ that isn't used to produce HCl or any other product just gets left over and added to the product. In this case, the H₂ is the excess reagent - it is present in excess. The 2HCl is the product, and the 4H₂ on the product side is the leftover.

Often, when doing an experiment, not all the reactants will mix with one another, even though in the chemical reaction, they will. The amount of product produced in the chemical reaction is the maximum amount of product that can be created in the reaction. It is called the theoretical yield. The actual amount experimentally created is called the actual yield. The percentage yield is the actual yield divided by the theoretical yield $converted to a percentage value$.

For example, if we had 5 moles of H₂ and 1 mole of Cl₂, and they reacted, we would expect to see 2 moles of HCl $Hydrochloric acid$ created, and 4 moles of Hydrogen gas left over. The theoretical yield of the equation would be the amount of product created $not including the leftovers$, which would be 2 moles HCl. However, if for some reason only 1.6 moles of HCl were created in the experiment, then the actual yield would be 1.6 moles HCl. The percent yield would be 1.6 ÷ 2.0 = 80%.

Practice

5 moles of Lithium are combined with 1 mole of O₂ to produce Li₂O.

• What is the limiting reagent? Oxygen Lithium
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• What is the excess reagent? Oxygen Lithium
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• What is the theoretical yield of the product? Type your answer as a number:
  
  
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• How many moles of the excess reagent are left over? Type your answer as a number:
  
  
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• When the above reaction is carried out in an experiment, the percentage yield is 75%. What's the actual yield of the product in this experiment? Type your answer as a number:
  
  
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