What is a Limiting Reactant?

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The Limiting reactant is a reactant (starting material in a chemical reaction) in a chemical reaction that determines the amount of product produced. The reactant in a chemical reaction that limits the amount of product that can be formed is known as the Limiting reactant. The reaction will stop when all of the limiting reactant is consumed. Limiting reactant is also known as “Limiting reagent”. It is important that the chemist know how to determine which reactant is the limiting reactant and to ensure he has enough of it to make the desired amount of product.

How to determine the amount of each reactant?

A balanced equation can help a scientist or a chemist know the proportion of each reactant. A balanced equation is one that reflects the law of conservation of mass, which states that mass of substances produced (products) by a chemical reaction, is always equal to the mass of the reacting substances (reactants). If we consider the chemical reaction for making water, the balanced equation is shown as below.

• 2H₂ + O₂ --> 2H₂O

It is clear that from the chemical reaction that twice as many hydrogen atoms are needed as oxygen atoms to make water. In order to find out the limiting reactant, a chemist must know the number of moles of each substance. A mole is equal to 6.023 * 10²³ units of the substance and weighs the same as the molecular weight of that substance. Molecular weight is the weight of all the atoms in a substance. In the above reaction, since the molecular weight of hydrogen is approximately equal to 2 grams, a mole of hydrogen molecules would also weigh approximately 2 grams and be roughly equal to 6.023 * 10²³ molecules of hydrogen. In the same way, the molecular weight of oxygen - approximately 32 grams –is roughly equal to one mole of oxygen molecules. Therefore, if the chemist has 2 grams of hydrogen and 32 grams of oxygen, then he knows that he has about a mole of each substance.

How to identify which reactant is the Limiting Reactant?

Once the equation is balanced properly and a clear knowledge of the proportion of each reactant is known is obtained, then it is simple to determine which reactant is the limiting reactant. For example, if the chemist determines that he has  1 mole of hydrogen and one mole of oxygen then the limiting reactant  is “hydrogen”. In the balanced equation for making water, we can see that it takes twice as many moles of hydrogen atoms than oxygen atoms to make water. In other words, each oxygen atom requires 2 hydrogen atoms in order to make water. The hydrogen would run out before the oxygen does, and once that happens, the reaction would come to an end.

What are some examples of Limiting reactants?

 Some of the reactions and their Limiting reactants are as shown below.

•  The balanced reaction where ammonia (NH₃) reacts with Oxygen (O₂) to give Nitrous oxide (NO) and Water (H₂O) is given as: 4NH₃ (g) + 5O₂ (g) --> 4NO (g) + 6 H₂O (g), where ‘g’ represents gas phase. It can be known that the limiting reactant of the above reaction is “Ammonia”, since ammonia is completely used up during the chemical reaction.
•  Consider the following chemical reaction between Benzene (C₆H₆) and oxygen (O₂) to produce carbon dioxide (CO₂) and water (H₂O) is shown as: 2C₆H₆ + 15O₂ --> 12CO₂ + 6H₂O. In this reaction oxygen is in excess and so the limiting reactant is Benzene, which is completely used up in the reaction. 
•  In a chemical reaction between Aluminium (Al) and oxygen to produce Aluminium oxide (Al₂O₃), the balanced equation is given as: 4Al (s) + 3O₂ (g) --> 2Al₂O₃(s), where‘s’ is the solid and ‘g’ is the gas. In this reaction, the limiting reactant is Aluminium, because it is totally consumed by the end of the reaction. 

What is an Excess reactant?

The reactant in a chemical reaction that is left over when a reaction stops as soon as the limiting reactant is completely consumed is known as excess reactant. The excess reactant is left over because there is nothing with which it can react. In the above chemical reaction, oxygen is in excess and so it is the excess reactant. 

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