What is a limiting reagent

Difference between limiting reagent and excess reagent

Key difference - limiting reagent vs. excess reagent

A chemical reagent is a chemical species that is required for a chemical reaction to take place. Sometimes this reagent compound is consumed as the reaction proceeds, but sometimes it is not. If this reagent is consumed during the reaction, it is called a reactant. The terms that limit the reagent and the excess reagent describe the consumption of these reagents during a reaction. The limiting reagent always determines the amount of product that we can get at the end of the reaction. In other words, the limiting reagent limits the formation of the product. The main difference between limiting reagent and excess reagent is that The amount of limiting reagent present in a reaction mixture is less than that of excess reagent.

Important areas

1. What is a limiting reagent?
- Definition, effects on the chemical reaction, examples
2. What is an excess reagent?
- Definition, effects on the chemical reaction, examples
3. What is the relationship between the limiting reagent and the excess reagent?
- Limitation of the reagent and excess reagent
4. What is the difference between the limiting reagent and the excess reagent?
- Comparison of the main differences

Key Terms: Excess Reagent, Limiting Reagent, Reactant, Reagent


What is a limiting reagent?

Limiting reagent is the reactant of a specific chemical reaction that limits the formation of the product. Therefore, the limiting reagent will determine the amount of product that will be formed after the reaction is complete.

Limiting reagent is completely consumed during the reaction. Hence, we can determine the amount of product that is formed by looking at the stoichiometric relationship between the limiting reagent and the product. The reaction ends after the limit reagent has been completely consumed. This is because the reaction mixture lacks one of the reactants.


The limiting reagent of a particular reaction can be determined with a simple calculation. If not, we can determine this by simply looking at the number of moles of reactants and their stoichiometric relationships obtained from the balanced chemical equation.

How to determine the limiting reagent of a reaction

Let's look at an example to understand this method.

Example: Consider the reaction between NaOH (0.40 g) and HCl (0.1 M, 10.00 ml), which produce sodium chloride and water.

  1. Write the balanced chemical equation for the reaction

NaOH(aq) + HCl(aq) → NaCl(G) + H2O(l)

  1. Calculate the number of moles of each reactant in the reaction mixture.

Amount of NaOH present = 0.40 g / 40 gmol-1

= 1 x 10-2mol

Amount of HCl present = 0.1 molL-1 x 10.00 x 10-3 L.

= 1 x 10-3mol

  1. Determine the stoichiometric relationship between reactants and products.

NaOH: HCl: NaCl = 1: 1: 1

  1. Calculate the amount of product that can be produced by each reactant. The reactant that gives a lesser amount of product is the limiting reagent.
  • Amount of NaCl produced by NaOH;

NaOH: NaCl = 1: 1

1 x 10-2mol: NaCl = 1: 1

NaCl = 1 x 10-2mol

  • Amount of NaCl produced by HCl;

HCl: NaCl = 1: 1

1 x 10-3mol: NaCl = 1: 1

NaCl = 1 x 10-3mol

Since HCl gives a small amount of product as NaOH, HCl is the limiting reagent.

What is an excess reagent?

Excess reagent is the reactant that is in excess in a reaction mixture. A certain amount of this reagent will be present after the reaction is complete. The excess reagent can be observed at the beginning of a reaction, as the reaction progresses and at the end of the reaction.

The concept of an excess reagent is useful in determining the amount of an unknown amount of a component that is present in a particular compound. For example, in titration procedures, we can add an excess of reagent that will react with the unknown compound and some of the reagent will remain after the reaction is complete. The amount of excess reagent can then be determined by titration with a suitable reagent. Since we know how much reagent is used in excess, we can determine the amount that reacted with the unknown component. This is known as the back titration method. Let's look at an example.

Example: A sample solution (10.00 ml) consists of an unknown amount of Ni+2 Ions. We add an excess amount of EDTA solution (0.1 M, 15.00 ml) to this sample. EDTA reacts with Ni+2 in a ratio of 1: 1. The amount of excess EDTA present in the sample can be determined using a standard Mg+2 Solution (0.1 M) in the presence of an EBT indicator and buffer with pH 10. Then we should calculate the amount of Mg+2 that reacted with the excess EDTA. Since we know the total amount of EDTA added to the sample, we can calculate the amount of EDTA reacted with Ni+2 Ions. With the ratio 1: 1 we can determine the amount of Ni+2 available in the original sample. In this reaction there is Ni+2 is the limiting reactant for the reaction.

Relationship between limiting reagent and excess reagent

A real reaction mixture (non-ideal reaction mixtures) always has a limiting reagent and an excess of reagent. This is because the reactants react with each other according to the stoichiometric relationship between them. However, sometimes all of the reactants are consumed during the reaction. In such cases there are no limiting or excess reagents.

Difference between limiting reagent and excess reagent

definition

Limiting reagent: Limiting reagent is the reactant of a specific chemical reaction that limits the formation of the product.

Excess reagent: Excess reagent is the reactant that is in excess in a reaction mixture.

consumption

Limiting reagent: Limiting reagent is completely consumed during a reaction.

Excess reagent: Excess reagent is not completely used up during a reaction.

Presence at the end of the reaction

Limiting reagent: Limiting reagent is not present at the end of the reaction.

Excess reagent: There will be some excess reagent at the end of the reaction.

Effects on the product

Limiting reagent: The limiting reagent limits the amount of product that will form from a reaction.

Excess reagent: The excess reagent has no effect on the product that forms from the chemical reaction.

Conclusion

The limiting reagent of a chemical reaction is very important in determining the amount of a product that is formed during a chemical reaction. The excess reagent does not affect the final product, but is important in back titration procedures. Although both are reactants, there are some differences between them. The main difference between the limiting reagent and the excess reagent is that the amount of the limiting reagent present in a reaction mixture is less than that of the excess reagent.

References:

1. Helmenstine, Anne Marie “What is an excess reactant? Review your chemistry concepts. “ThoughtCo,