Difference Between Limiting Reagent and Excess Reagent

Main Difference – Limiting Reagent vs Excess Reagent

A chemical reagent is a chemical species that is required in order for a chemical reaction to occur. Sometimes this reagent compound is consumed during the progression of reaction, but other times it is not. If this reagent is consumed during the reaction, it is called a reactant. The terms limiting reagent and excess reagent describe the consumption of these reagents during a reaction. The limiting reagent will always decide the amount of product that we can obtain 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 lower than that of the excess reagent.

Key Areas Covered

1. What is a Limiting Reagent
      – Definition, Effect on Chemical Reaction, Examples
2. What is an Excess Reagent
      – Definition, Effect on Chemical Reaction, Examples
3. What is the Relationship Between Limiting Reagent and Excess Reagent
      – Limiting Reagent and Excess Reagent
4. What is the Difference Between Limiting Reagent and Excess Reagent
      – Comparison of Key Differences

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

What is a Limiting Reagent

Limiting reagent is the reactant of a particular chemical reaction that limits the formation of the product. Therefore, the limiting reagent will decide the amount of product that is going to be formed after the completion of the reaction.

Limiting reagent is totally consumed during the reaction. Therefore, we can determine the amount of product that is going to be formed by looking at the stoichiometric relationship between the limiting reagent and the product. The reaction ends after the complete consumption of the limiting reagent. This is because the reaction mixture lacks one of the reactants.

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

How to Determine the Limiting Reagent of a Reaction

Let us consider an example to understand this method.

Ex: Consider the reaction between NaOH (0.40 g) and HCl (0.1 M, 10.00 mL) that produce Sodium chloride and water.

1. Write the balanced chemical equation for the reaction

NaOH_(aq)   +    HCl_(aq)   →  NaCl_(g)  +   H₂O_(l)

2. 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

3. Determine the stoichiometric relationship between reactants and products.

NaOH : HCl : NaCl   =   1 : 1 : 1

4. Calculate the amount of product that can be produced by each reactant. The reactant which gives a lower 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 low amount of product than NaOH, HCl is the limiting reagent.

What is an Excess Reagent

Excess reagent is the reactant that is present in excess in a reaction mixture. Some amount of this reagent will be present after the completion of the reaction. The excess reagent can be observed at the beginning of a reaction, at the progression of the reaction and at the end of the reaction.

The concept of excess reagent is useful in determining the amount of an unknown amount of a component present in a particular compound. For example, in titration methods, we can add a reagent in excess that will react with the unknown compound and some of the reagent will be left after the completion of the reaction. Then the amount of excess reagent can be determined by titrating it with a suitable reagent. Since we know the amount of reagent used in excess, we can determine the amount of it that reacted with the unknown component. This is called a back titration method. Let us consider an example.

Ex: A sample solution (10.00 mL) is composed of an unknown amount of Ni⁺² ions. We add an excess amount of EDTA (0.1 M, 15.00 mL) solution to this sample. EDTA reacts with Ni⁺² in 1: 1 ratio. 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 EBT indicator and pH 10 buffer.  Then we should calculate the amount of Mg⁺² that reacted with the excess EDTA. As we know the total amount of EDTA added to the sample, we can calculate the amount of EDTA reacted with Ni⁺² ions. Using the ratio 1: 1, we can determine the amount of Ni⁺² present in the original sample. In this reaction, Ni⁺² is the limiting reactant for the reaction.

Relationship Between Limiting Reagent and Excess Reagent

A real reaction mixture (not ideal reaction mixtures) will always have a limiting reagent and an excess reagent. This is because reactants react with each other according to the stoichiometric relationship between them. But sometimes, all reactants are consumed during the reaction. In such instances, there are no limiting or excess reagents.

Difference Between Limiting Reagent and Excess Reagent

Definition

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

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

Consumption

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

Excess Reagent: Excess reagent is not completely consumed 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: Some amount of excess reagent is present at the end of the reaction.

Effect on the Product

Limiting Reagent: The limiting reagent limits the amount of product that is formed from a reaction.

Excess Reagent: The excess reagent has no effect on the product formed from the chemical reaction.

Conclusion

The limiting reagent of a chemical reaction is very important in determining the amount of a product formed during a chemical reaction. The excess reagent has no effect on the final product but is important in back titration methods. Although both of them are reactants, there are some differences between them. The main difference between limiting reagent and excess reagent is that the amount of limiting reagent present in a reaction mixture is lower than that of the excess reagent.

References:

1. Helmenstine, Anne Marie. “What Is an Excess Reactant? Review Your Chemistry Concepts.” ThoughtCo, Available here. Accessed 24 Aug. 2017.
2. “Stoichiometry: Limiting reagent and excess.”, Available here. Accessed 24 Aug. 2017.

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1. “740453” (Public Domain) via Pixabay