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Effect of Concentration

Explains how concentrations of reactants and products drive the equilibrium of a reaction.

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Effect of Concentration

The pH in a solution can be influenced by the addition or removal of H+ ions

Credit: Ben Mills (Wikimedia: Benjah-bmm27)
Source: http://commons.wikimedia.org/wiki/File:Phenolphthalein-at-pH-9.jpg
License: CC BY-NC 3.0

Phenolphthalein is one of those chemicals that has one structure in a high acid environment and another structure in a low acid environment. If the hydrogen ion concentration is high, the compound is colorless, but turns red if the hydrogen ion concentration is low. By adding hydrogen ions to the solution or removing them through a chemical reaction, we can vary the color of the dye.

Effect of Concentration

A change in concentration of one of the substances in an equilibrium system typically involves either the addition or the removal of one of the reactants or products. Consider the Haber-Bosch process for the industrial production of ammonia from nitrogen and hydrogen gases.

N2(g)+3H2(g)2NH3(g)

If the concentration of one substance in a system is increased, the system will respond by favoring the reaction that removes that substance. When more N2 is added, the forward reaction will be favored because the forward reaction uses up N2 and converts it to NH3. The forward reaction speeds up temporarily as a result of the addition of a reactant. The position of equilibrium shifts as more NH3 is produced. The concentration of NH3 increases, while the concentrations of N2 and H2 decrease. After some time passes, equilibrium is reestablished with new concentrations of all three substances. As can be seen in Figure below, if more N2 is added, a new equilibrium is achieved by the system. The new concentration of NH3 is higher because of the favoring of the forward reaction. The new concentration of the H2 is lower. The concentration of N2 is higher than in the original equilibrium, but went down slightly following the addition of the N2 that disturbed the original equilibrium. By responding in this way, the value of the equilibrium constant for the reaction, Keq, does not change as a result of the stress to the system.

Adding more nitrogen in the Haber-Bosch process generates additional ammonia

Credit: CK-12 Foundation - Christopher Auyeung
License: CC BY-NC 3.0

The Haber-Bosch process is an equilibrium between reactant N2 and H2 and product NH3.[Figure2]

If more NH3 were added, the reverse reaction would be favored. This “favoring” of a reaction means temporarily speeding up the reaction in that direction until equilibrium is reestablished. Recall that once equilibrium is reestablished, the rates of the forward and reverse reactions are again equal. The addition of NH3 would result in increased formation of the reactants, N2 and H2.

An equilibrium can also be disrupted by the removal of one of the substances. If the concentration of a substance is decreased, the system will respond by favoring the reaction that replaces that substance. In the industrial Haber-Bosch process, NH3 is removed from the equilibrium system as the reaction proceeds. As a result, the forward reaction is favored so that more NH3 will be produced. The concentrations of N2 and H2 decrease. Continued removal of NH3 will eventually force the reaction to go to completion until all of the reactants are used up. If either N2 or H2 were removed from the equilibrium system, the reverse reaction would be favored and the concentration of NH3 would decrease.

The effect of changes in concentration on an equilibrium system according to Le Châtelier’s Principle is summarized in Table below.

Stress Response
addition of reactant forward reaction favored
addition of product reverse reaction favored
removal of reactant reverse reaction favored
removal of product forward reaction favored

 

 

Summary

  • The effects of concentration changes on an equilibrium are described.

Review

  1. In the Haber process, what happens if you add more hydrogen gas?
  2. You miscalculate and add too little nitrogen gas. Which way will the equilibrium shift?
  3. A mislabeled tank pumps in extra ammonia. What happens to the equilibrium?

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Use the resource below to answer the questions that follow.

 

 

 

 

 

  1. What is stress in an equilibrium reactant?
  2. What happens if more reactants are added?
  3. What happens if you remove product?

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Image Attributions

  1. [1]^ Credit: Ben Mills (Wikimedia: Benjah-bmm27); Source: http://commons.wikimedia.org/wiki/File:Phenolphthalein-at-pH-9.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: CK-12 Foundation - Christopher Auyeung; License: CC BY-NC 3.0

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