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.
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.
|addition of reactant||forward reaction favored|
|addition of product||reverse reaction favored|
|removal of reactant||reverse reaction favored|
|removal of product||forward reaction favored|
- The effects of concentration changes on an equilibrium are described.
- In the Haber process, what happens if you add more hydrogen gas?
- You miscalculate and add too little nitrogen gas. Which way will the equilibrium shift?
- A mislabeled tank pumps in extra ammonia. What happens to the equilibrium?
Use the resource below to answer the questions that follow.
- What is stress in an equilibrium reactant?
- What happens if more reactants are added?
- What happens if you remove product?