Factors that Affect Chemical Equilibrium

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  • 0:04 Reversible Reactions
  • 0:45 Disturbances in Equilibrium
  • 1:18 Concentration
  • 1:52 Temperature
  • 3:26 Pressure
  • 4:44 Lesson Summary
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Lesson Transcript
Instructor: Hemnath (Vikash) Seeboo

Taught Science (mainly Chemistry, Physics and Math) at high school level and has a Master's Degree in Education.

In this lesson, you'll learn how varying the conditions of concentration, temperature, and pressure will affect the reactants and products in a chemical reaction at equilibrium.

Reversible Reactions

When we think of a chemical reaction we think of reactants making products. Chemical reactions are equilibrium reactions. Equilibrium occurs when a certain proportion of a mixture exists as reactants, and the rest exists as products. In closed systems, equilibrium occurs when neither the products nor the reactants can escape to the outside environment.

However, when equilibrium has been reached, it does not mean that the reaction has stopped. It simply means that dynamic equilibrium has occurred, or the forward reaction is making products at the same rate that the backward reaction is making reactants. The word 'dynamic' means moving or changing and serves to remind us that the reaction has not stopped.

Disturbances in Equilibrium

A small disturbance in the equilibrium may shift the equilibrium position either to the right, forming more products, or shift it to the left, making more reactants. Any reaction by the system is temporary, and the system will eventually come back to equilibrium.

Henri Louis Le Chatelier was one of the first chemists to investigate the effects of concentration, temperature, and pressure on equilibrium. His investigations led him to the following conclusion:

''If a system in equilibrium is subjected to a change, processes will occur which tend to counteract the change imposed.''

Concentration

Consider the following reversible reaction:

A and B

We have a U-tube in which reactant A is on one side and product B on the other side.

A and B are in equilibrium

If we imagine that some amount of A is removed from the equilibrium reaction, it means that momentarily the contents of the U-tube will not be in equilibrium.

An amount of A has been removed.

Such a system cannot remain in this unbalanced situation. According to Le Chatelier's principle, the system will react in such a way as to return balance to this reaction. Thus, the backward reaction is in favor of re-establishing equilibrium.

Equilibrium is re-established

Temperature

Chemical reactions can either proceed with the absorption of energy (endothermic) or with the exit of energy (exothermic). In an endothermic reaction, energy can be considered a reactant. In an exothermic reaction, energy can be considered a product.

Consider this hypothetical reaction:

a plus b equals c

If the forward reaction is exothermic, the reaction will proceed with an increase in temperature; the backward reaction will automatically be endothermic and proceed with a decrease in temperature.

What will happen to the equilibrium if the temperature is increased? Well, the system will counteract by decreasing the temperature, favoring the backward reaction.

Now, what will happen to the equilibrium if the temperature is decreased? Here, the system will counteract by increasing the temperature, favoring the forward reaction.

Consider three identical sealed tubes containing dark brown nitrogen dioxide (NO2) in equilibrium with pale yellow dinitrogen tetroxide (N 2 O 4 ).

nitrogen dioxide equilibrium

Initially, all three tubes contain the same amounts of NO 2 and N 2 O 4 and have the same medium brown appearance.

  • Tube one is placed in ice water; this tube becomes pale yellow in color
  • Tube two is left at room temperature; this tube remains medium brown in color
  • Tube three is placed in hot water; this tube becomes dark brown in color

This shows that the equilibrium in the reaction moves toward the formation of the dark brown NO2 at higher temperatures. The equilibrium moves towards the formation of the yellow N2 O 4 at lower temperatures.

Pressure

Pressure is a factor that affects the equilibrium of gaseous systems.

Consider the Haber process:

Haber process

If we increase the total pressure of the system, it moves the molecules closer together. The additional pressure can be relieved if the molecules can react and reduce the total number of molecules present.

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