Chemical reactions occur as chemical compounds form and break apart. In a closed system, equilibrium results after a certain period of time. From this point forward, no changes in the concentration of the products and reagents can be observed anymore. In the following article, you learn which requirements are necessary for this state, what exactly characterizes this state, which meaning it has, and how one influences and uses it for certain purposes.
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Picture: “Balance (균형)” by Seongbin Im. License: CC BY-SA 2.0 The picture was cut.

Nature of Chemical Equilibrium

Definition of chemical equilibrium

The chemical equilibrium is the state of a chemical system at which a constant concentration of products and reagent is present. The reactions, which take place in homogeneous solutions, seem to have come to rest since no changes in concentrations of the participating substances can be determined. The substance turnover only occurs on the particle level, which is why it is also referred to as a dynamic equilibrium.

For each reaction, the position of the equilibrium, under certain surrounding conditions, is determined by a natural constant.

This form of the reaction is also known under the term reversible reaction, as it occurs in both directions and simultaneously. This condition results in the reaction equation for the reaction type of the equilibrium reaction to contain a double arrow. However, reversible reactions can only take place if none of the reaction partners leave the system.

Note: From a chemical standpoint, it is not correct to say that the same substance amounts of products and reagents are present. In the state of chemical equilibrium, the forward and backward reaction happens with the same velocity, which is why no changes in substance concentrations occur. Equalization of these facts has to be absolutely avoided!

Examples of equilibrium reactions

  • Pure water: H2O dissociates into H+ and OH. In pure water, there is equilibrium between H2O and the dissociated ions. Its position is very far on the side of H2 and this results in a pH-value of 7.
  • If glucose is solved in water at room temperature, a stable concentration relation results in 63 % β-glucose and 37 % α-glucose.

Requirements of chemical equilibrium

  • Closed or concluded system: Reversible reactions can only occur if none of the participating substances can escape.
  • Reversible reaction: If the reaction has begun and the first products have formed, an instant and an immediate backward reaction takes place so that the products are decomposed to their original substances again. The reaction velocities of the reaction partners adjust due to the back and forth swinging of the reactions until a constant relation of them has developed after a certain period of time.

Features of a chemical equilibrium

  • Forward and backward reaction happens simultaneously: dynamic equilibrium.
  • Identical reaction velocities [vforth = vback]
  • Adjustable from both sides
  • Original substances and reaction products are present simultaneously and in a constant concentration relation.
  • Incomplete substance turnover
  • Substance conversion only observable on the particle-level due to constant substance concentration
  • This applies: CRP / COS = constant
  • Catalyzers do not influence the location of the equilibrium.
  • Catalyzers shorten the time until the equilibrium is reached.

Development of a chemical equilibrium

In order to explain the equilibration of a reaction, you should be conscious of the meaning of the word reaction velocityMany reactions can go backward, as well as forwards. Reaction velocity is the change in substance concentration in a certain period of time. If forward and backward reactions occur simultaneously, which is typical for the equilibrium reaction, the following applies:

A + B C + D
Vforward = k1 • cA • cB
Vbackward = k• c• cD

[k= proportionality factor, c = concentration]

Thus, the following applies in chemical equilibrium:

Vforward = vbackward, and k1 • cA • cB = k2 • c• cD

If there is a reaction, which incompletely takes place in a closed system, and is also reversible (equilibrium reaction), the reaction initially has a high reaction velocity, as the concentrations of the original substances are high. The reaction velocity of the forward reaction gradually decreases since the substance concentrations of the reagents constantly decreases, while the backward reaction gains velocity since the substance concentrations of the products increase in the course of the reaction.

This process swings back and forth until a state is reached, at which the same amount of products and reagents is formed. In this state, the velocities of the forward and backward reactions are equal. This is why the reaction seems to have halted. Macroscopically, no changes can be observed as the chemical transitions only occur on the particle level.

Note: The greater the activity of the original substances, the greater is the velocity of the chemical reaction. In other words: the reaction velocity is directly proportional to the activity of the reagents.

The position of the chemical equilibrium is specific for each reaction and corresponds to a natural constant, which means that it cannot be changed. However, the time for equilibration can be shortened with the help of catalyzers.

This equilibration time is also specific for each reaction, but only at constant temperature conditions. The shortening of the time can be explained by the ability of the catalyzers to cause more “effective collisions” in their active state so that the chemical reaction is accelerated.

Law of Mass Action

The law of mass action or LMA offers the mathematical instrument to quantitatively describe the position of the chemical equilibrium.

If more than 50 % of the original substances react to products, the equilibrium is rather on the right side of the overall picture and one speaks of an equilibrium located on the right side.

Requirements of the law of mass action

  • Closed system
  • Equilibrated state

Calculation of the law of mass action

Calculation of the Law of Mass Action

k = equilibrium or mass action constant
c = substance concentration
A, B, C, D = reaction partners or their substance concentrations
a, b, c, d = stoichiometric numbers; can be taken from the reaction equation.

Explanation of the law of mass action

One main statement of the LMA is that the relation ‘k’ of the multiplied product concentrations to the multiplied reagent concentrations is constant for certain reactions under determined conditions. Thus, the quotient K of this equation is also referred to as equilibrium or mass action constant.

However, one has to consider that the LMA can only be applied to diluted solutions. In more concentrated solutions, there are deviations between the particles due to interactions. For example, the OH-ions cannot move in strong bases, as there is not enough solvent present for the ions. This makes one conclude that the position of the equilibrium fluctuates, depending on the concentration. Yet, the same concentrations develop in the equilibrium according to the LMA, which is why one always considers diluted solutions as ‘reference’.

Furthermore, the location of the equilibrium depends on temperature and, possibly, on the pressure relations. In the paragraph concerning “disturbances of the chemical equilibrium”, you will get further explanations.

The calculated equilibrium constant Kc has great importance for further calculations. On the bases of the value of Kc, one can calculate the transformed amount of reagents or the exploit of reaction products. Kc can be determined from experimental values.

Disturbances or Influencing the Equilibrium

If a chemical equilibrium is disturbed, an acceleration of the reaction occurs, which then eliminates or reverses the disturbance. This rule is also known as the principle of least constraint or Le Châtelier’s principle. This “constraint” means the disturbance of the equilibrium, which leads to the reaction to having to be compensated by acceleration.

If one applies a constraint to a system in equilibrium, the system shifts in the direction of a new equilibrium level so that the effect of the constraint becomes minimal; that is the smallest.

(Formulation of the principle of least constraint of Le Châtelier)

This disturbance can be triggered by different factors. As mentioned in the previous point, the position of the equilibrium can be changed by the deviation of temperature and pressure conditions. Besides these points, participating substance amounts also have an influence.

In order to correctly illustrate the way of the changes triggered by different factors, they are now examined individually.

Energy input

Energy input, e.g. via heating, results in a reinforced “uphill” reaction. This means that an increase in the formation of reagents occurs, which actually form products, and store their energy there, under “normal” conditions.

This means that an increase in temperature promotes the endothermic reaction and that the value of the equilibrium constant decreases. Analogously, the opposite happens in the event of a decrease in energy or temperature: The location of the equilibrium shifts in the direction of the products and the exothermic reaction is promoted.

Changes in amount of substance

For the sake of “rescue”, the following reactions occur at adding or removal of reaction partners: [reaction: A + B → C + D]

  1. Adding of original substances A or B → increased formation of the reaction products C and D
  2. Adding of the products C or D → increased formation of the reagents A and B
  3. Removal of A or B → increased formation of A or B
  4. Removal of C or D → increased product formation (C, D)

The increase in the concentration of a substance promotes its consumption and a decrease in concentration promote its reproduction.

Note: This means: If one wants to reach a 100 % product formation, one of the reaction products has to be removed from the reaction mixture completely and steadily. This also results in the fact that the change in substance concentration of a substance also always changes the concentrations of the other substances.

Via adding of acids, bases, or precipitants, the concentration of the reaction partners can, however, also be disturbed. In such a case, two coupled equilibrium reactions often occur parallel.

Changes in pressure conditions

If the participating substances in an equilibrium reaction in a closed system are gasses, a change in pressure results in a change in the location of the chemical equilibrium. If the reaction partners have another aggregate phase than gaseous, the equilibrium is not affected or changed. The background of this phenomenon is that changes in volume at reactions with non-gaseous substances are so small, that the dependency of the location of the equilibrium on the pressure can be neglected.

If an increase in pressure occurs at reactions, which take place under a decrease in volume, the chemical equilibrium shifts to the side of the products. The increase in pressure at a reaction, which takes place under an increase in volume, leads the location of the equilibrium to be shifted to the reagents.

A decrease in pressure promotes the reaction, which occurs under an increase in volume.

Review Questions

The solutions can be found below the sources.

1. Complete the following sentence: In chemical equilibrium…

  1. … The reactions take place in heterogeneous solutions.
  2. … There are as many reagents as products present.
  3. … Forward and backward reactions do not occur at the same time.
  4. … The reaction has reached stagnation, which means that no substance turnover takes place anymore.
  5. … The forward and backward reactions occur with the same velocity.

2. Which statement is true concerning the law of mass action?

  1. The LMA serves the quantitative description of the location of the chemical equilibrium.
  2. Absolute requirements for the validity of the LMA are the state of chemical equilibrium and an open system.
  3. Stoichiometric digits can be neglected at calculation since their effect is minimal on the location of the chemical equilibrium.
  4. The LMA only has great validity in pure, undiluted solutions.
  5. The reaction velocity can be determined by the equilibrium constant Kc.

3. The chemical equilibrium can be disturbed by different factors. To which of the following statements can you not approve of concerning this?

  1. An increase in temperature promotes the endothermic reaction and decreases the value of the equilibrium constant.
  2. If one adds additional original substances to a reaction, which is chemical equilibrium, more reaction products are formed.
  3. If one removes original substances from a reaction in this state, one increases the formation or the exploit of products.
  4. A decrease in pressure promotes the reaction, which occurs in an increase in volume.
  5. If one increases the pressure in reactions, which react under a decrease in volume, the location of the equilibrium shifts to the side of the products.
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