Equilibrium Point - Factors Affecting the Equilibrium Point

What is the equilibrium point?

The amount of reaction that has occurred when a system has reached equilibrium is called the equilibrium point. This varies from reaction to reaction. Factors affecting the equilibrium point are concentration, temperature, and pressure.

Effect of concentration on an equilibrium system

When the concentration of one or more reactants in a system at equilibrium is increased, the equilibrium of the system is disturbed. Then the forward reaction takes place more and the reactants become products. Now the system is back to equilibrium. Also, if the concentration of reactants is reduced, the products become reactants by performing the reverse reaction and the system becomes balanced.

Likewise, when the concentration of the product is increased, the reaction is directed in the direction of the reactant, making the reverse reaction takes place more. The forward reaction takes place more when the product concentration is reduced. That is, the reaction is directed toward the product. Thus, the system returns to equilibrium.

FeCl₃ and NH₄SCN (Ferric chloride and thiocyanate) equilibrium system

When FeCl₃ and NH₄SCN are mixed, [Fe(SCN)]²⁺ complex is formed. This complex is a blood-red color.

The effect of concentration on the equilibrium of the above system can be studied as follows. First, the resulting aqueous solution is diluted until clear and divided into four parts. One sample is taken as a control (reference) and an amount of FeCl₃, NH₄SCN, and NaOH are added respectively to the other three samples. The color of each system is compared with the reference.

It can be observed that the red color is increased in the samples which are added FeCl₃, NH₄SCN. That is because more Fe(SCN) has formed. That is because the forward reaction has increased. According to Le Chatelier’s principle, when increasing the concentration of reactants, the reaction is forwarded in the product’s direction.

When the sample is added NaOH, it will form Fe(OH)₃ precipitate. So, Fe³⁺ is removed from the medium. It can be observed that the intensity of the red color decreases. That is because the reverse reaction has increased. when the concentration of reactants decreases.

CrO₄^2- and Cr₂O₇^2- (Chromate and dichromate) equilibrium system

This system is in the following equilibrium,

The chromate ion (CrO₄^2-) is yellow in color and the Dichromate ion (Cr₂O₇^2-) is orange in color. When an acid is added to the above system, it can be observed that the system becomes orange color. That means the forward reaction has increased and more dichromate ions have been formed. Because the concentration of reactants has been increased.

When a base is added to the system, OH^- ions react with H⁺ ions and form water (H₂O). the concentration of products is increased. So, the system turns into yellow color. That is because the reverse reaction has increased.

BiCl₃ and BiOCl (bismuth chloride and bismuth oxychloride) equilibrium system

BiOCl is a white color precipitate. When the water is added to the above system, the amount of white precipitate is increased. When it is added HCl, the amount of the white precipitate is decreasing. Thus, the forward reaction is more favorable when adding reactants and the reverse reaction is more favorable when adding products.

Effect of temperature on an equilibrium system

Let’s consider the following reaction. We take the forward reaction as exothermic and the reverse reaction as endothermic.

When the temperature of the above equilibrium system is increased, the equilibrium is disrupted. To reduce (somewhat) the increasing temperature, the reaction favors the endothermic direction (heat absorbing direction). That means the reverse reaction is increasing. Then the system became equilibrium again. 

When the temperature of the above equilibrium system is decreased, the equilibrium is disrupted. To increase (somewhat) the decreasing temperature, the reaction favors the exothermic direction (heat-releasing direction). That means the forward reaction is increasing. Then the system became equilibrium again. 

In an equilibrium system, when the temperature is increased the endothermic reaction is more favorable. When the temperature is decreased, the exothermic reaction is more favorable. By increasing or decreasing the forward and reverse reactions, the system became a new equilibrium.

NO₂ and N₂O₄ (Nitrogen dioxide and Dinitrogen tetroxide) equilibrium system

NO₂/N₂O₄ system is in the following equilibrium.

N₂O₄ is a colorless gas and NO₂ gas is red-brown. Also, the forward reaction is endothermic, and the reverse reaction is exothermic. The effect of temperature on the above system can be observed as follows, 

Three samples of NO₂ gas with the same intensity of color are taken to three boiling tubes. Samples are labeled as A, B, and C.

After a while, it can be observed that the intensity of the red-brown color is decreased in sample B compared to the reference. This is because the reverse reaction has increased and more N₂O₄ has formed. According to Le Chatelier’s principle, the reaction has forwarded into an exothermic direction when the temperature is decreased.

As the same, it can be observed that the intensity of the red-brown color is decreased in sample C compared to the reference. This is because the forward reaction has increased and more NO₂ has formed. According to Le Chatelier’s principle, the reaction has forwarded into the endothermic direction when the temperature is increased.

Effect of pressure on a gaseous equilibrium system

There are two ways to increase the pressure of a gas. 

1. By decreasing the volume
2. By increasing the temperature (without changing the volume)
3. By adding more gas (without changing the temperature or volume)

Here we consider the increase of pressure by decreasing the volume. Let’s consider the following equilibrium,

Generally, the amount of molecules is increased, and there are more collisions between the molecules. That caused to increase in the pressure of the system. As the same when the number of molecules decreases, the pressure is also decreased. In the above system, there are four molecules in the reactant’s direction and there is only one molecule in the product direction. If the forward reaction is more favored, the pressure of the system is decreased. If the reverse reaction is more favored the pressure in the system is increased. Then the system becomes a new equilibrium.

Pressure affects only for gaseous system. If the number of molecules is equal on both sides of the reaction, such systems are not affected by the pressure. If there are liquids or solids on each side, those are not affected by pressure.

Studying the effect of pressure by changing the volume

N₂O₄ /NO₂ system can be used to study the effect of pressure. When the pressure is increased, the reverse reaction is a more favored one. When the pressure is decreased the forward reaction is a more favored one.

A sample of NO₂ gas is taken. The volume of the sample is decreased and increased using a piston. When the volume is decreased (the pressure is increased), the intensity of the red-brown color of the sample is decreasing. That means the amount of NO₂ has decreased and the reverse reaction has occurred more.

When the volume is increased (the pressure is decreased), it can be observed that the intensity of the red-brown color of the sample is increasing. That means the amount of N₂O₄ has decreased and more NO₂ has formed. So, the forward reaction has occurred more.