Factors Affecting Equilibrium

One of the principal goals of chemical synthesis is to maximize the conversion of the reactants to products while minimizing the expenditure of energy. This implies maximum yield of products at mild temperature and pressure conditions. If it does not happen, then the experimental conditions need to be adjusted.

Equilibrium constant, Kc is independent of initial concentrations. But if a system at equilibrium is subjected to a change in the concentration of one or more of the reacting substances, then the system is no longer at equilibrium; and net reaction takes place in some direction until the system returns to equilibrium once again. Similarly, a change in temperature or pressure of the system may also alter the equilibrium. In order to decide what course the reaction adopts and make a qualitative prediction about the effect of a change in conditions on equilibrium we use Le Chatelier’s principle.

Le Chatelier’s principle: It states that a change in any of the factors that determine the equilibrium conditions of a system will cause the system to change in such a manner so as to reduce or to counteract the effect of the change. This is applicable to all physical and chemical equilibria.

We shall study in detail the factors like

Ø Effect of change in concentration:

In general, when equilibrium is disturbed by the addition or removal of any reactant or products, Le Chatelier’s principle predicts that:

· The concentration stress of an added reactant or product is relieved by net reaction in the direction that consumes the added substance.

· The concentration stress of a removed reactant or product is relieved by net reaction in the direction that replenishes the removed substance.

or in other words,

“When the concentration of any of the reactants or products in a reaction at equilibrium is changed, the composition of the equilibrium mixture changes so as to minimize the effect of concentration changes”.

Let us take the reaction,

N₂₍_g) + 3H_2(g) = 2NH₃

If concentration of reactants dinitrogen or dihydrogen is increased, the reaction will proceed towards right hand side and the product ammonia obtained will be more. If the concentration of dinitrogen or dihydrogen is decreased, the reaction will proceed towards left hand side and reactants of dinitrogen or dihydrogen will be obtained back i.e. production of ammonia will decrease. Here, it is necessary to remember that 1 mole dinitrogen combines with 3 moles of dihydrogen in this reaction and forms 2 moles of ammonia. Hence, increase in concentration of dihydrogen rather than dinitrogen, will give more product. We take another example of heterogeneous equilibrium. If solid CaCO₃ is heated in closed vessel, the following decomposition reaction will occur.

CaCO_3(s) ⇋ CaO_(s) + CO₂₍_g)

Hence, if more CaO_(s) is to be obtained then, the CO₂₍_g) formed in the reaction should be removed because CO_2(g), gas can combine with solid CaO_2(g) and carry out reverse reaction then proportion of product will decrease. Hence, by removing CO₂₍_g) from the reaction vessel, more CaO_(s) can be obtained.

Ø Effect of change in pressure:

The change in pressure can be carried out by increasing or decreasing the concentration of the gas or respectively decreasing or increasing the volume of the vessel. By carrying out this type of change, there will be change in proportions of gaseous reactants or products or total products.

Le Chatelier's principle can also be applied to such reactions. In heterogeneous equilibrium if we do not take into consideration the effect of pressure on solid or liquid substances in equilibrium, it can work because their volumes and concentration are independent of change of pressure. Let us take the following example.

CO₍_g) + 3H_2(g) ⇋ CH_4(g) + H₂O_(g)

In this reaction 1 mole of reactant CO₍_g) reacts with three moles of reactant H_2(g) and forms 1 mole product CH_4(g) and 1 mole product H₂O_(g). Thus, 2 moles product is obtained from 4 moles of reactants. Hence, there is decrease in number of moles during reaction.

Total pressure will be double because PV = constant. The concentrations or pressures of reactants or products are increased. Hence, according to Le Chatelier's principle, the equilibrium will try to attain the original state. As pressure is doubled and 2 moles of products are obtained and 4 moles of reactants, there is decrease in number of moles and the reaction will go in forward direction.

As a reverse of this, if in reaction CaCO_3(s) ⇋ CaO_(s) + CO₂₍_g)the number of moles of product increases (from 0 to 1). If pressure is increased by addition of CO₂₍_g)then, the reaction will become reverse reaction and will decrease the product.

Ø Effect of inert gas addition:

If the volume is kept constant and an inert gas such as argon is added which does not take part in the reaction, the equilibrium remains undisturbed. It is because the addition of an inert gas at constant volume does not change the partial pressures or the molar concentrations of the substance involved in the reaction. The reaction quotient changes only if the added gas is a reactant or product involved in the reaction.

Ø Effect of temperature change:

Whenever an equilibrium is disturbed by a change in the concentration, pressure or volume, the composition of the equilibrium mixture changes because the reaction quotient, Q_c no longer equals the equilibrium constant, K_c. However, when a change in temperature occurs, the value of equilibrium constant, K_c is changed.

In general, the temperature dependence of the equilibrium constant depends on the sign of ∆H for the reaction.

· The equilibrium constant for an exothermic reaction (negative ∆H) decreases as the temperature increases.

· The equilibrium constant for an endothermic reaction (positive ∆H) increases as the temperature increases.

Temperature changes affect the equilibrium constant and rates of reactions. Production of ammonia according to the reaction,

N₂₍_g) + 3H_2(g) ⇋ 2NH_3(g)

∆H = -92.38KJ mol^-1

Is an exothermic process. According to Le Chatelier’s principle, raising the temperature shifts the equilibrium to left and decreases the equilibrium concentration of ammonia. In other words, low temperature is favorable for high yield of ammonia, but practically very low temperatures slow down the reaction and thus a catalyst is used.

Ø Effect of catalyst:

The use of suitable catalyst always helps to increase the rate of reaction, viz. iron powder is used as the catalyst in the production of ammonia by Haber's process. Use of catalyst is associated with chemical kinetics because it affects the rate of reaction. The function of catalyst is to decrease the energy of activation. Hence, the reaction easily moves forward towards product. During this reaction, the energy of activation decreases but has no effect on equilibrium constant, that is, more proportion of product cannot be obtained. Let us examine the case of reaction of ammonia gas obtained by combination of dinitrogen gas and dihydrogen gas with the help of Haber's process.

N₂₍_g) + 3H_2(g) ⇋ 2NH_3(g)

∆H = -92.38KJ mol^-1

In the above reaction, total 2 moles product are obtained from total 4 moles of reactants. Hence, according to Le Chatelier's principle, increase in pressure is advantageous to get more ammonia but for the reactions occurring in closed vessel and so the pressure has to be kept limited. In addition, this reaction is exothermic and so according to Le Chatelier's principle, the decrease in temperature is advantageous but decrease in temperature affects the rate of reaction.

By making compromise with these two and using catalyst whereby the energy of activation is decreased and increasing rate of reaction, more possible product is obtained in less time. Hence, Haber used iron powder as catalyst in the production reaction of ammonia and satisfactory results were obtained. In the production of ammonia, the values of optimum pressure and temperature are 200 bar and 773 K temperature respectively and iron powder is used as catalyst. If the value of equilibrium constant of the reaction K is very low then the use of catalyst is not fruitful or helpful.

Ø Ionic equilibrium in solution:

Under the effect of change of concentration on the direction of equilibrium. There are numerous equilibria that involve ions only. In the following sections we will study the equilibria involving ions. It is well known that the aqueous solution of sugar does not conduct electricity. When common salt (sodium chloride) is added to water it conducts electricity. Also, the conductance of electricity increases with an increase in concentration of common salt. Michael Faraday classified the substances into two categories based on their ability to conduct electricity.

Fe^3+(aq) + SCN^-(aq) ⇋ [Fe(SCN)]^2+(aq)

One category of substances conduct electricity in their aqueous solutions and are called electrolytes while the other do not and are thus, referred to as nonelectrolytes. Faraday further classified electrolytes into strong and weak electrolytes. Strong electrolytes on dissolution in water are ionized almost completely, while the weak electrolytes are only partially dissociated. It should be noted that in weak electrolytes, equilibrium is established between ions and the unionized molecules. This type of equilibrium involving ions in aqueous solution is called ionic equilibrium. Acids, bases and salts come under the category of electrolytes and may act as either strong or weak electrolytes.

Types of electrolyte	example
(1) Strong electrolyte	Aqueous solution of HCl, NaOH, KCl, NaCl etc.
(2) Weak electrolyte	Aqueous solution of CH₃COOH, NH₄OH, CH₃NH₂ etc.,