Question

What is Le Chatelier's Principle and how is it important especially in chemical industry?

250 or more words please explaining.

Answer 1

According to Le Châtelier's principle, When any system at equilibrium is subjected to change in concentration, temperature, volume, or pressure, then the system readjusts itself to counteract (partially) the effect of the applied change and a new equilibrium is established.

In chemistry, the principle is used to manipulate the outcomes of reversible reactions, often to increase the yield of reactions.

Example for this is helps us to predict the yield of ammonia, when the pressure or the temperature is changed. The principle used is if the conditions of a closed system at equilibrium change, the equilibrium position will move to the direction that will oppose this change. In this way, either the forward or the backward reaction will be favoured, thus producing a higher or a lower yield of product.

The uses of this principle in chemistry

Effect of change in concentration:

If we change the concentration of a chemicals in the reaction, that will alter the equilibrium to the side that would reduce that change in concentration. The reaction system will try to partially oppose the change affected to the original state of equilibrium. Because of this the rate of reaction, extent, and yield of products will be altered.

Eg: Production of methanol from carbon monoxide and hydrogen equilibrium

CO + 2 H₂ ⇌ CH₃OH

If we increase the concentration of carbon monoxide in the reaction system, we can assume that the amount of methanol will increase based on Le Chatelier's principle.

Effect of change in temperature:

Based on this principle if we take the production of ammonia (the Haber process) as example,

N₂(g) + 3 H₂(g) ⇌ 2 NH₃(g)    ΔH = -92 kJ mol⁻¹

Because this reaction is exothermic, it produces heat:

N₂(g) + 3 H₂(g) ⇌ 2 NH₃(g) + heat

If the temperature is increased, the equilibrium position will move to the left, as to oppose the increase in temperature (the forward reaction is exothermic). The backward reaction is favoured and therefore a lower yield of ammonia is produced.

If the reaction is in low temperature, tere is a chance of good yield. But the rate of the reaction will decrease. so, it is necessary to set the temperature at a compromise value to get reasonable rate with good yield.

Effect of change in pressure:

The equilibrium concentrations of the products and reactants may depend on the partial pressures of the products and reactants, but if the numbers of moles of gaseous reactants are equal to number of moles of gaseous products, pressure has no effect on equilibrium.

In the case of ammonia production using the haber process, If the pressure is increased, the equilibrium position will move to the right, as to oppose the increase in pressure (there are fewer moles of gas on the right side (2 moles) of the equation than the left side (4 moles)). The forward reaction is favoured and therefore a higher yield of ammonia is produced.

Effect of change in volume:

Effect of change in volume is almost same as that of change in pressure. Changing the volume of the reaction system changes the partial pressures of the products and reactants in the reaction and can affect the equilibrium concentrations. There is no effect on a chemical reaction where the number of moles of gas is the same on each side of the chemical reaction equation.

Effect of adding an inert gas:

Non-reactive gas does not change the equilibrium of reaction equation, as the inert gas appears on both sides of the chemical reaction equation. Example, if A and B react to form C and D, but X does not participate in the reaction, {\displaystyle {\ce {{{\mathit {a}}A}+{{\mathit {b}}B}+{\mathit {x}}X<=>{{\mathit {c}}C}+{{\mathit {d}}D}+{{\mathit {x}}X}}}} it is true that the total pressure of the system increases, the total pressure does not have any effect on the equilibrium constant; rather, it is a change in partial pressures that will cause a shift in the equilibrium. If, however, the volume is allowed to increase in the process, the partial pressures of all gases would be decreased resulting in a shift towards the side with the greater number of moles of gas.

Effect of a catalyst:

A catalyst is used to increases the rate of a reaction. Catalyst will not consume in the reaction. The use of a catalyst does not affect the position and composition of the equilibrium of a reaction, because both the forward and backward reactions are sped up by the same factor.

In the Haber process for the synthesis of ammonia (NH₃), iron and molybdenum is used as the catalyst to increase the rate of the reaction.