21 Chemistry -- Chemical Equilibrium

State and explain Lichateliers principle and discuss its application.

1 Answers

1. Le-Chatelier's principle

Equilibrium state of a reversible reaction can be disturbed by changing temperature, pressure, volume and concentration of reaction system. This fact was thoroughly studied by a French Chemist Henry Louis Le-Chatelier in 1884. He made a generalization which is termed as Le-Chatelier's principle. This principle states that, ‘when a system in equilibrium is subjected to a change in temperature, pressure, volume and concentration, the equilibrium shifts in such a direction so as to nullify (neutralize) the effect of the change.’

The important conclusions of the principle can be expressed as:

a. Effect of change in the temperature

Increasing the temperature shifts the equilibrium to that direction where heat is absorbed i.e. increase in temperature favors the endothermic side of reaction in the equilibrium. On the other hand, decreasing the temperature, shifts the equilibrium to that direction where heat is evolved, i.e. decrease in temperature favors the exothermic side of reaction in the equilibrium.

b. Effect of change in the pressure and volume

Increase in pressure shifts the equilibrium to that direction where volume is decreased. Similarly, decrease in pressure shifts the equilibrium to the direction having more volume. So, the reactions which proceed by the decrease in the volume, are favored at high pressure. The reactions which proceed by increase in the volume, are favored at low pressure.

c. Effect of change in the concentration

If any of the component is added from outside its concentration is increased and the equilibrium will shift in that direction where the added substance is consumed. This means, increasing the concentration of reactants shifts the equilibrium to the right hand side and increasing the concentration of the products shifts the equilibrium to the left hand side. However, if the concentration of a substance is decreased, the equilibrium will shift in a direction which will produce more of this substance.

d. Effect of addition of inert gas

Effect of inert gas can be studied at constant volume and at constant pressure.

i. At constant volume

When inert gas is added in a system at equilibrium at a constant volume, partial pressure of individual gas remains constant that is why there is no effect of the addition of inert gas at equilibrium.

ii. At constant pressure

When inert gas is added in a system at equilibrium at a constant pressure, the partial pressure of the individual gas gets decreased. That is why addition of inert gas at a constant pressure is equivalent to the decrease in pressure. In this case, the equilibrium shifts in the direction, where the volume is increased.

2. Applications of Le-Chatelier's principle

Le-Chatelier's principle is entirely a general law and is applicable to physical as well as chemical equilibrium. This law has much importance for the prediction of the conditions to get maximum yield of the desired products in a reversible reaction.

A. Application of Le-Chatelier's principle in physical equilibria

i. Let us take an example of melting of ice.

a. Effect of change in temperature

Since the melting of ice is endothermic process, increase in temperature shifts the equilibrium in forward side to form more water and decrease in temperature shifts the equilibrium backward to form more ice.

b. Effect of change in pressure

For the given mass of water, ice has more volume than water. On increasing the pressure the equilibrium shifts to that side where volume is decreased, i.e. the equilibrium shifts towards water. Increase in pressure decreases the melting point of ice to form more water. On decreasing the pressure, the equilibrium shifts to the side having more volume, i.e. the equilibrium shifts towards the formation of ice. It can be said that, on decreasing the pressure, the freezing of water increases to form more ice. This implies that, melting of ice is favored at high temperature and high pressure, and the reverse change i.e. freezing of water to ice is favored at low temperature and low pressure.

ii. Let us consider dissolution of a gas in water.

Since the dissolution of gas is an exothermic process and proceeds by the decrease in the volume, the favourable condition for the dissolution of the gases is low temperature and high pressure. This is the reason that aerated drinks are kept under high pressure and low temperature.

B. Application of Le-Chatelier's principle in chemical equilibria

1. Let us take an example of synthesis of ammonia by Haber's process,

a. Effect of change in temperature

The above reaction is exothermic in forward direction and endothermic in backward direction. On increasing the temperature, equilibrium shifts to backward side(endothermic side of reaction) and on decreasing the temperature, the equilibrium shifts to forward side (exothermic side of reaction) so as to nullify the effect of change according to the Le-Chatelier's principle. Therefore, the formation of ammonia from N2 and H2 is favoured at low temperature.

Though low temperature is favourable here, at very low temperature, equilibrium is reached slowly therefore, from the economic point of view 450°C is optimum temperature.

1 volume of nitrogen reacts with 3 volume of hydrogen (total number of moles of reactants=4) to form 2 volume of ammonia. Hence, the reaction takes place by decrease in volume. According to Le-Chatelier's principle increase in pressure favors the side where volume is decreased i.e. towards the forward side. So, increase in pressure shifts the equilibrium to forward and more ammonia will be formed. On the other hand, decrease in pressure shifts the equilibrium to backward and production of ammonia will be low.

c. Effect of change in concentration

When concentration of N2 or H2 is increased at equilibrium, the equilibrium is shifted to utilize added N2 or H2 i.e. equilibrium shifts to forward direction. Similarly on adding more NH3 at equilibrium, the equilibrium shifts to backward direction. Hence, formation of NHs is favoured by,

i. Low temperature (optimum temperature is about 450 ℃)

ii. High pressure

iii. High concentration of N2 and H2.

2. Let us consider the reversible reaction

In the above reaction, the volume is increased at the right hand side. Decrease in the pressure shifts this equilibrium at right hand side. So, the decomposition of CaCO3 is favoured at low pressure.

For the reaction CO+2H2 --------> CH3OHa. Kp > Kc b. Kp< Kc c. Kp =Kc d. Kp =Kc =0

Why is ammonia termed as a base although it does not have OH- ions in it?

One of the many reasons I would love to explain here is according to Lewis concept of bases.

The concept of acids, bases and salts, we have been discussing throughout the secondary education level was the Arrenhius Concept of Acids and Bases but which is not the only one describing these properties of compounds.

There are several other concepts such as:

Lewis concept of Acids and Bases
Bronsted-Lowry concept of Acids and Bases
Arrenhius concept of Acids and Bases

Lewis explains bases as the...

What is meant by concentration quotient of chemical reaction under what condition, it isequal to equilibrium constant?

The ratio of the product of concentrations of substances produced (products) to that of reactants is known as concentration quotient. It is denoted by Q. For example, for a reversible reaction. At equilibrium, Q becomes equal to K (equilibrium constant).

Can equilibrium be achieved between water and its vapor in an open beaker?

No, equilibrium cannot be achieved between water and its vapour in an open beaker. If one attempts to do so, he/she needs to maintain the equilibrium of water and vapour present in the given space.

Water vapour, being gas and lighter than air, rises up in the atmosphere and easily escapes away from the beaker. Thus, water vapour produced escapes into the surrounding and no equilibrium can be maintained between water and its vapour.

Why do we sweat more on a humid day?

Humidity is the measure of water vapor content in the air. Humidity also is reflected by dew point. Dew point is the temperatures air must be cooled in order to reach saturation.

When you have a high dew point which indicates more moisture present in the air, the body takes a longer time to naturally cool off.

The human body cools off by the process of evaporative cooling. Evaporative cooling is temperature reduction resulting from the evaporation of a liquid. Also, on a more scientific note,...

Why ice melts slowly at higher altitude?

For a given mass of water, ice has more volume than water. The atmospheric pressure is less or decreases at higher altitudes. On decreasing the pressure the equilibrium shifts to the side having more volume; the equilibrium shifts toward the formation of ice . It can be stated that on decreasing the pressure the freezing of water increases to form more ice so the melting of ice is favored at high temperature and high pressure. Hence, ice melts slowly at higher altitudes due to low pressure.

What is meant by chemical equilibrium? Discuss the characteristics of chemical equilibrium.

Chemical equilibrium is the state of the system in which there is no observable change in the properties of system with time. The rate of forward reaction is equal to the rate of backward reaction and there is no change in the observable properties of the reaction system which is achieved only in a reversible reaction performed in a closed system.

The characteristics of chemical equilibrium are :-

Chemical equilibrium is dynamic in nature. When a reversible reaction has attained equilibrium...

State and explain the law of mass action. Derive an expression for the equilibrium constantfor a reversible reaction.

The law of mass action is defined as at constant temperature , the rate of a forward reaction is directly proportional to the product of the molar concentration of the reactants each raised to a power equal to its stoichiometric coefficient that appear in the balanced chemical equation.For example , considerthe following general reaction; aA+ bB Products

Hence, according to the law, rate of reaction = k [A]^a[B]^b, where 'a' and 'b' represents the stoichiometric coefficient of reactants A and B...

Derive the law of mass action for the expression:nA + mB------>pC + qD.

The dissociation of CaCO3 is suppressed at high pressure explain.

The dissociation of CaCO3 is suppressed at high pressure. In the above reaction, zero volume of reactants react to form the one volume of gaseous product CO2. With the increase in pressure, the equilibrium shifts towards the direction to decrease the volume. Since the left hand side has low volume so the reaction proceeds towards the left hand side. Hence the dissociation of CaCO3 is suppressed due to the addition reaction of CaO and CO2.

How does the solubility of a gas in liquid vary with pressure?

The solubility is a measure of the concentration of dissolved gas particles in a liquid and is the function of gas pressure. As you increase the pressure of the gas, the collision frequency increases and the solubility goes up. As you decrease the pressure, the solubility goes down due to decrease in collision frequency.

Study the equilibrium of the reactionPCI5(g)-------->PCl3 (g) + Cl2(g) --- HeatHow is the equilibrium of the reaction is affected by the temperature, pressure andintroduction of the chlorine gas?

Since the reaction is endothermic in forward direction, the increase in temperature shifts the equilibrium towards the forward direction whereas the decrease in temperature shifts the equilibrium towards the backward direction.

In the above equilibrium, one mole of reactant reacts to give two moles of product. So, equilibrium takes place by the increase in volume. Decrease in pressure shifts the equilibrium in the forward direction whereas the increase in pressure shifts the equilibrium in the...

For a reaction, A + B → 2C2 moles of A and 3 moles of B are allowed to react. If equilibrium constant is 4 at 400° C, then the mole of C at equilibrium is

What are the differences between Dynamic Equilibrium and Static Equilibrium?

The differences between dynamic equilibrium and static equilibrium are :-

Dynamic Equilibrium	Static Equilibrium
1. This type of equilibrium is irreversible in nature.	1. This type of equilibrium is irreversible in nature.
2. In dynamic equilibrium, the forward and backward reaction rates are equal .	2. In static equilibrium, the forward and backward reaction rates are zero.
3. This equilibrium implies that reactants and products are still participating in chemical...

ask mattrab