Equilibrium:
Most of the chemical reactions when carried out in a closed vessel do not go to completion under given set of conditions of temperature and pressure. In fact, in all such cases, in the initial state, only the reactants are present but as the reaction proceeds, the concentration of the reactants decrease and that of products increases. Finally, a stage is reached when no further change in concentration of reactants and products is observed. At this stage, there is a balance between reactants and products and the reaction appears to have stopped. This state is called equilibrium state.
Therefore, “Equilibrium is the state at which the concentration of the reactants and products do not change with time”.
The equilibrium may establish in physical processes such as evaporation of water, melting of solids, dissolution of salts, etc as well as in chemical reactions. Therefore, equilibrium may be classified as:
i) Physical equilibrium:
Equilibrium set up in physical processes is called physical equilibrium. e.g. equilibrium in melting ice to water, evaporation of water to steam, dissolution of sugar.
ii) Chemical equilibrium:
Equilibrium set up in chemical processes is called chemical equilibrium. e.g. equilibrium in decomposition of calcium carbonate, reaction between hydrogen and iodine.
CaCO3 ——> CaO + CO2
H2 + I2 ——> 2 HI
Equilibrium in the physical changes:
The various equilibria which can exist in any physical system are:
- Solid —-> Liquid
- Liquid —-> Gas
- Solid —-> Gas
Let us consider this equilibrium briefly-
Solid-Liquid Equilibrium:
Let us place some ice and water in a perfectly insulated thermos flask at 273 K and normal atmospheric pressure. Since the flask is insulated, there will be no exchange of heat between its contents and the surroundings. As long as the temperature remains constant, there is no change of mass of ice and water. It appears that nothing is happening in the system. But if we could observe the individual molecules of ice and water, we would notice that a considerable activity is still taking place. Some molecules from the liquid water join on to the ice and at the same time, some molecules of ice go into the liquid. These two processes act in opposite directions to each other. Since there is no change in mass of ice and water, the numbers of molecules going from water to ice are the same as the number of molecules going from ice to water. In other words, the rate of transfer of molecules from ice into water (melting) and the rate of transfer of molecules from water into ice (freezing) become equal. Therefore, the system shows constancy in concentration and is termed as equilibrium state. This state is represented as:
H2O (s) ——-> H2O (l)
Ice water
At equilibrium, Rate of melting = Rate of freezing
It may be noted that ice and water are in equilibrium only at a particular temperature and pressure. The temperature at which the solid and the liquid phases are at equilibrium at atmospheric pressure is called the normal freezing point of that substance. The freezing point of a substance changes slightly with pressure.
Liquid- Gas Equilibrium:
Let us consider the equilibrium between liquid water and its vapours during evaporation of water in a closed vessel.
When liquid water is placed in a closed vessel at room temperature, it starts evaporating. As the process continues, more and more water molecules escape from liquid to the vapour state and pressure starts increasing. The change of pressure can easily be measured with the help of a manometer attached to the vessel. As evaporation continues, the pressure goes on increasing and the level in the right limb starts rising.
After some time, it has been observed that pressure becomes constant indicating that no more water is evaporating even though some liquid water is still present. This state of constant pressure can be attained for any period of time provided the temperature remains constant. This means that the amount of water does not decrease any further and amount of water vapours does not increase any more. This represents an equilibrium state and may be represented as:
H2O (l) ————> H2O (g)
Water vapours
Initially, the rate of condensation is less than the rate of evaporation. But as evaporation proceeds, the concentration of molecules in vapour phase increases and therefore, the rate of condensation also increase. Finally, the rate of evaporation becomes equal to the rate of condensation. Thus, at equilibrium,
Rate of evaporation = Rate of condensation
The pressure exerted by the vapours in equilibrium with the liquid at a particular temperature is called equilibrium vapour pressure or vapour pressure of the liquid. This is constant at a particular temperature but varies with varying temperature. With the increase in temperature, the vapour pressure increases, however, it is independent of the amount of water in the vessel.
Equilibrium involving Solids in Liquids and Gases in Liquids:
i) Solids in liquids:
When we add a small quantity of sugar to about 100 ml of water at 273 K, sugar dissolves. Such a solution is said to be unsaturated solution. However, if we continue adding sugar to the same amount of water at the same temperature, a stage is reached when no more sugar appears to dissolve. At this point, the solution is said to be saturated. Once the solution becomes saturated, then the concentration of sugar in the solution becomes constant. This indicates that a state of equilibrium has been reached between the undissolved sugar and dissolved sugar. This may be represented as:
Sugar (in solution) ——>Sugar (solid)
At this stage, the number of molecules of sugar going into the solution (dissolution process) becomes equal to the number of sugar molecules depositing back on the undissolved sugar from the solution (precipitation process).
Thus at equilibrium,
Rate of dissolution of sugar = Rate of precipitation of sugar
ii) Gases in liquids:
Gases can be dissolved in suitable liquids. At a given temperature, a liquid can dissolve only a certain definite mass of the gas. This suggests that a state of equilibrium exists between the molecules in the gaseous state and the molecules dissolved in the liquid. For example, when carbon dioxide is dissolved in soda water, the following equilibrium exists:
CO2 (g) —–> CO2 (in solution)
The solubility of a gas in the liquid depends on the pressure. The dependence of the solubility of a gas and its pressure above the solvent is given by a law known as Henry’s law.
According to Henry’s law, the mass of a gas dissolved in a given mass of a solvent at a particular temperature is proportional to the pressure of the gas above the solvent.
m α p
Or m = k p
Where,
m= mass of gas dissolved
p = Pressure of the gas
k = Proportionality constant
Note: The solubility of a gas in a liquid decreases with increase in temperature.
General Characteristics of Physical Equilibrium:
The important characteristics of equilibria involved in physical processes are as follows:
1) Equilibrium can be established only in case of closed system, i.e. the system should neither gain matter from the surrounding nor lose matter to the surroundings.
2) Equilibrium is always dynamics in nature.
3) All measurable properties of the system become constant at equilibrium.
4) When equilibrium is attained, there exists an expression of concentration of the substances involved in equilibrium which become constant at a given temperature.
5) Each equilibrium is characterized by a constant called equilibrium constant. The value of equilibrium constant indicates the direction and extent of the process.
Equilibrium in Chemical processes:
A chemical reaction is said to be taken place when the concentrations of the reactants decreases and that of the products increases with time.
There are many chemical reactions which do not proceed to complete i.e. the reactants are not completely converted into products. In such reactions, only a part of the reactants get converted into products are called reversible reactions. In a reversible reaction, both reactants and the products are present in equilibrium with each other.
Therefore, the equilibrium between different chemical species present in the same or different phases is called chemical equilibrium.
The chemical equilibrium may be classified into homogeneous and heterogeneous depending upon whether the reaction involves only one phase or more than one phase respectively.
1) Homogeneous Chemical Equilibria:
A reversible reaction in which all the reactants and the products at equilibrium are in the same phase is referred to as homogeneous equilibrium. e.g.
i) Gas Phase:
H2 (g) + I2 (g) ——> 2HI (g)
N2 (g) + 3H2 (g) —–> 2 NH3 (g)
SO2 (g) + O2 (g) ——> 2 SO3 (g)
N2 (g) + O2 (g) ——-> 2 NO (g)
ii) Liquid Phase:
Here all the reactants and the products are liquid and immiscible with each other.
CH3COOH (l) + CH3CH2OH (l) ——–> CH3COOC2H5 (l) + H2O (l)
Heterogeneous Chemical Equilibria:
A reversible reaction involving equilibrium between various chemical species present in two or more phases is said to be a heterogeneous chemical equilibrium. e.g.
i) Decomposition of KClO3 on heating to form KCl and oxygen.
KClO3 (s) ——–> KCl (s) + O2 (g)
ii) Thermal dissociation of CaCO3.
CaCO3 (s) ——> CaO (s) + CO2 (g)
iii) Reaction of steam and iron.
3Fe (s) + 4H2O (g) ——> Fe3O4 (s) + 4 H2 (g)
Characteristics of Chemical Equilibrium:
Characteristics of chemical equilibrium are as follows:
1) A chemical equilibrium can be established only in case of closed system, i.e. the system should neither gain matter from the surrounding nor lose matter to the surroundings.
2) A chemical equilibrium can be attained from either side of a reversible reaction.
3) The properties of the system become constant at equilibrium and remain unchanged thereafter. e.g. in the decomposition of CaCO3 and evaporation of H2O in a closed vessel, pressure becomes constant at equilibrium.
4) Chemical equilibrium is a dynamic equilibrium i.e. during the course of reversible reaction, a stage is reached when the concentration of the reactants and products become constant, called state equilibrium. At equilibrium, the concentration of reactants and products may or may not be equal but remain unchanged with time.
This is however, do not mean that the reaction has stopped at the equilibrium. In fact, at this stage the reaction proceeds in such a way that the rate of formation of products from reactants is equal to the rate of formation of reactant from the products. Such a state of equilibrium is termed as Dynamic equilibrium as shown in the figure-(a).
5) The state of equilibrium in any reversible reaction is characterized by a constant called equilibrium constant (K). The value of equilibrium constant depends upon the nature of reaction, temperature and pressure.