→ The adsorption of gases on solid adsorbents depends upon following factors: 1. Nature of Gas being adsorbed : The nature of physical adsorption is not specific. So, each gas is adsorbed at the surface of solid in low or high amount. But the gases, which are easily liquified, such as SO2, HCl, CO2, NH3 etc. are readily adsorbed than stable gases such as H2, N2, O2, etc., The rate of liquification of the gas depends on critical temperature.

→ Critical Temperature : The minimum temperature above which a gas cannot be liquified even at high pressure may be is called critical temperature. Greater the critical temperature, the liquification of gas will be easily taken place. Due to this, it will be readily adsorbed.

Example : The decreasing order of adsorption of gases on 1 g activated charcoal is as follows:

→ Surface Area of the Adsorbent : Extent of adsorption depends upon the surface area of the adsorbent i.e., larger the surface area, larger will be extent of adsorption. For example, finely divided metals like platinum, palladium etc. and porous substances (charcoal, silica gel) have larger surface area and show larger extent of adsorption.

→ Actually surface area depends on the size of particles. Smaller the size of particle, larger will be the surface area “The surface area per gram of the adsorbent is called specific surface area of the adsorbent”. Adsorption a specific surface area of the adsorbent.

→ The adsorbing power of various metals are as follows:
Colloidal Pd > Pt > Au > Ni

→ Effect of Pressure : At constant temprature, on increasing pressure the extent of adsorption of gases increases. At low temperature, it is observed that the extent of adsorption of a gas increases very rapidly with pressure. It is found that at low temperature, the extent of adsorption is directly proportional to pressure. But this fact is not applicable at high temperature.

→ “The relationship between amount of adsorption and pressure of gas at constant temperature is called adsorption isotherm”.

→ It is represented by an equation or graph. The amount of adsorption is represented by $$\frac{x}{m}$$
The graph between $$\frac{x}{m}$$ and pressure (p) is obtained as following:

→ According to graph, the amount of adsorption (x/m) increases on increasing pressure. At equilibrium pressure (Ps), am reaches it maximum value and no more adsorption takes place even if the pressure is further increased. This state is called saturation state and the corresponding pressure (Ps) is called saturation pressure. In this stage, adsorbed gas will make a molecular layer and adsorption is a reversible process in which the amount of adospriton will be equal to the amount of desorption at Ps pressure.

→ Effect of Temperature : As we know that the adsorption is an exothermic process hence on increasing temperature, extent of adsorption decreases at a constant pressure.

→ The physical adsorption or physisorption decreases on increasing temperature because of breaking of weak van der Waal’s forces due to increase in kinetic energy of adsorbed gaseous molecules.

→ The chemical adsorption or chemisorption first increases with increase in temperature upto a certain limit i.e, it reaches maximum at certain temperature and then decreases on increasing temperature further. It indicates that chemisorption is similar to a chemical reaction and needs activation energy. This fact thus prooves that chemical adsorption involves formation of chemical bonds between adsorbate and adsorbent.

→ The relationship between the extent of adsorption and temperature at constant pressure is known as adsorption isobar. The above given curves are examples of adsorption isobar.

→ Activation of Adsorbent : That process in which adsorbing power of an adsorbent increases, is known as activation of adsorbent. Due to this activation the specific surface area of an adsorbent increases. Activation can be done by any of the following methods :

• By making surface of the adsorbent rough: It can be done by mechanical rubbing. Due to rubbing surface, area increases and hence adsorption increases.
• By removing the already adsorb gases : For example, charcoal can be activated by heating it in superheated steam or in vacuum at a temperature between 623 to 1273 K. The charcoal obtained by the process is known as activated charcoal.
• Adsorbent in course grain form can be powdered further. The adsorbent is broken into too fine powder that the gases can be penetrate into it easily for adsorption.

→ As we know that adsorption is directly proportional to pressure i.e., as pressure increases, extent of adsorption increases.

→ “The relation between the amount of substance adsorbed by the adsorbent and the equilibrium gas pressure at constant temperature is known as adsorption isotherm.” This relation can be expressed in the form of a curve. The extent of adsorption is expressed by $$\left(\frac{x}{m}\right)$$ where x is the mass of adsortate and m is the mass of adsorbent.

→ In 1909, Freundlich gave an empirical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature.

→ According to Freundlich, as pressure is increased the extent of adsorption also increases. When the value of extent of adsorption i.e., (x/m) becomes maximum then the pressure at this point is known as saturation pressure (Ps). At saturation pressure, the extent of adsorption will be maximum. On further increasing the pressure, the extent of adsorption does not increases. Now the value of extent of adsorption becomes constant. It can be understand easily as :
At low pressure : At low pressure, the graph is nearly straight i.e.,
$$\frac{x}{m}$$ ∝ p1
$$\frac{x}{m}$$ = kp1 (Where k = constant) ……….(1)

→ At high pressure : At high pressure, the extent of adsorption becomes independent of the values of pressure (p). Hence,

$$\frac{x}{m}$$ ∝ p0
$$\frac{x}{m}$$ = k ………. (2)

→ At Intermediate pressure : At intermediate pressure the extent of adsorption will depend on Praised to powers between 1 and Oie, fraction. Hence,
$$\frac{x}{m}$$ ∝ p1/n
$$\frac{x}{m}$$ = kp1/n Where n > 1 ………….. (3)
In above equation n is a positive integer. Here in equations ‘n’ and ‘ are two constants and their values depend on the nature of adsorbate and adsorbent at a particular temperature. This equation (1) is known as Freundlich adsorption isotherm.
The value of kand n can be calculated as follows.
Taking logarithms on both sides of equation(1), we get
log = $$\frac{x}{m}$$ = log k + $$\frac{x}{m}$$ log P

→ The above equation is also known as Freundlich adsorption isotherm equation. The validity of Freundlich isotherm equation can be proved by plotting a curve, where $$\frac{x}{m}$$ is taken on y-axis and log pis taken on x-axis. This graph between log x/m vs log p should give straight line. If it comes to be a straight line, the Freundlich isotherm is valid otherwise not. The slope of the straight line gives the value of 1/n and the intercept on y-axis gives the value of log k.

Limitations of Freundlich adsorption Isotherm :

The main drawbacks of Freundlich adsorption isotherm is as follows:

• It is empirical and has no theoretical basis.
• Freundlich adsorption isotherm is valid only at low pressure. It fails at high pressure.
• The values of constant’ and ‘n are not temperature dependent. They vary with temperature.