SURFACE CHEMISTRY पृष्ठ रसायन
Surface chemistry deals with phenomena that occur at the surfaces or interfaces. सतह या अंतरापृष्ठ पर होने वाली परिघटनाओं की पढ़ाई इसमें होती है।
The interface or surface is represented by separating the bulk phases by a hyphen or a slash. For example, the interface between a solid and a gas may be represented by solid-gas or solid/gas.
Due to complete miscibility, there is no interface between the gases.
The bulk phases that we come across in surface chemistry may be pure compounds or solutions. Solutions का अर्थ तरल पदार्थ ही नहीं होता है, जैसे शर्बत। Stainless Steel भी एक सोल्यूशंस है। इसमें लोहा सोलवेंट है जिसमें कार्बन या अन्य धातुएँ सूक्ष्म मात्र में घुली होती हैं।
The interface is normally a few molecules thick but its area depends on the size of the particles of bulk phases – Both the phases.
Many important phenomenon, noticeable amongst these being corrosion, electrode processes, heterogeneous catalysis, dissolution and crystallisation occur at interfaces. The subject of surface chemistry finds many applications in industry, analytical work and daily life situations.
In this lesson we will deal with the following:
- Adsorption अधिशोषण
- Catalyst उत्प्रेरक
- Colloidal Solutions कोलाइडी विलयन
Adsorption अधिशोषण
It is the phenomenon of attracting and retaining the molecules of a substance on the surface of a solid resulting into a higher concentration on the surface than in the bulk.
यह परिभाषा आपके पुस्तक में दिया गया है इसलिए कक्षा व इंतहान के लिए हम अपने ज्ञान को यहीं तक सीमित रखने के लिए विवश हैं। परंतु competitions के लिए जान लेना उचित होगा कि Adsorption गैस – गैस के बीच में नहीं हो सकता है; उसके अलावा यह इन सभी के बीच में होता है –
Solid – gas, solid – liquid, liquid – gas, liquid – liquid (proved the liquids are immescible) & even between solids – solids. Please note that adsorption between two solids is not a rare phenomenon but a widespread phenomenon.
We ought to know 3 terms here:
- Adsorbate – The substance that is adsorbed.
In physisorption it is held by weak van der Waals forces.
In chemisorption it is held by a strong chemical bond.
- Adsorbent – The substance on which adsorption occurs.
- Adsorption isotherm – It is defined as the relationship between the pressure of the gas and extent of adsorption at a constant temperature.
Almost all solids adsorb gases. The extent of adsorption depends upon nature of gas and solid both, as well on the surface area of the solid, pressure of gas and temperature of the system.
Freundlich, 1909, gave an empirical relationship which came to be known as Freundlich adsorption isotherm.
The relationship can be expressed by the following equation –
x/m = k.p1/n (n>1)
x is the mass of gas adsorbed on mass m of the adsorbent at presssure p. k and n are constants depending on the nature of the adsorbent and the gas at a particular temperature.
Limitations of Freundlich adsorption isotherm
Experimentally it was determined that extent of gas adsorption varies directly with pressure, and then it directly varies with pressure raised to the power 1/n until saturation pressure, Ps is reached. Beyond that point, the rate of adsorption saturates even after applying higher pressure. Thus, the Freundlich adsorption isotherm fails at higher pressure. But it holds good at lower pressures.
Adsorption from Solution Phase
Solids can adsorb solutes from solutions also. It has been observed that in the case of adsorption from solution phase, the extent of adsorption:
- decreases with an increase in temperature.
- increases with an increase of surface area of the adsorbent.
- depends on the concentration of the solute in solution.
- depends on the nature of the adsorbent and the adsorbate.
The precise mechanism of adsorption from solution is not known. Freundlich’s equation approximately describes the behaviour of adsorption from solution with a difference that instead of pressure, concentration of the solution is taken into account, i.e., the same equation is used. Here the pressure, p is replaced by Concentration, C. So,
x/m = k.C1/n (n>1)
SORPTION
It is a term used to indicate the process of taking up of a gas or liquid by a solid without specifying whether the process is adsorption or absorption or a combination of both as in the example of chalk dipped in ink.
CHALK & INK EXAMPLE:- Dip a chalk stick in ink. The chalk stick gets coloured. Break it. What we observe. The Chalk stick is wet but the colour has not entered into it – It is white. Why? When dipped the solute molecules i.e. the pigment molecules get adsorbed over the chalk but the solvent portion of the ink gets absorbed inside the Chalk stick.
DIFFERENCE BETWEEN ADSORPTION AND ABSORPTION
In adsorption, the substance is concentrated only at the surface and does not penetrate through the surface to the bulk of the adsorbent, while in absorption, the substance is uniformly distributed throughout the bulk of the solid.
MECHANISM OF ADSORPTION
Adsorption arises due to the fact that the surface particles of the adsorbent are not in the same environment as the particles inside the bulk. Inside the adsorbent all the forces acting between the particles are mutually balanced but on the surface the particles are not surrounded by atoms or molecules of their kind on all sides, and hence they possess unbalanced or residual attractive forces.
These residual attractive forces of the adsorbent are responsible for attracting the adsorbate particles on its surface.
The extent of adsorption increases with the increase of surface area per unit mass of the adsorbent at a given temperature and pressure.
Another important factor featuring adsorption is the heat of adsorption. During adsorption, there is always a decrease in residual forces of the surface, i.e., there is decrease in surface energy which appears as heat.
Adsorption, therefore, is invariably an exothermic process. In other words, ∆H of adsorption is always negative.
In addition to that, when a gas is adsorbed, the freedom of movement of its molecules become restricted. This amounts to decrease in the entropy of the gas after adsorption, i.e., ∆S is negative.
Adsorption is thus accompanied by decrease in enthalpy as well as decrease in entropy of the system.
For a process to be spontaneous, the thermodynamic requirement is that, at constant temperature and pressure, ∆G must be negative, i.e., there is a decrease in Gibbs energy. On the basis of equation, ∆G = ∆H – T∆S, ∆G can be negative if ∆H has sufficiently high negative value as – T∆S is always positive (as discussed above ∆S is negative, so – – = +).
Thus, in an adsorption process, which is spontaneous, ∆H is highly negative in the beginning. As the adsorption proceeds, value of ∆H becomes less and less negative ultimately ∆H becomes equal to T∆S and ∆G becomes zero. At this state equilibrium is attained.
TYPES OF ADSORPTION
As discussed earlier there 2 types. Let us see there salient features in the table given below. TO BE CONTD. PL VISIT AGAIN