- Surface chemistry deals with phenomena that occur at the surfaces or interfaces. The interface or surface is represented by separating the bulk phases by a hyphen or a slash.
Example: Interface between a solid and gas may be represented by solid-gas or solid/gas. Due to complete miscibility, there is no interface between the gases.
- The interface is normally a few molecules thick but its area depends on the size of the particles of bulk phases. Many important phenomena, noticeable amongst these being corrosion, electrode processes, heterogeneous catalysis, dissolution and crystallisation occur at interfaces.
- In this Unit, you will be studying some important features of surface chemistry such as adsorption, catalysis and colloids including emulsions and gels.
- There are several examples which reveal the fact that the surface of a solid (or liquid) has a tendency to attract and retain the molecules of other immiscible phase with which it is brought into contact. These molecules remain only at the surface and do not go deeper into the bulk.
- This tendency of accumulation of molecular species at the surface than in the bulk of a solid (or liquid) is termed adsorption. The molecular species or substance which concentrates or accumulates at the surface is termed adsorbate and the material on whose surface the adsorption has taken place is called adsorbent.
- The reverse process i.e. removal of adsorbed substance from the surface is called desorption.
- The adsorption of gases on the surface of metals is called occlusion.
- The term sorption is employed when adsoption as well as absorption take place simultaneously.
Question 1: What do you mean by the term “Sorption”?
- Sorption is a physical and chemical process by which one substance becomes attached to another.
- The reverse of sorption is desorption.
Distinction Between Adsorption And Absorption
- In adsorption the concentration of the adsorbate increases only at the surface of the adsorbent, while in absorption the concentration is uniform throughout the bulk of the solid
- Adsorption is due to the fact that the surface particles of the adsorbent are in different state than the particles inside the bulk. Inside the adsorbent all the force 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 forces of the adsorbent are responsible for attracting the adsorbate particle on its surface.
- Adsorption is a surface phenomenon, whereas absorption is a bulk phenomenon.
- Adsorption occurs only at the surface of adsorbent, whereas absorption occurs throughout the body of the material.
Table: Difference between Adsorption and Absorption
Types of Adsorption
(A) Positive and Negative Adsorption
- When the concentration of the adsorbate is more on the surface of the adsorbent than in the bulk, it is called positive adsorption.
- When the concentration of the adsorbate is less relative to its concentration in the bulk, it is called negative adsorption.
(B) Physisorption and Chemisorption
- When a gas is adsorbed at the surface of a solid by week forces (Van der Waal's forces), it is called physical adsorption.
- When a gas is held on the surface of a solid by forces similar to those of a chemical bond, it is called chemical adsorption or chemiosorption. The chemical bonds may be covalent or ionic in nature. Chemisorption has a rather high energy of activation and is, therefore, often referred to as activated adsorption.
- Sometimes these two processes occur simultaneously and it is not easy to ascertain the type of adsorption.
- A physical adsorption at low temperature may pass into chemisorption as the temperature is increased.
- For example, hydrogen is first adsorbed on nickel by van der Walls' force. Molecules of hydrogen then dissociate and hydrogen atoms are held on the surface by chemisorption.
Potential energy map of Physisorption and Chemisorption
Table: Comparison of Physisorption and Chemisorption
Question 2:Which of the following is not characteristic of chemisorption?
- Chemisorption involves formation of chemical bonds between adsorbate and adsorbent molecules.
- Once the valency is satisfied, the adsorbent molecules can’t form bond with more adsorbate molecules. Thus only one layer is formed.
Characteristic of Adsorption
- Molecules at the surface of a solid, a metal, or a liquid experience in net inward force of attraction with free valencies.
- Adsorption is accompanied by evolution of heat. The amount of heat evolved when one mole of a gas is adsorbed on a solid, is known as molar heat of adsorption. Its magnitude depends upon the nature of the gas.
- The magnitude of gaseous adsorption depends upon temperature, pressure, nature of the gas and the nature of the adsorbent.
- Adsorption decreases with increase in temperature, since it is accompanied by evolution of heat.
- The adsorption increases with increase in pressure, since adsorption of gas leads to decrease in pressure.
- More readily soluble and easily liquifiable gases HCl, Cl2 , SO2 and NH3 are adsorbed more than the so called permanent gases such as H2 , O2 , N2 etc. because Vander Waal's forces involved in adsorption are much predominant in the former gases than the latter category of gases.
Factors which affect the extent of adsorption on solid surface
➢ Nature of the adsorbate (gas) and adsorbent (solid)
- Porous and finely powdered solid e.g. charcoal, fullers earth, adsorb more as compared to the hard non-porous materials. Due to this property powdered charcoal is used in gas masks used in coal mines.
- Gases with high critical temperature are ads or bed at higher extent as compared to gases with lower critical temperatures.
Table: Volumes of gases at N.T.P., adsorbed by 1g of charcoal at 288 K
➢ Surface area of the solid adsorbent
- The extent of adsorption depends directly upon the surface area of the adsorbent, i.e. larger the surface area of the adsorbent, greater is the extent of adsorption.
➢ Effect of pressure on the adsorbate gas
- An increase in the pressure of the adsorbate gas increases the extent of adsorption.
- At low temperature, the extent of adsorption increases rapidly with pressure.
- At low pressure, the extent of adsorption is found to be directly proportional to the pressure.
- At high pressure (closer to the saturation vapour pressure of the gas), the adsorption tends to achieve a limiting value.
➢ Effect of temperature
- As adsorption is accompanied by evolution of heat, so according to the Le-Chatelier’s principle, the magnitude of adsorption should decrease with rise in temperature.
- The amount of heat when one mole of the gas is adsorbed on the adsorbent is called the heat of adsorption.
➢ Adsorption from Solutions
Solid surfaces can also adsorb solutes from solutions.
A few examples are:
- When litmus solution is shaken with charcoal, it becomes colour less because the dye of the litmus solution is adsorbed by charcol.
- When the colourless Mg(OH)2 is precipitated in the presence of magneson reagent (a blue coloured dye). it acquires blue colour because the dye is adsorbed on the solid precipitate. The extent of adsorption from solution depends upon the concentration of the solute in the solution, and can be expressed by the Freundlich isotherm. The Freundlich adsorption isotherm for the adsorption from solution is, xm = kc1/n where, x is the mass of the solute adsorbed, m is the mass of the solid adsorbent, c is the equilibrium concentration of the solute in the solution, n is a constant having value greater than one. k is the proportionality constant, (The value of k depends upon the nature of solid, its particle size, temperature, and the nature of solute and solvent etc.) The plot of x/m against c is similar to that of Freundlich adsorption isotherm.
The above equations may be written in the following form:
where c, is the equilibrium concentration of the solute in the solution.
➢ Factors affecting adsorption from solution
- The adsorption from solutions by solid adsorbents is found to depend upon the following factors:
(i) Nature of the adsorbate and the adsorbent.
(ii) The adsorption decreases with temperature.
(iii) Greater the surface area of the adsorbent greater is the adsorption.
(iv) Concentration of the solute in the solution.
Question 3:When the temperature is lowered and pressure is raised, the adsorption of a gas on a solid?
The adsorption of gases increases with increasing pressure and decreases with the decrease in press. On the other hand it decreases with increasing temperature and increases with increase in temperature.
Applications of Adsorption
The phenomenon of adsorption finds a number of applications. Important applications are given as follows:
- Production of high vacuum: A bulk of charcoal cooled in liquid air is connected to a vessel which has already been exhausted as for as possible by a vacuum pump. The remaining traces of air are adsorbed by the charcoal to produce a very high vacuum.
- In Gas masks: It is a device which consists of activated charcoal or a mixture of adsorbents. This apparatus is used to adsorb poisonous gases (e.g. Cl2, CO, oxide of sulphur etc.) and thus purify the air for breathing is coal mines.
- For desiccation or dehumidification: Certain substances have a strong tendency to absorb water such as silica and alumina (Al2O3). These substances can be used to reduce/remove water vapours or moisture present in the air. Silica gel is also used for dehumidification in electronic equipment.
- Removal of colouring matter from solution: Animal charcoal removes colours of solutions by adsorbing coloured impurities. It is also used as decolouriser in the manufacture of cane sugar.
- Heterogeneous catalysis: Mostly heterogeneous catalytic reactions proceed through the adsorption of gaseous reactants on solid catalyst.
(i) Finely powdered nickel is used for the hydrogenation of oils.
(ii) Finely divided vanadium pentaoxide (V2O5) is used in the contact process for the manufacture of sulphuric acid.
(iii) Pt, Pd are used in many industrial processes as catalyst.
(iv) Manufacture of ammonia using iron as a catalyst.
- Separation of inert gases
Due to the difference in degree of adsorption of gases by charcoal, a mixture of inert gases can be separated by adsorption on coconut charcoal at different low temperatures.
- Softening of hard water
The hard water is made to pass through a column packed with zeolite (sodium aluminium silicate)
- De-ionisation of water
For softening water can be de-ionised by removing all dissolved salts with the help of cation and anion-exchanger resin.
- In curing diseases
A number of drugs are adsorbed on the germs and kill them or these are adsorbed on the tissues and heat them.
- Cleaning agents
Soap and detergents get adsorbed on the interface and thus reduce the surface tension between dirt and cloth, subsequently the dirt is removed from the cloth.
- Froth floatation process
(i) A low grade sulphide ore is concentrated by separating it from silica and other earthy matter by this method.
(ii) The finely divided ore is added to water containing pine oil and foaming agent.
(iii) The air is bubbled through the mixture.
(iv) The foam formed rises to the surface on which mineral particles wetted with oil are adsorbed while earthy matter settle down at the bottom.
- In adsorption indicators
Surface of certain precipitates such as silver halide, have the property of adsorbing some dyes like eosin, fluorescein etc. In this case of precipitation titrations (for example AgNO3 Versus NaCl) the indicator is adsorbed at the end point producing a characteristic colour on the precipitate.
- Chromatographic analysis
The phenomenon of adsorption has given an excellent technique of analysis known as chromatographic analysis. The technique finds a number of applications in analytical and industrial fields. Chromatographic technique is based on differential adsorption of different constituents of a mixture.
- In dyeing
Many dyes get adsorbed on the cloth either directly or by the use of mordants. “Catalyst is a substance which speeds up and speeds down a chemical reaction without itself being used up at the end of the reaction and the phenomenon is known as catalysis.