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BUFFER SOLUTIONS-1.pptx
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- 2. Table of contents 1.Adsorption 2.Typesof adsorption 3.Adsorption isotherms 4.Applications of adsorption
- 3. ADSORPTION The accumulationof molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption. The material on the surface of which the adsorption takes place is called adsorbent. Eg: Charcoal, Silica gel, Alumina The molecular species or substance, which concentrates or accumulates at the surface is termed as adsorbate Eg: H2, N2 and O2 gases Examples of adsorption: 1.silica gel adsorbs water vapour 2.Adsorption of a dye(methylene blue) by charcoal The process of adsorption is an exothermic process and is invariably accompanied by evolution of heat
- 4. • ∆Hadsorption is always–ve ∆Sadsorption is –ve For process of adsorption to occur i.e., ∆G to be –ve, ∆H must be > T∆S. i.e., ∆H should have sufficiently high value (>T∆S) so that ∆G becomes negative. Types of Adsorption 1.Physisorption accumulation of gases on the surface of a solid occurs on account of weak van der waal’s forces Eg: H2 and N2 gases adsorb on coconut charcoal. 2.Chemisorption accumulation of gases on the surface of a solid on account of chemical bond formation Eg: formation of iron nitride on the surface when the iron is heated in N2 gas at 623 K
- 5. Adsorption Isotherm Anadsorption isotherm is a graph that represents the variation in the amount of adsorbate(x) adsorbed on the surface of the adsorbent with the change in pressure at a constant temperature. FREUNDLICH ADSORPTION ISOTHERM LANGMUIR ADSORPTION ISOTHERM BET ADSORPTION ISOTHERM DUBININ RADUHKEVITCH (D-R) ADSORPTION ISOTHERM: TEMKIN ADSORPTION ISOTHERM
- 6. Freundlich Adsorption Isotherm The variation of extent of adsorption (x/m) with pressure (P) at a particular temperature was given mathematically by Freundlich in 1909. From the adsorption isotherm, following observations can be easily made (i) At low pressure (ii) At high pressure (iii) Thus, in the intermediate range of pressure The above equation is known as Freundlich adsorption equation where K and n are the constants which depend on the nature of the gas and adsorbent The value of ‘n’ is generally greater than one and thus the reciprocal, 1/n, can have values between 0-1.
- 7. Taking log bothsides of equation, we get, The equation above equation is comparable with comparable with equation of straight line, y = m x + c where, m represents slope of the line c represents intercept on y axis •Freundlich adsorption isotherm is only applicable at low pressures. Experimentally it was determined that extent of adsorption varies directly with pressure till saturation pressure Ps is reached. Beyond that point rate of adsorption saturates even after applying higher pressure. Thus Freundlich Adsorption Isotherm failed at higher pressure. Limitations of Freundlich adsorption isotherm
- 8. Langmuir Adsorption isotherm Surface of the solid is made up of elementary sites, each of which can adsorb one gas molecule surface of the solid is homogeneous (same affinity) Mono-layer adsorption only Adsorbed gases behave ideally (No lateral interactions between adsorbate molecules) Heat of adsorption is constant and equivalent for all sites Adsorbate gas molecules are localized (do not move around on the surface) assumptions of Langmuir adsorption isotherm
- 9. P is thepressure of the gas. Below given expression is known as Langmuir Adsorption Equation. Where •θ is the fraction of the surface covered by the adsorbed molecule. •K1 is an equilibrium constant known as the adsorption coefficient. •{𝑲𝟏 = (𝑲𝒂/𝑲𝒅) = rate constant for adsorption/ rate constant for desorption}