Surface Chemistry Revision Notes - IIT JEE/NEET Preparation | Nucleon

Surface Chemistry

  • Introduction
  • Surface chemistry is that branch of chemistry which deals with study of the phenomena occuring at the surface
    or interface, i.e. at the boundary separating two bulk phases. In this chapter our main emphasis will be on three
    important topics related to surface chemistry, viz., adsorption, colloids and emulsions.

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  • Types of adsorption
  • When the particles of the adsorbate are held to the surface of the adsorbent by the physical forces such as van
    der Waal’s forces, the adsorption is called physical adsorption or vanderwaals adsorption

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  • Adsorption of gases on solids
  • The extent of adsorption of a gas on a solid surface is affected by the following factors

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  • Freundlich Adsorption isotherm
  • The variation of extent of adsorption (x/m) with pressure (P) was given mathematically by Freundlich

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  • Langmuir Adsorption Isotherm
  • It is based on the assumption (a) that every adsorption site is equivalent and (b) that the ability of a particle to
    bind there is independent of whether or not nearby sites are occupied

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  • Adsorption from solutions
  • The process of adsorption can take place from solutions also. It is observed that solid adsorbents
    adsorb certain solutes from solution in preference to other solutes and solvents

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  • Applications of adsorption
  • In gas masks : Activated charcoal is generally used in gas masks to adsorb poisonous and toxic gases from air.
    These masks are commonly used by the miners because there are poisonous gases like CO, CH4 etc. in the
    atmosphere in coal mines

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  • Types of Catalysis
  • Positive Catalysis : A substance which increase the rate of chemical reaction is called positive catalyst
    and this process called positive catalysis

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  • Promoters/Activators
  • Substance which themselves are not catalyst but its presence can increase
    the catalytic activity of catalyst

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  • Characteristics of Catalysis
  • A Catalyst remains unchanged in mass and chemical compositions at the end of reactions. However its
    physical state can be change

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  • Adsorption Theory of Heterogeneous Catalyst
  • Adsorption Theory of Heterogeneous Catalyst

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  • Some Industrial Catalytic reactions
  • Some Industrial Catalytic reactions

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  • Colloid Solution
  • Colloid State : A substance is said to be in colloidal state when the size of the particle of
    disperse phase is greater than particle of true solution and less than that of suspension solution
    particle

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  • Colloids
  • A colloid is a heterogeneous system in which one substance is dispersed (dispersed
    phase) as very fine particles in another substance called dispersion medium. The solution
    and colloid essentially differ from one another by particle size

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  • Classification of colloids
  • On the bases of physical state of D.P. and D.M. colloidal solution may be divided into eight system.

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  • DISTINCTION BETWEEN LYOPHILIC AND LYOPHOBIC COLLOIDS
  • DISTINCTION BETWEEN LYOPHILIC AND LYOPHOBIC
    COLLOIDS

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  • Multimolecular, macromolecular and associated colloids
  • Multimolecular, macromolecular and associated colloids

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  • Associated colloids (Micelles)
  • There are some substances which at low concentrations behave as normal strong electrolytes but at higher
    concentrations exhibit colloidal behaviour due to the formation of aggregated particles. The aggregated particles
    thus formed are called micelles

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  • The Cleansing Action of Soaps
  • It has been mentioned earlier that a micelle consists of a hydrophobic hydrocarbon like central core. The cleansing
    action of soap is due to these micelles, because oil and grease can be solubilised in their hydrocarbon, like
    centres which are not otherwise soluble in water. This is shown diagrammatically in Fig. The dirt goes out along
    with the soap micelles

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  • Preparation of lyophobic colloidal sols : Condensation methods
  • Preparation of lyophobic colloidal sols :
    Condensation methods

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  • Dispersion Methods
  • In these methods large particles of the substance are broken into particles of colloidal dimensions in the presence
    of dispersion medium. These are stabilized by adding some suitable stabilizer. Some of the
    methods employed are given below

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  • Purification of Colloidal Sols
  • The colloidal sols obtained by various methods are impure and contain impurities of electrolytes and other
    soluble substances. These impurities may destabilise the sol. Hence, they have to be removed. A very important
    method of removal of soluble impurities from sols by a semipermeable membrane is known as dialysis

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  • Important properties of colloidal sols
  • Colligative Properties :
    Colloidal sols show the colligative properties viz. relative lowering of vapour pressure, elevation in boiling point,
    depression in freezing point and osmotic pressure. However, due to high average molecular masses of colloidal
    particles, mole fraction of the dispersed phase is very low. Hence, the values of the colligative properties observed
    experimentally are very small. Only osmotic pressure measurements are used in determining the molecular
    mass of polymers

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  • Mechanical Properties
  • Brownian movement: Robert Brown, a botanist, discovered in 1827 that pollen grains placed in water do not
    remain at rest but move about continuously and randomly. Later on, this phenomenon was observed in case of
    colloidal particles when they were seen under an ultramicroscope

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  • Electrical Properties (Electrophoresis)
  • The particles of the colloids are electrically charged and carry positive or negative charge. The dispersion medium
    has an equal and opposite charge making the system neutral as a whole. Due to similar nature of the charge
    carried by the particles, they repel each other and do not combine to form bigger particles. That is why, a sol is
    stable and particles do not settle down.

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  • Electrosmosis
  • Electrosmosis

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  • Sedimentation potential or Dorn potential
  • Sedimentation potential or Dorn potential

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  • Isoelectric point
  • Isoelectric point

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  • Streaming potential
  • Streaming potential

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  • Charge on colloidal particles
  • Colloidal particles are either positively charged or negatively charged. This charge is due to preferential adsorption
    of either positive or negative ions on their surface

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  • Electric Double Layer Theory or Helm-holtz Electric double layer
  • Electric Double Layer Theory or Helm-holtz Electric double layer

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  • Coagulation/Flocculation
  • The presence of small amounts of appropriate electrolytes is necessary for the stability of the colloids. However,
    when an electrolyte is added in larger concentration; the particles of the sol take up the ions which are oppositely
    charged and thus get neutralised

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  • Factor-Affecting Coagulations
  • Nature of sols : The lyophobic colloid can easily coagulate because it is a less stable colloid, but lyophilic
    colloids coagulate hardly by the addition of electrolyte due to protective layer of D.M. surrounding the colloidal
    particle

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  • Protective colloidal sols
  • Lyophilic colloidal sols are much more stable than lyophobic colloidal sols. This is due to the
    extensive solvation of lyophilic colloidal sols, which forms a protective layer outside it and thus
    prevents it from forming associated colloids

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  • Gold Number
  • Zpsigmondy (1901) introduce a term called gold number it is defined as ‘’the minimum amount of the protective
    colloid in milligrams which when added to 10 ml of a standard gold sol is just sufficient to prevent a colour change
    from red to blue on the addition of 1 ml of 10% sodium chloride solution

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  • Applications of Colloids
  • Colloids including emulsions find a number of uses in our daily life and industry. Some of the uses are given
    below.

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  • STEM TECHNOLOGY
  • The size and shape of the colloidal particles is determined with the help of an electron microscope which has much more

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