CBSE Class 12th Chemistry Notes: Surface Chemistry (Part – I)Jagran Josh
Surface Chemistry is the most important chapter of CBSE Class 12th Chemistry. Every year 2 to 3 conceptual questions are frequently asked in CBSE Class 12th Board exams. So, students must have deep understanding of the chapter in order to score maximum marks in CBSE Class 12th Chemistry Board Examination.
Here, you will get revision notes on CBSE Class 12 Chemistry: Chapter 5 - Surface Chemistry. These notes will give you a quick glance of the chapter.
The main topics covered in these quick notes are:
o Definition of
• Surface chemistry
o Difference between adsorption and absorption
o Physisorption and chemisorption
o Adsorption of gases on solids
o Effect of temperature on adsorption
o Adsorption from solutions
o Types of catalysis
• Shape-selective catalysis
• Enzyme catalysis
The notes on the chpater are as follows:
Surface chemistry is a branch in chemistry which deals with the study of the phenomenon occurring at the interface of two phases. Surface represents the physical boundary of any condensed phase like liquid or solid. It separates one phase from the other.For example: Interfaces existing between two immiscible liquids like oil and water; between a metal and a gas like platinum and hydrogen; a liquid and a gas etc. The surface or interface between two phases is represented by a hyphen or slash. For example: Between the solid and the liquid, the interface is represented by solid/liquid. There is no interface between the gases as they are completely miscible with one another.
The phenomenon of attracting and retaining the molecules of a substance on the surface of a liquid or a solid resulting into a higher concentration of the molecules on the surface is called adsorption. It is also called the positive adsorption. The surface on which adsorption takes place is called the adsorbent and the substance which is being adsorbed is called the adsorbate.
Types of adsorption:
Adsorption is of two types:
(i) Physical adsorption (physisorption): When a gas is held (adsorbed) on the surface of a soild by Vander Waals forces without forming any chemical bond between adsorbate and adsorbent it is called physical adsorption. E.g., Adsorption of CO2 gas on the surface of charcoal.
(ii) Chemical adsorption (Chemisorption): When a gas is held (adsorbed) on the surface of a solid by forces similar to those of a chemical bond the type of adsorption is called chemical adsorption. This type of adsorption leads to the formation of a surface compound.
Differences between Physisorption and Chemisorptions
Only van der Waals forces forces are present between adsorbate and surface of adsorbent.
It usually takes place at a low temperature and decreases with increasing temperature.
It is reversible.
It is related to the ease of liquefaction of the gas.
It forms multi-molecular layers.
It does not require any activation energy.
Chemical bonds are formed between adsorbate and surface of adsorbent.
It takes place at a high temperature.
It is irreversible.
The extent of adsorption not related to liquefaction of the gas.
It forms mono-molecular layers.
It requires activation energy.
Desorption is the reverse of adsorption which involves the removal of the substance adsorbed from the surface. The process of desorption can be carried out by:
(i) Reducing the pressure and (ii) By heating
In absorption, the concentration of the molecules of a substance is more in the bulk than at the surface and the molecules in the bulk are uniformly distributed. If the concentration of the molecules of a substance is more in the bulk but molecules are not uniformly distributed, then it is called negative adsorption. When both adsorption and absorption takes place together and are indistinguish-able, then we use the term called Sorption.
Adsorption of gas on solid surface:
Almost all solids adsorb gases to some extent. Charcoal in gas mask adsorbs poisonous gases on its surface. Silica gel adsorbs moisture and is used for drying air.
The extent of adsorption of a gas on solid is expressed as x/m.
where, x = mass of the adsorbate, m = mass of the adsorbent
Factors affecting the extent of adsorption:
(i) Nature of the solid (adsorbent)
(ii) Nature of the gas (adsorbate)
(iii) Pressure of the gas
(v) Activation of adsorbent
In short, adsorption depends on the nature of the adsorbent. More the roughness of a surface, more it has number of pores hence adsorbs more number of gases than the smooth surface. Most common adsorbents are silica gel and activated charcoal. The extent of adsorption also depends on the surface area of the solid. The greater the surface area of the solid, the greater would be the adsorption.
The extent of adsorption of a gas on a solid is also affected by the pressure of the gas. A graph between x/m and the pressure P at constant temperature is known as the adsorption isotherm.
A catalyst is a substance which increases the rate of reaction without participating in the reaction. The catalyst is recovered chemically unchanged at the end of the reaction. Catalysts are always used in very small amounts and speed up the reaction by decreasing the potential energy activation barrier. A catalyst does not affect the equilibrium constant as it speeds up the reaction in both the directions whether forward or backward by the same factor.
Catalysis and its types:
The phenomenon of increase in the rate of a reaction with the help of a catalyst is known as catalysis.
It is of two types:
(i) Homogeneous catalysis: When the catalyst and the reactants are in the same phase then the type of catalysis is called the homogeneous catalysis.
(ii) Heterogeneous catalysis: When the reactants and the catalyst are in different physical states then the catalysis is termed as the heterogeneous catalysis.
Important features of the catalyst:
(i) Activity: It represents the capacity of s catalyst to speed up the reaction. It depends upon the strength of chemisorption to a large extent. It also depends on the surface area of the catalyst.
(ii) Selectivity: The ability of a catalyst to direct a reaction to yield a particular product excluding others.
Zeolites as shape selective catalyst:
The shape selective catalysis depends upon the structure of the pores present in the catalyst and the size of the reactant and product molecules. Zeolites are silicates with number of cages or pores. It catalysis only those reactant molecules having a shape and size similar to cages. Example is ZSM-5 used for converting alcohol directly into gasoline. Zeolites are mainly used in petrochemical industries for cracking of hydrocarbons and isomerisation.
Enzymes as biological catalysts:
Enzymes are biological catalysts which are more efficient than chemical catalysts. They can speed up the reaction rates by 108 to 1020 times. Enzymes are also very specific in their action.The specific action of enzyme is due to the presence of a specific and active site present on its surface. The substrate (reactant) binds to this active site through intermolecular forces. Enzymatic reaction follows lock-and-key mechanism.