Emulsions are intimate mixtures of two immiscible liquids, one of which is dispersed in the other in the form of droplets. In the relative proportions of the two, the one in excess acts as the external phase and the other as the internal or dispersoid phase. It can be said that two types of emulsions can be switched by simply changing the ratio of the dispersed phase and the dispersion medium.
If color is noticed when you look at a drop of the emulsion, it is an oil-in-water type emulsion. When kL p >> 1 i.e. when the pressure is very large or the entire surface is covered, then equation (5) changes. the adsorption reaches its limiting value, but in the intermediate range of pressure the adsorption will follow a relation. A catalyst increases the reaction rate by lowering the activation energy of the rate-determining step.
Promoters: A substance which (though not a catalyst per se) promotes the activity of the catalyst is called a promoter or activator and the process is known as Activation. When the poison is adsorbed on the catalyst surface in preference to reactions, the poisoning is called temporary. Autocatalyst: There are some reactions where one of the products of the reaction acts as the catalyst itself.
The reaction is catalyzed by the NO2 ion formed as a side reaction according to the reaction. Homogeneous catalysts, which are in the same phase with the reactants as with the base, catalyze the hydrolysis of an ester. A homogeneous reaction catalyzed in the gas phase is the aryl oxidation of sulfur dioxide in the presence of nitrogen monoxide.
If an acid catalysis is a reaction, the reactions said to be subject to acid catalysis in which generally hydronium ion is acid catalyst in water inversion of sugar cane anger and hydrolysis of esters are some examples of acid catalyzed reactions. The reaction rates are found to be proportional to the concentration of H+ (or H3O+) ions which are the actual catalysts in acid catalyzed reactions and the concentration of the reacting molecules or ion. Acid-base catalysis: The reactions catalyzed by acids and bases are specifically called acid-base catalyzed reactions.
For example, the mutarotation of glucose in aqueous solutions is catalyzed by acids and bases. The reaction rate is proportional to the H+ ion concentration below pH 3 and proportional to the OH- ion concentration at a pH around 6. Reactions that are catalyzed by some acids (or only H3O+ ions) are said to be acid-specific catalysis.
The reactions catalyzed only by the base catalyst OH– ions are said to be specific hydroxyl ion catalysis. Conversion of acetone to diacetonyl alcohol or base hydrolysis of esters are examples of hydroxyl ion catalysis.
It is assumed that in this reaction nitric oxide combines with one of the reactants to form intermediate (NO2). Heterogeneous catalysts, which are in a different phase than the reactants, as with platinum, catalyzed the oxidation of ammonia to nitrogen monoxide. When the catalyst is in a different phase than the reactants, it is called a heterogeneous catalyst.
This is also known as surface catalysis because the reaction starts at the surface of the solid catalyst. These catalysts have a very large surface area in the order of 1 to 500 m2 per gram for contact. Many reactions that occur on a metal surface, such as the decomposition of HI on gold or the decomposition of N2O on platinum, are zero-order reactions because the rate-determining step takes place on the surface itself.
Thus, despite the huge surface area, when reactant gas molecules cover the surface, the rate does not increase with increasing reactant concentrations. The most important example of heterogeneous catalysis is: the addition of H2 to carbon-carbon (C=C) double bonds of organic compounds to form C-C bonds. However, the reaction becomes rapid even at room temperature in the presence of finely divided nickel, platinum or palladium as a catalyst.
The catalyzed reactions probably proceed through the following steps. a) H2 and ethane molecules approach the metal surface and are adsorbed on the metal surface. With a mixture of copper, zinc oxide as catalyst and Cr2O3 as promoter, methanol is prepared from CO and H2. iv). In the contact process, using V2O5 as a catalyst, SO2 is converted into SO3. v) Using nickel as a catalyst, ethanol is dehydrated to aldehyde.
The adsorbed molecules dissociate to form active species such as free radicals that react faster than the molecules. Adsorbed molecules cannot move freely and therefore collide with other molecules on the surface. The above steps were previously explained with the example of hydrogenation of ethene to ethane.
This depends on the strength of adsorption (chemisorption) in such a way that it allows the adsorption of other reactants and desorption of product molecules. In the presence of platinum, the reaction between H2 and O2 occurs at a high rate, although in the absence of catalyst it proceeds very slowly. For example, for the reaction between CO and H2, different products are obtained using different catalysts.
C ) Acrolern (CH2 = CH CHO) is selectively obtained from the reaction between propylene and O2 in the presence of bismuth molybdate catalyst.
A small amount of enzyme can induce the breakdown of a large amount of substrate. This refinement of the original lock and key model is known as the induced fit model. The reactions in zeolites depend on the structure and size of the cavities (cages) and pores (tunnels) present therein.
The phenomenon of association of the surfactants in the aqueous bulk phase is called micellization. The limiting value for the decrease in surface tension with concentration at infinite dilution (C→0) is called the surface activity. Proteins can also be considered Zwitterionic (ammonium + carboxylates) surfactants. The head group in nonionic surfactants is usually derived from an uncharged group of polyoxyethylene.
Colloids are classified into eight different classes based on the state of aggregation of the dispersed phase and the dispersion medium. Lyophilic colloids: The colloidal solution in which the particles of the dispersed phase have a great affinity (or fondness) for the dispersion medium are called lyophilic colloids. Lyophobic Colloids: Colloidal solutions in which the particles of the dispersed phase have an aversion but no love or affinity for the dispersion medium are called lyophobic colloids.
In fact, the color of the colloidal solution depends on the wavelength scattered by the dispersed phase particles and it changes with the change of the particle size. It was later found that the particles in the dispersed phase of all colloidal solutions also show zig-zag motion. Movement increases with increase in temperature, decrease in size of suspended particles and decrease in viscosity of the medium. one).
Charge on colloidal particles: The negative or positive charge on the colloidal particles is of the same type in a colloidal solution. Due to the large size of pores of the ordinary filter papers, the colloidal particles can move through them. The electrical charge on colloidal particles may be due to dissociation of the surface molecules.
This phenomenon of precipitation or coagulation of salt particles with the addition of excess electrolyte is known as flocculation. The coagulation behavior of various electrolytes can be explained based on the Hardy-Schulze law given as:. i) Coagulation of the sol is effective with ions that carry a charge opposite to that of the colloidal particles.
S 3 (negative sol) Fe(OH) 3 (positive sol) Electrolyte Valency
The presence and nature of charge on solar particles can be detected by the phenomenon of electrophoresis. Precipitation of colloids: While traces of electrolytes are essential to the stability of a sol, the addition of a relatively large amount of electrolytes makes the sol unstable; the colloidal particles grow in size and precipitate. These ions are called active ions. ii) With increasing valence of the coagulating ion, the precipitating power of the active ion increases.
The flocculation values of some common electrolytes for a positively and a negatively charged sol are given in table. Apart from the addition of the electrolyte, the coagulation of a colloidal sol can also be brought about by prolonged dialysis, electrophoresis and mixing of two colloids of opposite charge in equivalent proportions. Protective action of colloids: A lyophobic sol is prevented from precipitating when a lyophilic sol is added to a lyophobic sol, whereby the latter becomes more stable and less sensitive to the electrolytes.
The lyophilic salt thus plays a protective role, usually by forming an adsorbed layer that completely covers the particles of the lyophobic salt, so that it practically behaves like a lyophilic salt. It is defined as the mass in milligrams of dry lyophilic salt added to 10 ml of standard gold salt to prevent its coagulation (i.e. a change in color from red to blue) after the addition of 1 ml of 10% sodium chloride solution. The values of the golden numbers and their reciprocals for some lyophilic colloids are given in the table.
The protective action of a lyophilic sol on a lyophobic colloid against coagulation may be due to association or united action of particles of the two sols. Applications of Coagulation: Based on the neutralization of charge on the dispersed phase particles by suitable agents, colloidal sols find many applications as indicated below. Cottrell precipitator: Smoke and dust particles from air pollutants cause health problems in industrial environments.
Using the principle of electrophoresis, the smoke or dust particles from air or waste gases can be removed by discharge, deposition or deposition. Sewage disposal: Sewage has particles of colloidal sol with an electrical charge in the form of dust, waste sludge, etc.
