Chemical Kinetics, Surface Phenomena, and Material Properties
Pseudo First Order Reactions
The order of a reaction is sometimes altered by conditions. Consider a chemical reaction between two substances when one reactant is present in large excess. For example, during the hydrolysis of 0.01 mol of ethyl acetate with 10 mol of water, the amounts of the various constituents at the beginning (t = 0) and completion (t) illustrate this phenomenon, known as a pseudo-first-order reaction.
Collision Theory of Chemical Reactions
Although the Arrhenius equation is applicable under a wide range of circumstances, collision theory, developed by Max Trautz and William Lewis in 1916-18, provides greater insight into the energetic and mechanistic aspects of reactions. It is based on the kinetic theory of gases. According to this theory, reactant molecules are assumed to be hard spheres, and reaction is postulated to occur when molecules collide with each other. The number of collisions per second per unit volume of the reaction mixture is known as collision frequency (Z). Another factor affecting the rate of chemical reactions is activation energy (Ea). For a bimolecular elementary reaction A + B → Products, collision theory explains the reaction rate based on these factors.
Adsorption Fundamentals
Several examples reveal that the surface of a solid tends to attract and retain molecules of the phase it contacts. These molecules remain only at the surface and do not penetrate deeper into the bulk. The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption. The molecular species or substance that concentrates or accumulates at the surface is termed the adsorbate, and the material on whose surface the adsorption takes place is called the adsorbent.
Characteristics of Adsorbents
Adsorption is essentially a surface phenomenon. Solids, particularly in a finely divided state, have a large surface area. Therefore, materials like charcoal, silica gel, alumina gel, clay, colloids, and metals in a finely divided state act as good adsorbents.
Catalysis: Potassium Chlorate Decomposition
Potassium chlorate, when heated strongly, decomposes slowly, yielding dioxygen. The decomposition occurs in the temperature range of 653-873 K. This process can be influenced by catalysts.
Introduction to Colloids
A colloid is a heterogeneous system in which one substance (dispersed phase) is dispersed as very fine particles in another substance called the dispersion medium.
Emulsions: Types and Formation
Emulsions are liquid-liquid colloidal systems, representing the dispersion of finely divided droplets in another liquid. Shaking a mixture of two immiscible or partially miscible liquids creates a coarse dispersion of one liquid in the other, known as an emulsion. Generally, one of the liquids is water. There are two main types of emulsions:
- Oil dispersed in water (O/W type)
- Water dispersed in oil (W/O type)
Abundance and Occurrence of Metals
Elements vary in abundance. Among metals, aluminium is the most abundant, being the third most abundant element in the Earth’s crust (approximately 8.3% by weight). It is a major component of many igneous minerals, including mica and clays. Many gemstones are impure forms of Al2O3; impurities range from Cr (in ruby) to Co (in sapphire). Iron is the second most abundant metal in the Earth’s crust. It forms various compounds, and its diverse uses make it a very important element. Iron is also an essential element in biological systems.