Archives November 2013

Van-t Hoff Differential MethodThe van 't Hoff equation in chemical thermodynamics relates the change in the equilibrium constant, Keq, of a chemical equilibrium to the change in temperature, T, given the standard enthalpy change, ΔHo, for the process. It was proposed by Dutch chemist J.H. van't Hoff in 1884. Under standard conditions The van't Hoff equation is based on the assumption that the enthalpy and entropy are constant with temperature changes. In practice, the equation is experimentally approximate in that both enthalpy and entropy changes of a process (reaction) vary (each differently) with temperature. Its accuracy is determined in accounting for the curvature in the standard enthalpy changes over temperature. A major use of the equation is to estimate a new equilibrium constant at a new absolute temperature assuming a constant standard enthalpy change over the temperature range. Under standard conditions, the van't Hoff equation is    more...

Rate of ReactionConceptsThe concentration-time plots showed that each species in the reaction has its own rate of change in concentration. The reactants have a negative slope, because they are being consumed in the reaction. Products have a positive slope, because they are being formed in the reaction. For the hypothetical example reaction A + 2 B   →   3 C the stoichiometric coefficient for species B is twice that of species A; thus the concentration of B will decline twice as fast as that of species A. Similarly, the concentration of species C increases three times as fast as the concentration of A decreases. Conceptually there should be a single, unambiguous rate for a reaction. How might such a rate be defined given the highly varied rates of change for the various species in the reaction?The rate of reaction, r, is defined to be the slope of the concentration-time more...

In chemical kinetics, the order of reaction with respect to a given substance (such as reactant, catalyst or product) is defined as the index, or exponent, to which its concentration term in the rate equation is raised. For the typical rate equation of form r; =; k[mathrm{A}]^x[mathrm{B}]^y..., where [A], [B], ... are concentrations, the reaction orders (or partial  reaction orders) are x for substance A, y for substance B, etc. The overall reaction order is the sum x + y + .... For example, the chemical reaction between mercury (II) chloride and oxalate ion  2 HgCl2(aq) + C2O42-(aq) → 2 Cl-(aq) + 2 CO2(g) + Hg2Cl2(s)   has the observed rate equation  r = k[HgCl2]1[C2O42-]2  In this case, the reaction order with respect to the reactant HgCl2 is 1 and with respect to oxalate ion is 2; the overall reaction order is 1 + 2 = 3. As is true for many more...

The Effect of a Catalyst Reversible chemical reactions will eventually achieve equilibrium. At equilibrium, the rate of the formation of products is equal to the rate of the formation of reactants. Reaction progress towards this equilibrium state can be accelerated through the addition of catalysts or heat to the system.Catalysts are compounds that can accelerate the progress of a reaction without being consumed. Common examples of catalysts include acid catalysts and enzymes. A schematic depiction of the effect of a catalyst on a reaction coordinate can be seen in the figure below. Catalysts allow reactions to proceed by a different reaction pathway involving a lower-energy transition state. By lowering the energy of the rate-limiting step (the transition state), catalysts reduce the required energy of activation and allow a reaction to proceed and, in the case of a reversible reaction, reach equilibrium more rapidly. Visit   http://www.studyadda.com/videos/xii-class-chemistry-lectures/chemical-kinetics/effect-of-catalyst/1757 All the video lectures at more...

Thermal Conduction In heat transfer, conduction (or heat conduction) is the transfer of heat energy by microscopic diffusion and collisions of particles or quasi-particles within a body due to a temperature gradient. The microscopically diffusing and colliding objects include molecules, electrons, atoms, and phonons. They transfer microscopically disorganized kinetic and potential energy, which are jointly known as internal energy. Conduction can only take place within an object or material, or between two objects that are in direct or indirect contact with each other. Conduction takes place in all forms of ponderable matter, such as solids, liquids, gases and plasmas. Steady state  The state of the rod in which no part of the rod absorbs heat to raise its own temperature is called the steady-state. In steady state there is no effect of specific heat on the process of conduction, but the temperature of cross- section depends only on the thermal-conductivity more...

Pascal's law or the principle of transmission of fluid-pressure is a principle in fluid mechanics that states that pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that the pressure variations (initial differences) remain the same. The law was established by French mathematician Blaise Pascal.Pascal's principle is definedThis principle is stated mathematically as:   is the hydrostatic pressure (given in pascals in the SI system), or the difference in pressure at two points within a fluid column, due to the weight of the fluid; ρ is the fluid density (in kilograms per cubic meter more...

Density The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume. The symbol most often used for density is ρ (the lower case Greek letter rho). Mathematically, density is defined as mass divided by   where ρ is the density, m is the mass, and V is the volume. In some cases (for instance, in the United States oil and gas industry), density is loosely defined as its weight per unit volume,although this is scientifically inaccurate – this quantity is more properly called specific weight.  Visit http://www.studyadda.com/videos/ix-class-physics-lectures/floatation/density/1744 All the video lectures at StudyAdda are exclusively developed and taught by Mr. Lalit Sardana(IIT-JEE,AIR 243) and Mrs. Shweta Sardana(AIPMT Trainer, MSc., MPhil (Gold Medalist)), Sardana Tutorials).

In chemistry isomerization (also isomerisation) is the process by which one molecule is transformed into another molecule which has exactly the same atoms, but the atoms have a different arrangement e.g. A-B-C → B-A-C (these related molecules are known as isomers). In some molecules and under some conditions, isomerization occurs spontaneously. Many isomers are equal or roughly equal in bond energy, and so exist in roughly equal amounts, provided that they can interconvert relatively freely, that is the energy barrier between the two isomers is not too high. When the isomerization occurs intramolecularly it is considered a rearrangement reaction.    In n-alkanes, no carbon is bonded to more than two other carbons, giving rise to a linear chain. When a carbon is bonded to more than two other carbons, a branch is formed. The smallest branched alkane is isobutane. Notice that isobutane has the same molecular formula, C 4 H more...

 ï»¿ In chemistry, Le Chatelier's principle, also called Chatelier's principle or "The Equilibrium Law", can be used to predict the effect of a change in conditions on a chemical equilibrium. The principle is named after Henry Louis Le Chatelier and sometimes Karl Ferdinand Braun who discovered it independently. It can be summarized as:   If a chemical system at equilibrium experiences a change in concentration, temperature, volume, or partial pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established.  ï»¿ This principle has a variety of names, depending upon the discipline using it. See, for example, homeostasis. It is common to take Le Chatelier's principle to be a more general observation, roughly stated:   Any change in status quo prompts an opposing reaction in the responding system. In chemistry, the principle is used to manipulate the outcomes of reversible reactions, often to increase the yield of reactions. In pharmacology, the binding of ligands to the receptor may shift more...

For a general chemical equilibrium      the thermodynamic equilibrium constant can be defined such that, at equilibrium,       where curly brackets denote the thermodynamic activities of the chemical species. The logarithm of this expression appears in the formula for the Gibbs free energy change for the reaction. If deviations from ideal behaviour are neglected, the activities may be replaced by concentrations, [A], and a concentration quotient, Kc.       Kc is defined which is equal to the thermodynamic equilibrium constant divided by a quotient of activity coefficients. For ideal behaviour this quotient has a value of 1, and Kc = K (Kc appears here to have units of concentration more...