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“[I was] ...the instrument destined to take away from the world one of its greatest calamities..." Edward Jenner, scientist and doctor As Edward Jenner (1749-1823) was growing up in England, smallpox had again become prevalent and was ravaging London and the countryside. Jenner became a doctor, practicing in Gloucestershire where he became interested in the link between cowpox and smallpox. Milkmaids who contracted the non- deadly cowpox seemed immune to smallpox, and Jenner, intrigued by this, began to investigate the link. In May 1796, milkmaid Sarah Nelmes contracted cowpox from her cow, and pus-filled blisters covered her hands and arms. She visited Dr. Jenner. Realizing his opportunity to test the protective properties of cowpox on someone who had not contracted smallpox, Jenner took some pus from Sarah and applied it to scratches made on the arm of a young boy, James Phipps. Some days later Phipps came down with a more...

Resistance The electrical resistance of a circuit component or device is defined as the ratio of the voltage applied to the electric current which flows through it.  If the resistance is constant over a considerable range of voltage, then Ohm's law, I = V/R, can be used to predict the behavior of the material. Although the definition above involves DC current and voltage, the same definition holds for the AC application of resistors.Whether or not a material obeys Ohm's law, its resistance can be described in terms of its bulk resistivity. The resistivity, and thus the resistance, is temperature dependent. Over sizable ranges of temperature, this temperature dependence can be predicted from a temperature coefficient of resistance.  Resistivity  The electrical resistance of a wire would be expected to be greater for a longer wire, less for a wire of larger cross sectional area, and would be expected to depend upon more...

Chemical Kinetics Chemical kinetics deals with the rates of reactions. In order for a reaction to occur, a collision must occur; the collision must be of sufficient energy to break the necessary bonds and be of proper orientation. This is the basis of reaction rates. Factors effecting reaction rates are concentration of the reacting species found in the rate law, temperature and the presence or absence of a catalyst.  A rate law of a reaction is a mathematical expression relating the rate of a reaction to the concentration of either reactants or products. One may not look at an overall reaction, and determine the rate law. The rate law may be theoretically determined from the rate determining step (slow step) of the reaction mechanism. Many chemical reactions actually require a number of steps in order to break bonds and form new ones. These series of steps are called the reaction mechanism. more...

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).