Catalysis

Basra University for Oil and Gas (BUOG)-IRAQ- Oil & Gas Engineering College, 2020

A catalyst is a substance that affects the rate of a reaction but emerges from the process unchanged. A catalyst usually changes a reaction rate by promoting a different molecular path (“mechanism”) for the reaction. A Catalyst speeds up a reaction, although strictly speaking, a catalyst can either accelerate or slow the formation of a particular product species. A catalyst changes only the rate of a reaction; it does not affect the equilibrium.

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Catalysts are substances that increase the rate of a reaction by providing a low energy "shortcut" from reactants to products. In some cases, reactions occur so slowly that without a catalyst, they are of little value. Nearly all reactions that occur in living cells require catalysts called enzymes- without them, life would be impossible. Enzymes allow many chemical reactions to occur within the homeostasis constraints of a living system. Enzymes function as organic catalysts. A catalyst is a chemical involved in, but not changed by, a chemical reaction. Many enzymes function by lowering the activation energy of reactions. By bringing the reactants closer together, chemical bonds may be weakened and reactions will proceed faster than without the catalyst

The branch of chemistry, which deals with the study of rate of chemical reactions, the factors affecting the rate of reactions and the mechanism of the reaction is called chemical kinetics. Rate of Reaction Rate of a chemical reaction is the change in the concentration of any one of the reactants or products per unit time. It is expressed in mol L-1 s-1 or Ms-1 or atm time-1 units. This rate of reaction is known as average rate of reaction (r av). (r av can be calculated by dividing the concentration difference by the time interval). Instantaneous Rate of Reaction Rate of a chemical reaction at a particular moment of time, is known as instantaneous rate of reaction. Slowest step of a reaction was called rate determining step by van't Ho ff. For expressing the rate of a reaction where stoichiometric coefficients of reactants or products are not equal to one, rate of disappearance of any of the reactants or the rate of appearance of products is divided by their respective stoichiometric coefficients. Thus for the reaction , 2HI(g) → H 2 (g) + I 2 (g) Rate of reaction = = = Rate Law Expressions According to the law of mass action, For a chemical reaction, aA + bB → Products Rate α [A] a [B] b = k[A] a [B] b But experimentally, it is observed that the rate of reaction is found to depend upon 'α' concentration terms of A and 'β' concentration terms of B Then, Rate α [A] α [B] β = k[A] α [B] β where, [A] and [B] molar concentrations of A and B respectively and k is the velocity constant or rate constant. The above expression is known as rate law. Rate Constant The proportionality constant k, in rate law expression is called rate constant or velocity constant. Rate constant may be defined as the specific rate of reaction when the molar concentrations of the reactants is taken to be unity, i.e., Rate = k, if [A] = [B] = 1

TO INVESTIGATE THE CAUSE OF REACTION RATE DIFFERENCE

Introduction: The rates of chemical reactions and the ability to control those rates are crucial aspects of life. Chemical kinetics is the study of the rates at which chemical reactions occur, the factors that affect the speed of reactions, and the mechanisms by which reactions proceed. The reaction rate depends on the reactants, the concentrations of the reactants, the temperature at which the reaction takes place, and any catalysts or inhibitors that affect the reaction. If a chemical reaction has a fast rate, a large portion of the molecules react to form products in a given time period. If a chemical reaction has a slow rate, a small portion of molecules react to form products in a given time period.