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chapter 18 - Chapter 18 Elementary Chemical Kinetics...

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Chapter 18: Elementary Chemical Kinetics Questions on Concepts Q18.1) Why is the stoichiometry of a reaction generally not sufficient to determine reaction order? The order of a rate equation for a chemical reaction indicates the concentration dependence of the rate. The order is dependent on the mechanism of the reaction, and not on the proportionality of the reaction (i.e. the number of reactant molecules required to generate a specific number of product molecules). Q18.2) What is an elementary chemical step, and how is one used in kinetics? An elementary reaction step is a chemical process that occurs in a single step. Elementary reaction steps are combined to construct complex reaction mechanisms. Q18.3) What is the difference between chemical and physical methods for studying chemical kinetics? In chemical methods, a chemical process is used to quantify molecular concentrations. For example, a portion of the reaction sample may be removed, the reaction terminated, and the concentration of reactants or products is then measured. Physical methods involve measurement of some physical property of the system that changes as the reaction proceeds. Physical methods are in general more convenient, as the probe for the physical change are often located within the reaction vessel permitting measurement without having to disrupt or terminate the chemical reaction. Q18.4) What is the method of initial rates, and why is it used in chemical kinetics studies? In the method of initial rates a chemical reaction is initiated using a specific set of reactant concentrations and the reaction rate is measured. Subsequent measurements are in which the concentration of the reactant of interest is changed but all of the other concentrations are let the same. This process is repeated with varying initial concentrations of the reactant of interest while holding the other reactant concentrations constant. The variation in reaction rate with change in concentration of a single species can be used to determine the order of the reaction with respect to the species being changed. Finally, the reaction rates are measured at early times before the reactant concentrations have undergone substantial change from their initial value. 18-1
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Q18.5) What is a rate law expression, and how is it determined? The reaction rate law expression is a mathematical equation that expresses the reaction rate in terms of reactant concentrations and the microscopic rate constant of the reaction. Reaction rate law expressions must be determined experimentally. Q18.6) What is the difference between a first-order reaction and a second-order reaction? In a first order reaction the reaction rate depends linearly on the concentration of a single reactant species. In a second order reaction, the reaction rate depends either on a product of the concentration of two different reactant species with each concentration to the first power, or on the square of the concentration of a single reactant concentration.
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