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Unformatted text preview: Chapter 13: Chemical Kinetics Chemical kinetics explains how chemical reactions occur by studying their rates and mechanisms. -explains how the speeds of different chemical reactions vary and how slow reactions can be accelerated by introducing catalysts- provides the basis for modifying conditions to carry out chemical reactions with reasonable speed- the central conceptual method is to relate the rate of a reaction to the amount of reactants present-major goal is to discern the way rate depends on the concentrations of the reacting species and to deduce the mechanism of a reaction from experimental knowledge of that rate- reaction mechanism- a series of elementary reactions, together with their rates, that describe the detailed steps by which a reaction proceeds 13.1 Rates of Chemical Reactions reaction rate depends on concentration of reactants, temperature, and the physical forms of the reactants Measuring Reaction Rates A kinetics experiment measures the rate at which the concentration of a substance taking part in a chemical reaction changes with time if the reaction is slow enough, we can allow it to proceed for a measured time and then abruptly stop it by rapidly cooling the reaction mixture to a low enough temperature for rapid reactions, we use the absorption of light as a probe to determine the concentration of the absorbing species (reactants) at a series of times Average reaction rate- the change in the concentration of a product of a reaction divided by the time interval over which the change occurs and also divided by the coefficient of the product in the chemical equation for the reaction if concentration is measured in moll-1 , reaction rate has units of molL-1 s-1 graphical method: the average rate is the slope of the straight line connecting the concentrations at the initial and final points of a time interval Instantaneous reaction rate- the average rate, obtained over progressively shorter time intervals as t a pproaches 0, the rate becomes the slope of the line tangent to the curve at time t slope is written as derivative of concentration with respect to time: Initial reaction rate- the rate of a chemical reaction at the moment that it begins, before any products can start to react in reverse direction For the general reaction, aA+bB cC+ dD , Average reaction rate = change in concentration change in time Instantaneous rate = lim t [A] t + t [A] t = d [A]...
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- Spring '08