Chemical kinetics is the aspect of chemistry that deals with the speed or rate of chemical reactions and the mechanisms by which they occur. The rate of a chemical reaction is a measure of how fast the reaction occurs, and it is defined as the change in the amount or concentration of a reactant or product per unit time. The mechanism of a reaction is the series of steps or processes through which it occurs. Most experimental techniques for determining reaction rates involve measuring of the rate of disappearance of a reactant, or the rate of appearance of a product. For a reaction in which the reactant Y is converted to some products: Rate = Concentration of Y at time t2 −Concentration of Y at time t1/t2 −t1 Rate = Δ [Y]/ Δt where [Y] indicates the molar concentration of the reactant of interest, and Δ refers to a change in the given amount. Rate for a reactant, by this definition, is a negative number. For a product, it is positive. The value of the rate at a particular time is known as the instantaneous rate and will be different from the average rate. Its value can be obtained from the plot of concentration (mol/L) vs. time (s) as the slope of a line tangent to the curve at a given point. Consider the following kinetic data for the decomposition of N2O5 to gaseous NO2 and O2 at 40°C (see table 16-3). A plot of [N2O5] vs. time is shown in figure 16-2. From this curve, the instantaneous rate of reaction at any time t can be obtained from the slope of the tangent to the curve. This corresponds to the value of Δ [N2O5]/ Δt for the tangent at a given instant. The instantaneous rate at the beginning of the reaction (t =0) is known as the initial rate.
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