Experimental Determination of Rate

Ethyl acetate is a liquid with a fruity odor that belongs to a class of organic (carbon-containing) compounds called esters. 

To obtain the rate of a reaction, you must detemine the concentration of a reactant or prodnct during the course of the reaction. One way to do this for a slow reaction is to withdraw samples from the reaction vessel at various times and analyze them. The rate of the reaction of ethyl acetate with water in acidic solution was one of the first to be determined this way.  

This reaction is slow, so the amount of acetic acid produced is easily obtained by titration before any significant further reaction occurs. 

An experiment to follow the concentration of N2O5 as the decomposition proceeds 

The total pressure is measured during the reaction at 4S°C. Pressure values can be related to the concentrations of N2O5, NOj, and O2 in the flask. 

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Gas-phase reactions which result in nucleophilic displacement at a saturated, or an unsaturated, carbon centre have been observed in positive and negative ion chemistry. By far, the most widely occurring case is the formal analog of the Sn2 reaction initially reported by Bohme and Young (1970). The experimental determination of rate constants for SN2 reactions has received a great deal of attention as has the mechanistic point of view including the interpretation of the potential energy surface for the gas-phase reaction. 

Rate coefficients k(E) have been obtained in this way for decompositions of benzene, thiophene and benzonitrile over an energy range of some electron volts. These were the first direct experimental determinations of rate coefficients, k(E), for ionic decompositions. Moreover, a range of energies was accessible, since the times at which rates were measured extended over the 3 orders of magnitude, which contrasts with the later PIPECO experiments in which rates have been measured only in the microsecond time-frame. 

The first four sections of this chapter describe the experimental determination of rate laws and their relation to assumed mechanisms for chemical reactions. Now we have to find out what determines the actual magnitudes of rate constants (either for elementary reactions or for overall rates of multistep reactions), and how temperature affects reaction rates. To consider these matters, it is necessary to connect molecular collision rates to the rates of chemical reactions. We limit the discussion to gas-phase reactions, for which the kinetic theory of Chapter 9 is applicable. 

To illustrate the experimental determination of rate constants and mechanisms we turn to another set of reactions important in the mechanism of the Belousov-Zhabotinskii reaction. The overall reaction for the bromination of malonic acid is... 

By integration of the rate equations, it is possible to obtain expressions that describe changes in the concentration of reactants or products as a function of time. As described below, integrated rate equations are extremely useful in the experimental determination of rate constants and reaction order. 

Experimental determinations of rate constants for intramolecular electron-transfer, for a series of related donor-bridge-acceptor (DBA) molecules in which the donor, the acceptor, or the bridge has been varied, usually show a fairly smooth relation between the ergonic-ity (—AC ) of the reaction and ket or AG (. Often the plot of log ket against -AG is linear over the observable range (usually limited by that of AG ) sometimes it is appreciably curved. Such plots can be used to examine the classical and semi-classical Marcus equations. 

Monitoring the kinetics of RAFT polymerization, specifically the rate of monomer depletion, does not allow discrimination between these models or the various hybrid models that have been proposed. Klumpermaim and Heuts re-examined the molecular orbital calculations of Coote and coworkers and experimental determinations of rate parameters associated with RAFT... 

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