Rate constant measurement

McFarland M, Albritton D L, Fehsenfeld F C, Ferguson E E and Schmeltekopf A L 1973 Flow-drift technique for ion mobility and ion-molecule reaction rate constant measurements. I. Apparatus and mobility measurements J. Chem. Phys. 59 6610-19... 

Thus curvature in an Arrhenius plot is sometimes ascribed to a nonzero value of ACp, the heat capacity of activation. As can be imagined, the experimental problem is very difficult, requiring rate constant measurements of high accuracy and precision. Figure 6-2 shows a curved Arrhenius plot for the neutral hydrolysis of methyl trifluoroacetate in aqueous dimethysulfoxide. The rate constants were measured by conductometry, their relative standard deviations being 0.014 to 0.076%. The value of ACp was estimated to be about — 200 J mol K, with an uncertainty of less than 10 J moE K. ... 

It is conventional to take as the activation volume the value of AV when P = 0, namely —bRT. (This is essentially equal to the value at atmospheric pressure.) Pressure has usually been measured in kilobars (kbar), or 10 dyn cm 1 kbar = 986.92 atm. The currently preferred unit is the pascal (Pa), which is 1 N m 1 kbar = 0.1 GPa. Measurements of AV usually require pressures in the range 0-10 kbar. The units of AV are cubic centimeters per mole most AV values are in the range —30 to +30 cm moP, and the typical uncertainty is 1 cm moP. Rate constant measurements should be in pressure-independent units (mole fraction or molality), not molarity. ... 

Pseudo-unimolecular rate constants measured at various temperatures, one of which is tabulated. 

Frequency factors arc often determined from data obtained within a narrow temperature window. For this reason, it has been recommended4 that when extrapolating rate constants less error might be introduced by adopting the standard values for frequency factors (above) than by using experimentally measured values. The standard values may also be used to estimate activation energies from rate constants measured at a single temperature. 

Table I. Comparison between Rate Constants Measured by the Pulsing Technique and the Pressure Method...

Using either of the above approaches we have measured the thermal rate constants for some 40 hydrogen atom and proton transfer reactions. The results are tabulated in Table II where the thermal rate constants are compared with the rate constants obtained at 10.5 volt cm.-1 (3.7 e.v. exit energy) either by the usual method of pressure variation or for concurrent reactions by the ratio-plot technique outlined in previous publications (14, 17, 36). The ion source temperature during these measurements was about 310°K. Table II also includes the thermal rate constants measured by others (12, 13, 33, 39) using similar pulsing techniques. 

Both the reactors are operated in batch, and the concentrations of components involved are measured online by electro-conductivity. Data interpretation is made by the kinetic equation of second order. The results obtained in the range of 25-45"C are given in Table 3. Again, the values for the rate constant measured in SCISR, ks, are S5 tematically higher than those in STR, ksr, by about 20%, and no significant difference betvi een the values for the active energy measured in SCISR and STR has been found. 

Graphing, using rate constants measured at several temperatures, is the most accurate method to find the value of. However, a good estimate of can be calculated from rate constants measured at just two temperatures ... 

The measurement of potential Hg methylation and MeHg demethylation is significantly more complex than the measurement of HgT and MeHg concentrations in ambient samples. Methodological considerations include the maintenance of redox and temperature condition of samples during measurement, an understanding of the time course of both processes, and an understanding of the impact of spike level on the methylation and demethylation rate constants. Measurement of methylation and demethylation also requires the use of a tracer, and the abiUty to measure that trace analytically. 

The reactions of the bare sodium ion with all neutrals were determined to proceed via a three-body association mechanism and the rate constants measured cover a large range from a slow association reaction with NH3 to a near-collision rate with CH3OC2H4OCH3 (DMOE). The lifetimes of the intermediate complexes obtained using parameterized trajectory results and the experimental rates compare fairly well with predictions based on RRKM theory. The calculations also accounted for the large isotope effect observed for the more rapid clustering of ND3 than NH3 to Na+. 

As seen in Equation 4, the concentration of free isocyanate as a function of time is dependent on both rate constants. Measuring the concentration of the isocyanate as a function of time during isothermal cure provides a convenient method to determine both rate constants. 

In the second investigation [34], involving a coumarin synthesis by Knoevenagel condensation, supported by rate constant measurements and activation energy calculations, it was found that the effect of MW was more important when the reaction was conducted in xylene - it was noticeably reduced in ethanol (Eq. (5) and Tab. 3.3). 

Rogers, J.D. (1989) Rate constant measurements for the reaction of the hydroxyl radical with cyclohexene, cyclopentene, and glutaraldehyde. Environ. Sci. Technol. 23, 177-181. 

The effect of multidipole interaction was observed and studied for a series of different reactions of polyatomic esters and alcohols with peroxyl radicals, hydroperoxides, and dioxygen. The results of rate constants measurements and estimation of AGnfJ are collected in Table 9.19. 

Taking into account the reaction mechanism proposed for the same reaction in water, eq (4) and (5), BrN3 should be the reactive intermediate formed in a preequilibrium step, the rate of consumption of the total bromine is therefore expressed by eq (6). At sufficiently high N3 concentration, the BrN3 concentration is simply given by eq (7) and that of total bromine by eq (8). The introduction of eq (7) and (8) in eq (6) gives eq (9) from which the observed third-order rate constant, measurable by monitoring the free Br2 and the observed... 

The last sentence of the paper under consideration here turned out to be illusory, because, as shown in Section 5.6, the rate-constants measured in nitrobenzene solution turned out to be not those for the attack of a carbenium ion on the double-bond of the monomer, but hybrids. 

This type of electrode is a particularly powerful analytical tool since by performing steady-state measurements alone, it can measure faster rate constants than any other method. For a second-order reaction, the RRDE can reliably and reproducibly determine rate constants as fast as 10 mol dm ) s, while the maximum first-order rate constant measurable with the RRDE is about 10 s . A further advantage of the RRDE is the way that steady-state currents are measured (see below), whereas other methods of determining such high values ofk require the measurement of transients. 

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