Relative-rate techniques

Relative rate techniques have the advantage that such relative measurements can be made with greater precision than absolute rate constant measurements because only relative, not absolute, concentrations of X, and X2 need be measured. Indeed, precisions of 5% or better are common using these techniques. Note, however, that increased precision does not necessarily imply increased accuracy. 

In the presence of an atmosphere of air, reaction (1) has been studied by several groups using relative rate techniques under conditions of steady photolysis (18-21). and by Hynes et al. QJ) who employed the pulsed laser photolysis of H202 with LIF detection of OH. Inspection of Table I reveals that the direct time-resolved study (12) yielded a significantly lower result for k2. Hynes et al. (17) discuss the likelihood of there being a secondary reaction channel for DMS in the competitive kinetics experiments which leads to the high rate constants. Indeed, this discrepancy is repeated for the cases of OH + CH3SH and OH + CS2 (see Table II). For this reason the rate constants under atmospheric conditions obtained by the direct technique are recommended for all three of these reactions. 

Kinetic studies of the reaction of 1-bromopropane with OH radicals have been previously performed [27,30,32,36,37]. Donaghy et al. [30], using the relative rate technique, found a rate constant of (11.8 3.0) x 10 cm molecule s while using C-C6H12 as the reference compound. The obtained atmospheric lifetime was 11-16 days. Teton et al. [32] via pulsed laser photolysis followed... 

Experiments have been carried out at York and McGill Universities. In the relative rate technique of bimolecular reactions, we follow two or more reactions, in which two or more molecules will form products with the same reactant, i.e. ... 

Compared to toluene, benzene is much less reactive towards Cl-atoms. There are only six studies of this reaction, and these are inconsistent with each other [16,17, 19,24,27,42]. In all studies except one, a relative rate technique was used to determine reaction rate constants at room temperature. Atkinson and Aschmann [16] irradiated Cl2-benzene-n-butane-air mixtures and from the relative loss rates of benzene and n-butane obtained the value of (1.5 0.9) x 10 cm /(molecules) for the Cl-atom reaction rate of benzene. Wallington et al. [17] and Noziere et al. 

We performed extensive studies to determine the Cl-atom rate constant of benzene. n-Butane and ethane were among first molecules that were used as references to measure the relative rate constant of Cl-atom reaction of benzene. By the end of these experiments, the concentration of reference molecules reduced close to the detection limits whereas no decay of benzene outside the uncertainty limits was observed. Although the relative rate technique is particularly powerful in the measurement of reactions with comparable rate constants, for reactions with very different rate constants, this method is less accurate. Since it has become clear that the Cl-atom rate constant of benzene is slow, chloromethane, dichloromethane, and trichloromethane (chloroform) were used as references. These sets of experiments consisted of 20 individual experiments in which the concentration of hydrocarbons ranged from 10 to 15 mTorr and the concentration of CI2 varied from 10 to 100 mTorr. The relative rate constants for chloromethane and dichloromethane have been previously measured by Niki et al. [65]. Specifically, they were combined as a check on the experimental procedures. No difference in the values of the rate... 

Oxidation rate constant k, for gas-phase second order rate constants, koe for reaction with OH radical, k os with NOj radical and ko3 with O3 or as indicated, data at other temperatures see reference photooxidation ty, = 2.4-24 h for the gas-phase reaction with hydroxyl radical in air, based on the rate of disappearance of hydrocarbon due to reaction with hydroxyl radical (Damall et al. 1976) koH = (4.64 0.14) X 10 cm molecule S at (299 2) K (relative rate technique to cyclohexane Atkinson et al. 1982 quoted, Atkinson 1985)... 

Experimental determinations from references [10, 12, 13] using relative rate techniques with n-butane as the reference compound. 

The relative rate technique has a few advantages over other methods, i.e. no absolute concentrations have to be measured, impmities do not generally disturb the measurements and the experiments can be carried out in the presence of several reaction partners (Barnes et ai,... 

Also the initiation of radical chains diuing the reaction process does not distrurb the measmements. However, the rate constant of the reference reaction, k2, has to be known. The following system illustrates the kinetics of the relative rate technique. 

Kinetic and mechanistic investigations on the NO3 radical oxidation of three vinyl ethers, methyl, propyl and butyl vinyl ethers have been performed in smog chambers (Klotz et ah, 2004, Georges et al, 2004). Analyses were performed using an in situ FTIR device coupled to a multi-reflection White-cell. Rate constants for the oxidation of methyl and propyl vinyl ethers with NO3 were determined using the relative rate technique and/or by a pseudoabsolute rate technique. Rate constants of (4 2) xio and (1.3 0.2) xio cm molecule" s" were obtained for the NO3 oxidation of methyl and propyl vinyl ethers, respectively. Mechanistic experiments also have been performed for methyl, propyl and butyl vinyl ethers. Main oxidation products are formaldehyde, a formate (methyl, propyl or butyl), nitrates and peroxynitrates (Figure 3). Formaldehyde and the formates have been quantified and their formation yields estimated to be around 50 %. 

The experimental techniques used to study the kinetics of OH radical reactions can be separated Into two distinct methods, namely, absolute and relative rate techniques. The absolute methods have employed discharge flow, flash photolysis, modulation-phase shift, and pulsed radlolysls systems, while a variety of differing chemical systems have been used to determine relative rate data. These techniques are briefly discussed below. 

It is difficult to control and define the levels of hydroxyl radicals in a reaction system and rate constants are often derived using competition kinetics sometimes referred to as relative rate techniques. In this experimental approach, a compound, U, whose reaction rate is not known is allowed to react with hydroxyl radical in the presence of a reference compound, R, whose reaction rate is known. Assuming the compounds are only reacting with OH, the proportionate loss of the reference... 

RR/FTIR relative rate technique/Fourier transform IR absorption... 

Rate coefficients for the reactions of OH and O3 with 2- and 3-methyl-3-butene-1,2-diol and l,2-epoxy-3-methyl-3-butene, the latter having been reported very recently as product from isoprene + O3 [11], were determined by use of a relative rate technique. In the O3 experiments sufficient CO was added to scavenge > 90 % of OH radicals eventually being formed. For the O3 reaction of the diols absolute measurements were carried out in addition by monitoring the decay of O3 under pseudo-first order conditions. Absolute and relative rate data were in good agreement. The average values at 295 2 K are ... 

Other studies on the reactivity of OH with various functional groups have been investigated. The absolute rate constants for the reaction of OH with a range of symmetrical acetals and 2-propoxyethanol have been measured (using the relative-rate technique). The mechanisms of OH addition to propene, imidazole, and phenol have been studied by practical and theoretical techniques. Thus, the rates... 

Paulson, S.E., Fenske, J.D., Sen, A.D., Callahan, T.W. A novel small-ratio relative-rate technique for measuring OH formation yields from the reactions of O3 with alkenes in the gas phase, and Its application to the reactions of ethene and propene. J. Phys. Chem. A103,2050-2059 (1999) Peiro-Garci a, J., Nebot-Gil, I. Ab initio study on the mechanism of the atmospheric reaction OH -I- O3 —> HO2 + O2. Chem. Phys. Chem. 4, 843-847 (2003a)... 

The kinetics of the reaction of chlorine atoms with 2-buten-l-ol have been studied using a relative rate technique by Rodriguez et al. (2008). The rate coefficient values obtained are presented in table n-D-8. The authors have reported an Arrhenius expression of... 

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