Rates and Rate Laws

This statement implies that the rate law constant does change with temperature, and that s correct. But we also have some simple models for how the rate law constant changes with temperature. 

Reactions don t just occur singly they occur sequentially or in parallel. We will consider how several processes occurring simultaneously affect the amounts of products and reactants. Finally, we recognize that most chemical reactions occur in discrete steps. The overall combination of these steps, called elementary processes, is what makes up the mechanism of a reaction. A proposed mechanism must be consistent with the experimentally determined rate law of a reaction. This requirement puts some restrictions on how we can expect a chemical reaction to occur on an atomic and molecular scale. 

Near the end of the chapter, we will consider two interesting types of reactions, branched reactions and oscillating reactions. Not only do such reactions have interesting kinetics, but they also have some fascinating applications. Finally, we will discuss a little bit of theoretical kinetics, to leave you with the idea that not all kinetics is phenomenological. More and more, basic physical chemical principles are applied at the molecular level in attempts to describe adequate models for chemical reactions—which are, after all, of fundamental interest to chemists. 

One of the most basic descriptions of a chemical reaction is how fast it goes. But when we speak of how fast a reaction goes, we are not thinking fast as in a velocity in meters per second. Rather, we are thinking about how quickly amounts (that is, moles) of reactants are converted into amounts (moles) of products. 010 quickness implies that time (in units of seconds, minutes, hours, days, and so on) will be a concern also. The rate of a reaction is an indication of how many moles of a reactant or product are reacted or produced over a period of time. 

Rates of reactions are a central issue in kinetics. Understand that it is difficult to predict before the fact how fast a reaction will be (although we will explore some of the factors that influence the rate of reactions). A lot of information about kinetics of reactions is experimentally determined. Reaction rates also provide the fundamental information needed to deduce the individual actions that reactant species take in order to make products. (We will consider this near the end of this chapter.) 

---

Outer-sphere (OS) reaction rates and rate laws can be defined for solvolysis of a given complex. Complex formation is defined as the reverse reaction—that is, replacement of solvent (S) by another ligand (L )- Following the arguments of... 

So far we have discussed weathering rates and rate laws from laboratory experiments on pure minerals. These laboratory studies are meant to provide insight for natural systems (rates and variables that affect these rates). We may first try to compare laboratory and field results. 

It is therefore intriguing to understand what is the particular role of the platinum/electrolyte interface in the Kolbe synthesis favoring that reaction path—Eqs. (39a)-(39c)—which is thermodynamically disfavored and unlikely to occur. A closely related reaction whose kinetics are easier to investigate with conventional electrode kinetic methods is the anodically initiated addition of N3 radicals to olefins, discovered by Schafer and Alazrak (275). The consecutive reactions, which follow the initial generation of the reactive intermediate, an Na radical, are somewhat slower than that of the Kolbe radicals, so that their rate influences the shape and potential of the current voltage curves which can be evaluated in terms of reaction rates and rate laws. 

Figure 2 shows that with about 0.5M isobutane at 100 °C. the rates and rate laws for oxidation in the gas phase and in solution are similar. 

For Problem 3-7(b) write the combined PFR mole balance on each species and rate law solely in terms of the molar flow rates and rate law parameters. 

Inherently, the FI A stopped-flow procedure should be an ideal vehicle to determine reaction rates and rate laws, provided that an experimental approach could be designed that allows resolving the individual contributions of physical dispersion and chemical kinetics. A comprehensive treatment of this problem was recently described by Hungerford et al. [838], who pointed out that although the single-line stopped-flow system (Fig. 4.15a cf. Fig. 4.11) allows optimization of solution conditions for measurement during a selected stopped-flow time interval (fs) by choosing... 

Nitric Oxide. Rates of nitric oxide exchange with CoI(NO)2(PPh3) in toluene solution are much greater than for nitric oxide exchange with Co(CO)3(NO) or with Fe(CO)2(NO)a in the gas phase. The reaction in toluene is zero-order in nitric oxide concentration, but in the gas phase the reaction is first-order in nitric oxide. A fuller understanding of these rate and rate-law differences may develop when the preliminary report on the solution studies is amplified in the promised full version which will include activation parameters. 

IBLG See questions from Reaction Rates and Rate Laws ... 

The effects of pH, ionic strength, reagent concentration ratios and deuterium substitution of the sulfur-bonded hydrogens on both the rates and rate laws for the methylene blue oxidations of mercaptoethanol and dithioerythritol have been determined. A free radical chain mechanism consistent with the observed kinetic behavior of the oxidation reactions is proposed. A key feature of the proposed mechanism is the formation of the sulfur-sulfur linkage of the disulfide in the reversible formation of a disulfide radical anion (RSSR) as a chain propagating step in the chain sequence. 

---