Speed elementary step

D) A catalyst is a substance that increases the rate of a chemical reaction withont itself being consumed. The catalyst may react to form an intermediate, but it is regenerated in a subsequent step of the reaction. The catalyst speeds up a reaction by providing a set of elementary steps (reaction mechanisms) with more favorable kinetics than those that existed in its absence. Choices A and C, even thongh they are true statements, are the resnlts of a lowered activation energy. 

In chemical reactions, the rate-determining step works in much the same way. From a conceptual standpoint, consider a reaction with two elementary steps where the first step is the slow one (the rate-determining step). The concentration of the reactants in the first step is far more important than the concentration of the additional reactants in the second step. That is because the second step can t start until the first step has been completed. In our fast-food example, it s the equivalent of saying that you can t speed up the process by adding more cashiers. You can have fifty cashiers, but if there are no french fries for them to pick up, they can t do anything. 

A catalyst speeds up a reaction by providing new pathways (elementary steps) with more favorable reaction kinetics than those that exist in the uncatalyzed reaction. Catalysts do so by interacting with the reactant in a reaction pathway that significantly lowers the activation energy, Ea, in comparison to the uncatalyzed reaction. 

A catalyst speeds up a reaction by providing a set of elementary steps with more favorable kinetics than those that exist in its absence. From Equation (13.8) we know that the rate constant k (and hence the rate) of a reaction depends on the frequency factor A and the activation energy —the larger A or the smaller E, the greater the rate. In many cases, a catalyst increases the rate by lowering the activation energy for the reaction. 

Figure 16.19 At highway toll booths, drivers must slow down and stop as tolls are paid. Although they can resume their speeds after paying the toll, the pause affects their overall rate of travel. In a similar way, the overall rate of a chemical reaction is dependent on how fast the slowest elementary step proceeds. 

In Section 14.3 we stressed that rate laws must be determined experimentally they cannot be predicted from the coefficients of balanced chemical equations. We are now in a position to understand why this is so. Every reaction is made up of a series of one or more elementary steps, and the rate laws and relative speeds of these steps dictate the overall rate law for the reaction. Indeed, the rate law for a reaction can be determined from its mechanism, as we will see shortly, and compared with the experimental rate law. Thus, our next challenge in kinetics is to arrive at reaction mechanisms that lead to rate laws consistent with those observed experimentally. We start by exarnining the rate laws of elementary reactions. 

Autocatalytic reactions in closed, homogeneous systems characteristically start slowly or at imperceptible speeds, accelerate to a maximum rate and then subside until all the material has been transformed. The most deeply studied example is the explosively rapid reaction between oxygen and hydrogen for which the observed rate is the consequence of a network of elementary steps with linear chain-branching. The acceleratory phase is dominated by the sequence HO + H2 -> H2O + H H + 02-> j O+H2 HO + H. The second of these three steps is the slowest, and at low temperatures it controls the rate. The overall stoichiometry corresponds to ... 

The total voluminal speed of an elementary step is the difference between the voluminal speeds of the jumps in two opposite directions, and it can be given as ... 

A majority of the reactions considered derive from the qnasi-chemical reactions listed in Chapter 2 (see section 2.5). For each type of reaction, we will give the elementary steps that proceed in the same zone and their volirmtnal speed. We will also introduce the voluminal speed of the equivalent step whose jnstification is given by the theorem of local pseudo-steady state mode (see section 7.9). 

The voluminal speed of this elementary step can be written as (defects being diluted) ... 

If 6 is the fraction of adsorbed sites, Fq the volume of the adsorbed phase, and 1-9 the recovered fraction, the voluminal speed of this elementary step can be written as (defects being diluted) ... 

---