Examville.com - Molecularity, Mechanism and Rate Law of a Reaction Review Notes

Molecularity, Mechanism and Rate Law of a ReactionFor a chemical reaction to occur, the reacting molecules should collide with each as said by collisions theory. Molecularity is defined as the number of reacting species (molecues, atoms, or ions), which collide simultaneously to bring about a chemical reaction. Depending on the number of reacting species reactions can be unimolecular, bimolecular or trimolecular. Unimolecular reaction: i) The decomposition of hydrogen peroxide involves single species, which undergoes the change to form products. ii) Decomposition of ammonium nitrite is also unimolecular reaction because it involves only one reacting species. Bimolecular reactions: When reactions involve the collision of two species, then it is called bimolecular reactions. Tri -molecular reactions: When reactions involve the collision of three species, then it is called trimolecular reactions. In the similar way, we can expect reactions, which involve the collision of four or more molecules. But it is observed that reactions involving three or more molecules are uncommon due to the fact that for these reactions simultaneous collision of three molecules is required. It means third molecule must collide with the other two molecules at the same time when they are in process of collision. The chances of the occurrence of such collisions are very rare. On this basis we can say that reactions involving three or more than three reacting particles should be slow. However, it is observed that some reactions involving more than three molecules are quite fast. For example consider a following reaction The experimental measured rate of this reaction is: Rate = k [Br-] [BrO3-] [H+] 2 Thus the rate if a reaction is of first order with respect to Br- and BrO3- ion and second order with respect to H+ ions and the overall reaction is of1+1+2 =4.But from the reaction it is clear that the reaction is to take place in single step, the 12 particles (5Br- ions, 1 BrO3- ion and 6H+ions) would have to collide simultaneously. Similarly, taking the case of potassium chlorate with ferrous sulphate in the presence of sulphuric acid involves ten species. The above reaction is appears to tenth order but actually it is a second order reaction. This means that if the reaction is to take place in single step, the 10 particles (1KClO3, 6 FeSO4 and 3H2SO4) would have to collide simultaneously. Such type of reactions is very small. Reactions with large number of collision will not occur generally. But if the reaction is found to be quite fast, it means that even though the balanced equation involves a large number of molecules, yet the reaction does not proceed by simultaneous collision of all these reacting particles. It can involve two or maximum three collisions simultaneous. Such type of reactions which take place through a sequence of two or more consecutive steps are called complex reactions .The detailed description of various steps by which reactant change into products is called mechanism of reaction. The steps which contribute to the overall reaction are called elementary processes. Mechanism and rate law: In case of multi step reaction each elementary step will take place its own distinctive rate. Some of the steps will be very fast and some of them will be slow. If one step takes place much more slowly than the other steps, it will definitely control the overall reaction rate. It means that all other steps have to wait for the slow step. The rate of the reaction will never be less than the rate of this slow reaction. The other reactions will under go completion and form products only after the occurrence of the slow step reaction. Thus, the rate of a reaction can be defined as the rate of reaction, which is determined by the slowest step in the sequence of complex reaction. In other words slowest step is the rate-determining step in the given mechanism. Consider a reaction between NO2 andF2 to form NO2F. From the experimental observations it is found that the rate of a reaction is proportional to the concentration of nitrogen peroxide and fluorine. This shows that rate-determining step in the mechanism of this reaction must be the reaction between NO2 and F2 only. So this reaction should be present in the mechanism of the reaction. In the above example slow step will determine the rate of reaction Each step in the above mechanism is called elementary reaction Sum of all the elementary reactions will give the net reaction or overall reaction. So rate of the above reaction can be written as: Rate = - dx/dt = k [NO2] [F2] This is the rate law for the reaction. Some More Examples 1) From the experimental observations it is found that the rate of a reaction is proportional to the square concentration of nitrogen peroxide. This shows that rate-determining step in the mechanism of this reaction must be independent of the concentration of CO. The proposed mechanism of this reaction will be The above reaction proceed in two steps but the rate of the overall reaction is determined by the first step which is the slower step Rate of the above reaction can be written as; Rate = k [NO2] 2 This is the rate law for the reaction. 2) Thermal decomposition of dinitrogen pentaoxide, Experimentally rate of the reaction is given as Rate of reaction = k [N2O5]. If the reaction were to be taken place by the collision of two N2O5 molecules then the reaction will of second order reaction Rate = k [N2O5] 2 But is found that rate law suggests it is of first order reaction. This means reaction is a complex reaction and takes place by two or more successive steps. Expected mechanism of the above reaction is Since slow step is the rate determining step, so above reaction of first order reaction. Rate = k [N2O5] 2 3) Reaction of NO and H2 as: Experimentally determined rate of the reaction is Rate = k [NO] 2 [H2] The rate law expression suggest that it is a complex reaction and will proceed in a sequence of steps which are as follows Since slow step is the rate-determining step, so above reaction is of third order reaction i.e. Rate = k [NO] 2 [H2] 4) Reaction of decomposition of hypochlorite (ClO-) Mechanism The first step is the slow and rate determining step. Thus, the rate of the reaction is Rate = k [ClO-] [ClO-] =k [ClO-] 2 So the above reaction is of second order reaction. 5) Reaction of decomposition of hydrogen peroxide The decomposition of H2O2 is catalyzed by iodide ion in alkaline medium. Alkaline medium Experimentally calculated rate of reaction is Rate = k [H2O2][I-] The rate of reaction is of first order with respect to both H2O2and I. This means that the reaction is a complex reaction and takes place in two or more successive steps. So the probable mechanism for the reaction will be; Since the first step is the slow, rate determining step. Thus, the rate of the reaction is: Rate = k [H2O2][I-] Distinction between Molecularity and order of reaction --------------------------------------------------------------------------------------------