Mechanisms and Molecularity Learning Goal: To understand how elementary steps ma
ID: 987593 • Letter: M
Question
Mechanisms and Molecularity Learning Goal: To understand how elementary steps makes up a mechanism and how rate law for an elementary step can be determined. Very often, a reaction does not tell us the whole story. For instance, the reaction NO_2(g) + CO(g) right arrow NO(g) + CO_2(g) does not involve a collision between an NO_2 molecule and a CO molecule. Based on experimental data at moderate temperature, this reaction is thought to occur in the following two steps: NO_2(g) + NO_2(g) right arrow NO_3(g) + NO(g) NO_3(g) + CO(g) right arrow CO_2(g) + NO_2(g) Each individuial step is called an elementary step. Together, these elementary steps are called the reaction mechanism. Overall, the resulting reaction is NO_2(g) + CO(g) right arrow NO(g) + CO_2(g) Notice that in the elementary steps NO_3 appears both as a product and then as a reactant; therefore it cancels out of the final chemical equation. NO_3 is called a reaction intermediate. Also notice that 2 molecules of NO_2 appear in the reactants of the first step and 1 molecule of NO_2 appears as product of the second step, the net effect leaves only 1 molecule of NO_2 as a reactant in the net equation. Over and rate law of a reaction The overall order of an elementary step directly corresponds to its molecularity. Both steps in this example are second order because they are each bimolecular. Furthermore, the rate law can be determined directly from the number of each type of molecule in an elementary step. For example, the rate law for step 1 is rate = k[NO_2]^2 The exponent "2" is used because the reaction involves two NO_2 molecules. The rate law for step 2 is rate = k[NO_3]^1 [CO]^! = k[NO_3][CO] because the reaction involves only one molecule of each reactant the exponents are commited. Analyzing a new reaction Consider the following elementary steps that make up the mechanism of a certain reaction: 3X right arrow E+F E+2M right arrowF+N What is the overall reaction? Express your answer as a chemical equation. Very often, a reaction does not tell us the whole story. For instancer, the reaction NO_2(g)+CO(g) right arrow NO(g)+CO_2(g) does not incolve a collision between anNO_2 molecule and a CO molecule. based on experimental data at moderate temperatures, this reaction is thought to occur in the following two steps: NO_2(g) + NO_2(g) right arrow NO_3(g) + NO(g) NO_3(g) + CO(g) right arrow CO_2(g) + NO_2(g) Each individual step is called an elementary step. Together, these elementary steps are called the reaction mechanism. Overall, the reaction is NO_2(g) + CO(g) right arrow NO(g) + CO_2(g) Notice that in the elementary steps NO_3 appears both as a product and then as a reactant; therefore it cancels out of the final chemical equation. NO_3 is called a reaction intermediate. Also noticethat 2 molecules of NO_2 appear in the reactants of the first step and 1 molecule of NO_2 appears as product of the second step, the net effect leaves only 1 molecule of NO_2 as a reactant in the net equation. Molecularity is the proper terms for "how the molecules collide" in a reaction. For example, step 1 is bimolecular because it involves the collision of two molecules. Step 2 is also bimolecular for the same reason. Unimolecular reactions involve only one molecule in the reactants. Though rare, collisions among three molecules can occur. Such a reaction would be called termolecular. What is the rate law for step 1 of this reaction? Express your answet in standard MasteringChemistry notation. For example, if the rate law is k[A][C]^3 type k*[A]*[C]^3. What is the rate law for step 2 of this reaction? Express your answet in standard MasteringChemistry notation. For example, if the rate law is k[A][C]^3 type k*[A]*[C]^3.Explanation / Answer
Eq.1 is 3X--->E+F
EQ.2 E+2M--->F+N
adding eq.1 and eq.2 gives 3X+E+2M----.:> E+2F+N
3X+2M----> 2F+N ( Overall reaction)
for Ist equation , rate K[X]3, K is rate constant
for the second equation, rate= K' [E] [M]2, K' is rate constant