the reaction : a2 + b = c + d occurs in the following steps : (a).a2 = 2a, (b0. a + b = x and (c). a + x = c+ d the order of this reaction is one. what is its molecularity. what one of the three steps is the slowest or rate determining step?
Answers
ANSWER
Rate law states the rate of a reaction is directly proportional to the reactant concentrations, each concentration being raised to some power. and, According to the law of Mass Action, the rate of a single (One Step) reaction is directly proportional to the concentration of the reacting substance.
Rate of law of a binocular single step reaction:-
A+B⟶Product
Rate=k[A][B]
And according to the law of mass action
Rate of a reaction α[A] [B]
Rate of a reaction k[A] [B]
NO
2
+F
2
⟶NO
2
F+F
NO
2
+F⟶NO
2
F
Net reaction: 2NO
2
+F
2
⟶2NO
2
F
In complex reactions, the order of the slowest elementary reaction gives the order of the complex reaction. In the above example, although the reaction proceeds through two steps but the rate of the overall reaction is determined by the first step which is the slower of the two steps. Thus
r=
dt
−dx
=k[NO
2
][F
2
]
Both order and molecularity have normally a maximum value of 3.
Molecularity in chemistry is the number of molecules that come together to react in an elementary (single-step) reaction and is equal to the sum of stoichiometric coefficients of reactants in this elementary reaction. Depending on how many molecules come together, a reaction can be unimolecular, bimolecular or trimolecular.
There are some chemical reactions whose molecularity appears to be more than three from stoichiometric equations, eg:
4HBr+O
2
⟶2H
2
O+2Br
HBr+O
2
⟶HOOBr
Step 1: HOOBr+HBr⟶2HOBr
Step 2: [HOBr+HBr⟶H
2
O+Br
2
]
In the reaction molecularity seems to be '5'. Such reactions involve two or more steps; each step has its own molecularity not greater than three, eg: in the above reaction, molecularity of each of the steps is 2. And talking about order, there have also been cases of fourth order reactions being reported in the past.
However the probability of such an occurrence is extremely low because if the order of kinetics is 4 then it means that 4 molecules are reacting with right orientation and activation energy, something rather uncommon. But then again, if you are just talking about possibility, even higher order reactions are possible. But majority, it is 3.
Molecularity of a complex reaction;
A+2b → C is 3
Since, one molecule of A and 2 molecules of B are reacting.
Hence, it is a trimolecular reaction. In general the order of the reaction cannot be predicted from the stoichiometric of the overall reaction.
Thus, order of the reaction may or may not be equal to the molecularity, are generally the same.