Write any reaction with fractional order.what is its rate law?
Answers
Here's an example.
Let's consider a reaction of the generic form:
3A⟶2C
Now, let's say we have some knowledge that this reaction proceeds by the following two-step mechanism:
Step one is fast and reversible.
A↽−−⇀k1k−12B
Step 2 is slow and irreversible.
A+B−→k2C
The rate of the reaction (expressed as the change in concentration of the product per unit time) can be expressed in terms of the concentrations of A and B.
rate=Δ[C]Δt=k2[A][B]
However, we would prefer to not use intermediates in our rate law. Since the first step is fast and reversible, we can use the law of mass action to create a relationship between the concentrations of A and B:
K1=k1k−1=[B]2[A]
[B]2=K1[A]
[B]=K1[A]−−−−−√
If we substitute into the rate law:
rate=Δ[C]Δt=k2[A]K1[A]−−−−−√=k2K1/21[A]3/2
The term k2K1/21=k2k1/21k1/2−1 becomes kobs, the observed rate constant of the overall reaction. The fractional order comes from the second step being rate-determining, but the first step being an equilibrium with stoichiometry that generates the fractional tern.
Rate=k[A]^1/4[B]^1/2
In the above case,order will be:3/4
Rate law is the expression in which the reaction rate is given in terms of molar concentration of the reactants with each term raised to some power which may or may not be equal to the stoichiometric coefficient of the reacting species in a balanced chemical reaction.