problems on first order reaction and zero order reaction and activation energy
Answers
Answer:
Zero-order kinetics is always an artifact of the conditions under which the reaction is carried out. For this reason, reactions that follow zero-order kinetics are often referred to as pseudo-zero-order reactions. Clearly, a zero-order process cannot continue after a reactant has been exhausted. Just before this point is reached, the reaction will revert to another rate law instead of falling directly to zero as depicted at the upper left.
There are two general conditions that can give rise to zero-order rates:
Only a small fraction of the reactant molecules are in a location or state in which they are able to react, and this fraction is continually replenished from the larger pool.
When two or more reactants are involved, the concentrations of some are much greater than those of others
This situation commonly occurs when a reaction is catalyzed by attachment to a solid surface (heterogeneous catalysis) or to an enzyme.
Example 1: Decomposition of Nitrous Oxide
Nitrous oxide will decompose exothermically into nitrogen and oxygen, at a temperature of approximately 575 °C
2N2O−→−−Δ,Ni2N2(g)+O2(g)(2.10.1)(2.10.1)2N2O→Δ,Ni2N2(g)+O2(g)
This reaction in the presence of a hot platinum wire (which acts as a catalyst) is zero-order, but it follows more conventional second order kinetics when carried out entirely in the gas phase.
2N2O⟶2N2(g)+O2(g)(2.10.2)(2.10.2)2N2O⟶2N2(g)+O2(g)
In this case, the N2ON2O molecules that react are limited to those that have attached themselves to the surface of the solid catalyst. Once all of the sites on the limited surface of the catalyst have been occupied, additional gas-phase molecules must wait until the decomposition of one of the adsorbed molecules frees up a surface site.
Enzyme-catalyzed reactions in organisms begin with the attachment of the substrate to the active site on the enzyme, leading to the formation of an enzyme-substrate complex. If the number of enzyme molecules is limited in relation to substrate molecules, then the reaction may appear to be zero-order.
This is most often seen when two or more reactants are involved. Thus if the reaction
A+B→products(1)(1)A+B→products
is first-order in both reactants so that
rate=k [A][B](2)(2)rate=k [A][B]
If BB is present in great excess, then the reaction will appear to be zero order in BB (and first order overall). This commonly happens when BB is also the solvent that the reaction occurs in.
Differential Form of the Zeroth Order Rate Law
Rate=−d[A]dt=k[A]0=k=constant(3)(3)Rate=−d[A]dt=k[A]0=k=constant
where RateRate is the reaction rate and kk is the reaction rate coefficient. In this example, the units of k