Question

Is zero order rxn will molecularity be equal to zero?

Answer 1

A first order reaction means that just one atom or molecule is involved in the slow step of a reaction. A second order reaction means that two atoms or molecules are involved in the slow step of a reaction. Azero order reaction infers that no molecules are involved in the slow step of a reaction. So, if a reaction doesn’t depend on any molecules, is a reaction even occurring? The answer is yes! Zero order reactions can occur when something outside of the reacting species is responsible for them reacting. For example, the halogenation of a hydrocarbon cannot occur without the presence of light. The rate of the reaction is completely dependent on the intensity of the light striking the reaction mixture and not on the concentration of the reacting species.



Another example of a necessary ingredient that is zero order is a spectator ion. Spectator ions do not take part in a reaction but they are needed as the counter-ions of required ingredients. Spectator ions have no effect on a reaction, which, by definition, means that they are zero order.

Now consider the following first order reaction:



The slow step of this reaction is completely dependent on the concentration of the (CH33)33 CCl and not on the OH−− with which it reacts. The NaOH is a necessary ingredient because the product cannot be made without it, but this reaction is zero order with respect to OH−−.

Saturated Catalysts

There is one other way in which a component can be zero order, and that occurs when a catalyst is saturated. Although catalysis is a very active area of research, we still don’t understand very well how catalysts work; but we know that by adding a catalyst we can speed up the rate of a reaction. Generally, catalysts act like any other component in a reaction, and their presence is usually first order; that is, the rate of reaction changes in proportion to the amount of catalyst present. But there is a condition, particular to catalysts, which we need to investigate. What happens when a catalyst cannot make a reaction go any faster? Why would this happen?



Catalysts act by bringing molecules together in such a way that is beneficial for a reaction to occur. In addition, they weaken or even break bonds that will allow new bonds to form. To do this, the catalyst must come into contact with reactant molecules. But, if a catalyst is completely surrounded with reactant molecules and working at full speed, adding more reactant molecules will not increase the speed of the reaction. In this special case, adding more reactant has no effect on the rate of the reaction. Under these conditions, the reactant appears to be zero order