[Isomer]

Catalysts are substances that are used industrially or within our bodies to....

A. lower the number of effective collisions between molecules

B. provide an alternate pathway for chemical changes


I put A, but the answer is B. Can somebody please explain why it doesn't lower the number of effective collisions? I thought there were less collisions, so it speeded the rxn up because of the fewer collisions. Also, how does it provide an alternate pathyway for chemical changes???

[wereworm73]

For the reaction to take place, the reagent molecules need to encounter each other.  If you reduce the number of intermolecular collisions (encounters), then you just slow down the reaction rate, which is the opposite of what a catalyst does.

[sdekivit]

B. provide an alternate pathway for chemical changes

the functions of enzymes in the body are:

1. to put the two reagents together in the right orientation
2. producing a stable intermediate by electron rearrangements which is otherwise not stable.
3. straining the bound substrate forcing a particular reaction to occur.

--> so it forces a reaction that would normally hardly takes place. due to a high activation energy).

[Mitch]

How would it lower the number of collisions? The number of collisions is related to the volume of the reaction-space and the speed of the reactants. How would a catalyst affect either the volume of your reaction vessel or slower the speed of a reactant in solution?

[P]

In a situation where 2 molecules have to come together (collide) in a certain direction for reaction to take place - a catalyst (such as a zeolite for example) can help align the molecules such that they collide aligned in the right direction for the desired reaction to take place. Thus - the effective number of collisions leading to the desired reaction will be increased by the presence of the catalyst.

zeolites have a cage like structure and in some cases only allow certain sized molecules to enter the  cage in a certain orientation. reactions take place in the cage with the molecules aligned in the desired way rather than a haphazard/random orientation.  (hope this makes sense and is relevant  )

[Donaldson Tan]

Kinetic theory requires chemical reaction proceed via collision. If there are less effective collisions, then the rate of chemical reaction must decrease. This rules out option (A).

Catalyst makes reactions faster by providing an alternative pathway with a lower activation energy. The rate constant is indirectly proportional to the exponential of the activation energy. (see Arrhenius Equation) This means the catalyst provides a alternate reaction route that exhibits a higher rate constant, thus exhibiting an overall increase in reaction rate.

I think the wording in option (B) is vague because it does not directly link the catalyst function to alternate reaction pathway.