Common Misconceptions
4 min read

Chemical Kinetics

- What you are getting from a statement might not be necessarily true. Let us burst some of the common misconceptions
1

Rate of reaction indicates the extent of completion of reaction.

Let us look at it this way, your family is on a trip and have to reach a place that is 200 km away. If they travel non-stop, their average speed is 50 km/h. Hence, they arrive at the destination in 4 hours. But what if they had few stoppages, because of which, their average speed becomes 40 km/h? They will now reach in 5 hours instead of 4. So, even if the speed reduces, the trip gets completed, albeit a bit slowly. The average speed indicates the speed at which the car was travelling to reach the destination. It is not indicative of the extent of the completion of the trip.
The same applies to the reactions. The rate of reaction only indicates the speed at which it is proceeding towards its completion. It doesn't provide any information about the extent of its completion.
2

Increasing the temperature lowers the activation energy.

Let's say that you want to buy chocolate that is of rupees 20, but you have only 10 rupees with you, so you won't be able to buy the chocolate at that very moment. So you go to your friend to lend you the remaining amount. Your friend only has a 20 rupee note with him. You ask him to give him the money and promise that you'll return the extra amount once you buy the chocolate. Now you have 30 rupees with you. The chocolate still costs rupees 10 only. But now you can buy it as you have more than 10 rupees.
The cost of the chocolate can be equated with the activation energy and the amount loaned to you can be equated with an increase in Kinetic energy of reactant molecules because of an increase in temperature.
When the temperature increases, the fraction of molecules whose kinetic energy exceeds the activation energy increases rapidly. This the reason that virtually all chemical reactions (and all elementary reactions) are more rapid at higher temperatures.
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3

A reaction which is of zero order with respect to one of the reactant can proceed without it

Zero-order reaction is a chemical reaction, wherein the rate of reaction does not vary with the increase or decrease in the concentration of the reactants.
Let's take an example of a reaction where there are two reactants, A and B. They combine to give AB as a product.

Let's assume, that the rate of reaction is dependent only on the concentration of A.
Then the rate law of the reaction will written as, . Where [A] is the concentration of A and n is the order of the reaction w.r.t A and [B] is the concentration of B and 0 here indicates that the order of the reaction w.r.t B is 0.
While we may describe the rate as being ''only dependent on [A]'', in reality, the reaction requires molecules of both A and B to form AB.
The rate of reaction may be affected by increasing or decreasing the concentration of A, but to ensure that the reaction takes place, B is required. In the absence of B, the rate of appearance of AB will be zero. And therefore, the reaction rate will too be zero.
Thus, even if the rate law only includes one reactant, other reactants must still be present for the reaction to proceed.
4

The presence of a catalyst will increase the amount of yield

Let's look at it this way, there is an older car which can seat 5 people and cannot go over 50Km/h and there is a brand new car, which can also seat 5 people, can easily travel at 100km/h.
If we need to travel a distance of 100 km, the older car can reach the destination in 2 hours whereas the brand new car can reach the destination in 1 hour, although the number of people they can carry remains the same.
Travelling in the older car can be thought of reaction without any catalyst and the brand new car as a reaction with a catalyst. Although both can carry only 5 people, the only difference is that those 5 people arrive at the destination in a shorter time while travelling in the brand new car.
Now according to Arrhenius equation,
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A catalyst increases the rate of reaction either by increasing A or by decreasing Ea or by doing both. Though catalyst is involved in the reaction, it does not appear in the overall reaction. Thus, its concentration remains constant throughout the chemical reaction. Also, it does not affect yield as it doesn't affect product formation.
Let's see an example to understand this better
The reaction below is for a very famous, Haber's process:-
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It is a reversible reaction which is catalyzed by Iron.
The catalyst does not affect the position of the equilibrium. Adding a catalyst doesn't produce a greater percentage of ammonia in the equilibrium mixture. Its only function is to speed up the reaction.
In its absence, the reaction is so slow that virtually no reaction happens. The catalyst ensures that the reaction is fast enough for a dynamic equilibrium to be set up within a very short time.