Molecularity of Reaction vs. Order of Reaction — What's the Difference?
By Tayyaba Rehman — Published on December 31, 2023
Molecularity of Reaction indicates the number of molecules in an elementary step, while Order of Reaction denotes how the rate depends on concentration. Both describe reaction kinetics.
Difference Between Molecularity of Reaction and Order of Reaction
Table of Contents
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Key Differences
Molecularity of Reaction is strictly associated with the number of molecules or ions involved in an elementary step of a reaction. It is always a whole number and helps in categorizing the reaction. The Order of Reaction, on the other hand, is an experimental quantity that shows how the rate of reaction depends on the concentration of reactants.
For the Molecularity of Reaction, reactions can be unimolecular, bimolecular, or termolecular, based on whether one, two, or three molecules are involved. However, the Order of Reaction can be zero, first, second, or any other whole, fractional, or even negative number, based on experimental observations.
It's essential to understand that while the Molecularity of Reaction is theoretical and based on an elementary reaction step, the Order of Reaction is determined experimentally, and it pertains to the overall rate of the reaction, not just an individual step.
One can never deduce the Order of Reaction merely by inspecting a chemical equation. However, the Molecularity of Reaction can be discerned by examining an elementary reaction step. It's also worth noting that while the Molecularity of Reaction pertains to individual steps, the Order of Reaction concerns the overall process.
In essence, while both the Molecularity of Reaction and Order of Reaction deal with reaction kinetics, they offer different perspectives. The former is a theoretical concept concerning individual reaction steps, while the latter is empirical, relating the reaction rate to reactant concentrations.
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Comparison Chart
Definition
Number of molecules in an elementary step.
How rate depends on concentration.
Determined
By inspecting an elementary step.
Experimentally.
Values
Whole numbers (e.g., 1, 2, 3).
Can be zero, whole, fractional, or negative.
Refers to
Individual elementary steps.
Overall reaction rate.
Theoretical or Experimental
Theoretical.
Experimental.
Compare with Definitions
Molecularity of Reaction
The count of molecules involved in an elementary reaction step.
A bimolecular reaction involves two molecules.
Order of Reaction
Experimental measure linking rate to concentration.
A first-order reaction rate is directly proportional to the concentration of one reactant.
Molecularity of Reaction
Represents molecules partaking in an individual step.
Termolecular steps in reactions are rare due to their Molecularity of Reaction of three.
Order of Reaction
Exponent indicating concentration's effect on rate.
In zero-order reactions, the rate remains constant regardless of the reactant concentration.
Molecularity of Reaction
Theoretical concept indicating molecule participation.
The reaction between two ions is bimolecular in its Molecularity of Reaction.
Order of Reaction
Not necessarily equal to molecule count in reaction.
A bimolecular step doesn't always lead to a second-order reaction.
Molecularity of Reaction
Integer value indicating involved molecules in a step.
A reaction involving a single molecule has a Molecularity of Reaction of one.
Order of Reaction
Describes how rate changes with concentration variation.
In a second-order reaction, rate varies with the square of a reactant's concentration.
Molecularity of Reaction
Classification based on molecule count in a basic step.
A unimolecular step has a Molecularity of Reaction of one.
Order of Reaction
Derived from experimental rate-concentration data.
Some reactions can have a fractional Order of Reaction.
Common Curiosities
Can Molecularity of Reaction be fractional?
No, Molecularity of Reaction is always a whole number.
Does the chemical equation always reveal the Order of Reaction?
No, the Order of Reaction is determined experimentally and cannot always be deduced from the chemical equation.
Are termolecular reactions common?
No, termolecular reactions, with a Molecularity of Reaction of three, are rare due to their complexity.
What is a zero-order reaction?
In a zero-order reaction, the rate remains constant regardless of the concentration of the reactant.
Is the Order of Reaction always a whole number?
No, the Order of Reaction can be whole, fractional, zero, or even negative based on experimental findings.
Which is a theoretical concept, Order of Reaction or Molecularity of Reaction?
Molecularity of Reaction is a theoretical concept, while Order of Reaction is based on experimental observations.
Can a reaction have the same Molecularity of Reaction and Order of Reaction?
Yes, it's possible, but not guaranteed, as the two concepts describe different aspects of the reaction.
How does the Order of Reaction differ from Molecularity of Reaction?
Order of Reaction is determined experimentally and shows how reaction rate varies with concentration, whereas Molecularity of Reaction is theoretical and counts molecules in a step.
What is Molecularity of Reaction?
Molecularity of Reaction indicates the number of molecules or ions involved in an elementary step of a reaction.
Can a bimolecular reaction have a first-order reaction rate?
Yes, the Order of Reaction is determined experimentally, so a bimolecular reaction might exhibit a first-order rate.
Can the Order of Reaction change during a reaction?
Yes, for some complex reactions, the effective Order of Reaction may change as conditions or concentrations change.
How many molecules are involved in a unimolecular step?
A unimolecular step involves just one molecule, as suggested by its Molecularity of Reaction.
How do we determine the Order of Reaction?
The Order of Reaction is determined through kinetic experiments that relate reaction rate to reactant concentrations.
Why is the Molecularity of Reaction limited to small whole numbers?
It's improbable for more than three particles (termolecular) to collide simultaneously and undergo a reaction due to spatial and energetic reasons.
Is the Order of Reaction always equal to the sum of the stoichiometric coefficients?
No, the Order of Reaction is an experimental value and may not necessarily equal the sum of the stoichiometric coefficients.
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Tayyaba RehmanTayyaba Rehman is a distinguished writer, currently serving as a primary contributor to askdifference.com. As a researcher in semantics and etymology, Tayyaba's passion for the complexity of languages and their distinctions has found a perfect home on the platform. Tayyaba delves into the intricacies of language, distinguishing between commonly confused words and phrases, thereby providing clarity for readers worldwide.
