Why is molecularity applicable only for elementary reactions whereas order is applicable for elementary as well as complex reactions?
Why is molecularity applicable only for elementary reactions whereas order is applicable for elementary as well as complex reactions?
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Solution and Explanation
Molecularity vs Order in Reactions
The concepts of **molecularity** and **order** are important in chemical kinetics. While both describe the behavior of reactions, they apply to different types of reactions. Below is an explanation of the differences between molecularity and order:
1. Molecularity:
Molecularity refers to the number of molecules (or atoms or ions) that collide or interact simultaneously in an elementary reaction. An elementary reaction is a single step reaction, where the reaction proceeds through a single, unbroken mechanism. For example, a simple bimolecular reaction: \[ \text{A} + \text{B} \rightarrow \text{C} \] has molecularity 2 because two reactant molecules, A and B, are involved in the collision.
Molecularity is defined only for **elementary reactions** because: - **Elementary reactions** occur in a single step, and the number of reacting species is directly related to the reaction's molecularity. - It cannot be applied to **complex reactions** because complex reactions consist of multiple elementary steps, and their overall molecularity is not well-defined in a single-step context.
2. Order of Reaction:
The order of a reaction refers to the power of the concentration terms in the rate law. It describes how the rate of the reaction depends on the concentration of reactants. The order is determined empirically through experiments, and it may not necessarily match the molecularity of the reaction. For example, a reaction may be: \[ \text{Rate} = k[\text{A}]^2[\text{B}] \] The order of the reaction is the sum of the exponents of the concentrations, in this case, 3 (second-order with respect to A and first-order with respect to B).
The order of reaction can be applied to **both elementary and complex reactions** because: - For **elementary reactions**, the order usually matches the molecularity. - For **complex reactions**, the order is determined experimentally, as these reactions consist of multiple elementary steps and the overall rate law may involve a combination of steps.
3. Key Differences:
- Molecularity is applicable only for elementary reactions because it refers to the number of reactant species involved in the single step mechanism.
- Order is applicable for both elementary and complex reactions, as it is based on experimental data and reflects how the rate depends on reactant concentrations.
Conclusion:
- **Molecularity** is defined only for elementary reactions, as it represents the number of reactant molecules involved in a single step. - **Order** can be determined for both elementary and complex reactions and reflects how the rate changes with the concentration of reactants, whether the reaction occurs in a single step or multiple steps.
Top Questions on Molecularity
- One mole of an ideal gas expands isothermally and reversibly to double its volume. If the expansion work done by the system is 1728.85 J, the temperature of the system is ____ K (rounded off to 2 decimal places).
Given:
Gas constant, \( R = 8.314 \) J·K\(^{-1}\)·mol\(^{-1}\)- GATE XL - 2024
- Chemistry
- Molecularity
- One mole of an ideal gas expands isothermally and reversibly to double its volume. If the expansion work done by the system is 1728.85 J, the temperature of the system is ____ K (rounded off to 2 decimal places).
Given:
Gas constant, \( R = 8.314 \) J·K\(^{-1}\)·mol\(^{-1}\)- GATE XL - 2024
- Chemistry
- Molecularity
Questions Asked in CBSE CLASS XII exam
- Let \( \vec{a} \) be a position vector whose tip is the point (2, -3). If \( \overrightarrow{AB} = \vec{a} \), where coordinates of A are (–4, 5), then the coordinates of B are:
- Your school is organizing a road safety awareness workshop for students of class IX – XII. As the head boy of your school, draft a notice informing the students about the workshop. Include other necessary details. You are Ashna/Ashish. Put your notice in a box.
- CBSE CLASS XII - 2025
- Letter Writing
Read the following passage carefully:
(1) Gossip. All humans partake in some form, despite the age-old adage, "If you have nothing nice to say, don't say anything at all." Whether it's workplace chatter, the sharing of family news or group texts between friends, it's inevitable that anyone who participates in the above, talks about other people.
(2) People tend to think of gossip as synonymous with malicious rumours, put-downs, or the breathless propagation of a tabloid scoop. But researchers often define it more broadly as "talking about people who aren't present," says an assistant professor of psychology. "It's something that comes very naturally to us" - an integral part of conversation, information sharing, and even community building.
(3) "It's not necessarily negative," adds a professor of psychology. "It can be positive or neutral." Some researchers argue that gossip helped our ancestors survive. Evolutionary psychologists first pioneered this idea, comparing gossip to the grooming primates engage in as a means of bonding.
(4) People are usually resistant to thinking about gossip as anything but bad. It is true that there are some types of gossip that should be avoided, such as gossip that is purely harmful and serves no purpose like mean comments on someone's way of dressing. In such a scenario no one benefits. Gossip can also be both active and passive. Active gossiping refers to directly sharing negative information about someone else. An active gossiper initiates the conversation and actively spreads it. Passive gossiping on the other hand involves more subtle behaviour like hinting, dropping suggestive comments or participating in gossip by listening without actively contributing, but still allowing the gossip to spread through their presence.
(5) It's interesting to note that gossiping also says something about relationships people have with one another. The act of gossiping often implies a level of trust and closeness with someone, which are key aspects of a good friendship.
(6) Research has found that gossip can stave off loneliness, while other studies have found it can facilitate bonding and closeness and serve as a form of entertainment. So, keep on talking. And when your conversation turns to gossip, as it inevitably will, remember that some good can come of it - with the right intentions, of course.
Answer the following questions, based on the above passage:- CBSE CLASS XII - 2025
- Reading Comprehension
Read the following passage carefully:
(1) Introduction: Humans are social creatures with a deep desire to interact with one another. Recent developments in technology have made it possible for billions of people worldwide to use cell phones to satisfy this need. The current study aims to analyse the impact of smartphone use among various categories of people in Kerala, India. The present study will provide an insight into the extent and magnitude of people's reliance on smartphones to communicate in this modern age and the dwindling nature of social relationships.
(2) Methodology: A google form created questionnaire was used to gather information on the subject from a sample of participants who were from various districts in Kerala, India.
(3) Survey Examination: Among the 131 study participants, 66 were men and the remaining 65 were women. For convenience, the study considered only the main purpose for which a smartphone is used by any given individual.
(4) Results:
Individuals differed in their interests and preferences regarding their choice of communication medium. Some preferred direct conversation, while others conveyed their messages through virtual mediums of communication, that did not involve any face-to-face interaction.
(5) Implications for Interventions: It was observed that individuals were interested in using their smartphones even when they were dining. This constant use can have several disadvantages such as reckless use of mobile phones, including prolonged hours of radiation exposure, over-calorie intake due to binge eating, leading to obesity, and other health problems. It is also a vital tool to achieve social progress.
The use of phones while travelling can sometimes detract from an individual's enjoyment of the serenity of the environment, the scenic beauty of picnic spots, and affect the way an individual behaves and interacts with his or her co-travellers.
(6) Conclusion: It is true that most of the population under different age and gender categories use smartphones every day, but there is no significant fall in the duration spent in direct face-to-face conversation with fellow beings, except in very few individuals. The majority of people agree that traditional face-to-face communication is more comfortable, reliable, and expressive than text messages, phone calls, or electronic mail.
Answer the following questions, based on the above passage:- CBSE CLASS XII - 2025
- Reading Comprehension
Read the following extracts and answer the questions
Aunt Jennifer's fingers fluttering through her wool
Find even the ivory needle hard to pull.
The massive weight of Uncle's wedding band
Sits heavily upon Aunt Jennifer's hand.
When Aunt is dead, her terrified hands will lie
Still ringed with ordeals she was mastered by.
The tigers in the panel that she made
Will go on prancing, proud and unafraid.
(Aunt Jennifer's Tigers)