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Chemical Kinetics Formula and Concepts for JEE Main 2025
In the field of physical chemistry named reaction kinetics, chemical kinetics is concerned with the speed at which chemical reactions occur.
It contrasts with thermodynamics, which deals with the direction in which a process occurs but does not reveal the rate of the process.
Chemical kinetics is the study of how experimental conditions affect the pace of a chemical reaction, yielding knowledge on the event's mechanism and transition stages, as well as the development of mathematical models that can also characterize the properties of a chemical reaction. In this guide, we will break down the essential concepts, formulas, and tips to help you master chemical kinetics for JEE Main 2025.
Rate of a Chemical Reaction and Factors Affecting Reaction Rate
Understanding the rate of a chemical reaction is essential in chemistry, as it helps us comprehend the mechanisms, kinetics, and controlling factors governing chemical processes. The rate of reaction represents the change in the concentration of reactants or the formation of products over time. Several factors influence the rate of reactions, including concentration, temperature, pressure, and the presence of a catalyst. In this comprehensive overview, we'll delve into these aspects, covering elementary and complex reactions, reaction order, rate law, rate constant, differential and integral forms of zero and first-order reactions, half-lives, the effect of temperature on reactions according to Arrhenius theory, activation energy, and the collision theory of bimolecular gaseous reactions.
Rate of a Chemical Reaction: The rate of a chemical reaction, often denoted as 'r,' is a measure of how fast reactants are being converted into products. It can be expressed as the change in concentration of a reactant or the formation of a product per unit time:
\[r = -\frac{\Delta[A]}{\Delta t}\]
Where $[\Delta[A]]$ represents the concentration of a reactant (which may decrease with time), and $\Delta t$ is the change in time.
Factors Affecting the Rate of Reactions: Several factors significantly influence the rate of reactions:
Concentration: The concentration of reactants affects the rate. An increase in reactant concentration generally leads to a faster reaction rate. This is due to the greater frequency of collisions between reactant particles.
Temperature: Temperature plays a critical role in reaction rate. As temperature rises, the kinetic energy of particles increases, leading to more effective collisions and higher reaction rates. The relationship between temperature and reaction rate is described by the Arrhenius equation.
Pressure: Pressure affects the rate of reactions involving gases. An increase in pressure, especially in gaseous reactions, results in more frequent collisions between reactant particles, leading to an increased rate.
Catalyst: A catalyst is a substance that increases the rate of a chemical reaction by providing an alternative reaction pathway with lower activation energy. It remains unchanged after the reaction, allowing it to facilitate multiple reactions.
If you want to read more about the Reaction Rates, then you can check out Vedantu’s page on it.
Elementary and Complex Reactions:
Elementary Reactions: These are simple, single-step reactions involving the collision of reactant molecules. They provide insights into the molecular-level mechanisms of reactions.
Complex Reactions: Complex reactions involve multiple elementary reactions occurring in a series. The overall rate of the complex reaction depends on the slowest elementary step, known as the rate-determining step.
Order of Reaction: An Introduction
The order of a reaction is a critical concept in chemical kinetics, integral to understanding reaction rates. It denotes the exponent to which the concentration terms in the rate equation are raised, revealing how changes in reactant concentrations influence the rate. For instance, if doubling the concentration doubles the rate, the reaction is first-order. Second-order reactions exhibit a proportional rate change with the square of the concentration change. Determining the order aids in formulating rate laws and predicting reaction outcomes. In the competitive landscape of JEE Main, a comprehensive grasp of reaction kinetics, including order determination, is indispensable for mastering chemical kinetics and excelling in the examination.
What is Order of Reaction?
The order of a reaction is a fundamental concept in chemical kinetics that describes the relationship between the concentration of reactants and the rate of a chemical reaction. It is expressed as a power (exponent) to which the concentration terms in the rate equation are raised. The order indicates how the rate of the reaction is affected by changes in the concentration of reactants.
There are three common types of Order of Reaction:
Zero Order:
The rate of the reaction is independent of the concentration of the reactant. The rate equation is often written as $ \text{Rate} = k $.
First Order:
The rate of the reaction is directly proportional to the concentration of a single reactant. The rate equation is typically written as $ \text{Rate} = k[A] $ or $ \text{Rate} = k[B] $, where $ [A] $ or $ [B] $ is the concentration of the reactant.
Second Order:
The rate of the reaction is directly proportional to the square of the concentration of a single reactant or to the product of the concentrations of two reactants. The rate equation can be written as $ \text{Rate} = k[A]^2 $, $ \text{Rate} = k[B]^2 $, or $ \text{Rate} = k[A][B] $.
The determination of the reaction order is crucial for understanding reaction mechanisms, formulating rate laws, and predicting the behaviour of chemical reactions under different conditions.
Order of Reaction Examples
Here’s a brief overview of the order of reaction with examples:
Zero-Order Reaction:
Example: Decomposition of N₂O₅
Rate is constant, independent of reactant concentration.
Rate = k
First-Order Reaction:
Example: Decomposition of Hydrogen Peroxide (H₂O₂)
Rate is directly proportional to the concentration of one reactant.
Rate = k[H₂O₂]
Second-Order Reaction:
Example: Reaction between NO₂ and CO
Rate depends on the square of the concentration of one reactant or the product of two reactants.
Rate = k[NO₂]² or Rate = k[NO₂][CO]
Fractional Order Reaction:
Example: Decomposition of HI
Rate is proportional to a fractional power of the concentration (e.g., Rate = k[HI]¹/₂).
Third-Order Reaction:
Example: Reaction between NO and O₃
Rate depends on the concentrations raised to the third power.
Rate = k[NO][O₃]
In each case, the rate law is determined by experimental data.
Order and Molecularity of Reactions:
Reaction Order: The order of a reaction describes how the concentration of reactants influences the rate. It is determined experimentally and can be zero, first, second, or even fractional order.
Molecularity: Molecularity refers to the number of reactant particles involved in an elementary reaction. It can be unimolecular, bimolecular, or termolecular.
Rate Law and Rate Constant: The rate law for a reaction relates the rate to the concentrations of reactants. It is determined experimentally and takes the form:
\[\text{Rate} = k[A]^m[B]^n\]
Where:
Rate - is the reaction rate.
[A] and [B] are the concentrations of reactants A and B.
k is the rate constant.
m and n are the reaction orders for A and B, respectively.
Differential and Integral Forms of Zero and First-Order Reactions:
Zero-Order Reaction: In a zero-order reaction, the rate is independent of the concentration of the reactants. The rate is constant, and the reaction follows a linear relationship between concentration and time. The differential form is
\[\frac{d[A]}{dt} = -k\]
Integral form:
\[[A]_t = [A]_0 - kt\]
First-Order Reaction: In a first-order reaction, the rate is directly proportional to the concentration of the reactant. The rate follows an exponential relationship with time. The differential form is
\[\frac{[A]}{[A]_0} = -kt\]
d[A]/dt=−k[A], while the integral form is ln
\[\ln\left(\frac{[A]_t}{[A]_0}\right) = -kt + \ln([A]_0)\]
Pseudo First Order Reaction: A pseudo-first-order reaction is a chemical process that behaves as if it were first-order, even when it involves multiple reactants. This simplification is often applied when the concentration of one reactant greatly exceeds that of the others, effectively making its concentration constant. By assuming the constant concentration of the dominant reactant, the reaction kinetics appear first-order, facilitating easier rate equation determination and simplifying reaction rate calculations. Pseudo-first-order reactions are commonly observed in fields such as environmental chemistry, where one reactant's concentration remains relatively stable compared to the others, leading to a simplified mathematical treatment of the reaction kinetics.
Characteristics and Half-Lives of Zero and First-Order Reactions:
Zero-Order Reaction Characteristics:
The rate is constant.
The half-life (t_{1/2}) is inversely proportional to the initial concentration:
$t_{1/2} = \frac{[A]_0}{2k}$
As the reaction progresses, the concentration decreases linearly.
First-Order Reaction Characteristics:
The rate is directly proportional to the concentration.
The half-life is constant and does not depend on the initial concentration:
\[t_{1/2} = \frac{0.693}{k}\]
As the reaction progresses, the concentration decreases exponentially.
Effect of Temperature on the Rate of Reactions - Arrhenius Theory:
The Arrhenius equation relates the rate constant (\[k = A e^{-\frac{E_a}{RT}}\])
Where:
$k \text{ is the rate constant,}$ $A \text{ is the pre-exponential factor,}$ $E_a \text{ is the activation energy,}$ $R \text{ is the gas constant,}$ $T \text{ is the temperature in Kelvin.}$
As temperature increases, the rate constant and the rate of reaction also increase exponentially.
Determination of Order of Reaction
The order of reaction in a chemical reaction is determined by studying the relationship between the rate of the reaction and the concentrations of reactants. There are several methods to determine the order:
Initial Rate Method: Measure the initial rates of the reaction with different initial concentrations of reactants.
Half-life Method: Determine the time taken for the concentration of a reactant to reduce to half. This method is applicable for first-order reactions.
Method of Isolation: Study the reaction at the beginning when the concentration of one reactant is much higher than the other.
Graphical Method: Plotting concentration vs. time or concentration vs. rate graphs can help determine the order visually.
The order of reaction provides crucial information about the reaction mechanism and aids in predicting how changes in concentration will affect the rate.
Collision Theory of Bimolecular Gaseous Reactions:
The collision theory explains the kinetics of bimolecular gaseous reactions. It proposes that for a reaction to occur, reactant particles must collide with sufficient energy and proper orientation. Factors like temperature, concentration, and catalysts influence the frequency and effectiveness of collisions, thus affecting the reaction rate.
Understanding the rate of a chemical reaction and the factors affecting it is crucial for predicting and controlling chemical processes. By studying the order, rate law, rate constant, and different order reactions, as well as the effects of temperature, students gain insight into the kinetics of reactions. Furthermore, the collision theory explains the underlying principles of reaction mechanisms. These concepts are essential for various fields, including chemical engineering, environmental science, and material science.
Applications of Chemical Kinetics for JEE Main 2025
Catalysis and Reaction Engineering: Kinetics is fundamental in designing catalysts and optimizing reactions for industrial applications, such as the production of ammonia in the Haber process.
Material Science: Kinetics aids in understanding processes like polymerization and the formation of new materials, which are crucial in manufacturing.
Pharmaceutical Industry: Chemical Kinetics is used to develop drugs with desired release rates, ensuring consistent therapeutic effects over time.
Battery and Energy Storage
Reaction rates in batteries, like lithium-ion cells, are analysed using kinetics to improve performance and lifespan.Explosives and Propellants: Understanding reaction rates is critical for designing explosives and rocket propellants with controlled energy release.
Climate Change Studies: Kinetics helps in modelling the chemical reactions involved in atmospheric processes, such as ozone depletion and greenhouse gas formation.
Wastewater Treatment: The rate of decomposition of harmful substances during chemical or biological treatment is studied using kinetics.
Forensic Science: Chemical Kinetics is applied to determine the time of death in forensic investigations by studying the rate of biochemical changes in a body.
Food Chemistry: It helps analyse cooking processes, fermentation rates, and nutrient breakdown for better food quality control.
Nuclear Reactions: Kinetics is essential in understanding and controlling nuclear reactions in reactors and medical applications.
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Conclusion
The JEE Main chapter on Chemical Kinetics is of paramount importance in the world of chemistry and beyond. It delves into the rates of chemical reactions, offering profound insights into reaction mechanisms, influencing factors, and the underlying principles governing reaction kinetics. Mastery of this chapter equips students with the ability to predict, control, and optimise chemical processes across various scientific and industrial domains. Understanding rate laws, reaction orders, activation energy, and the Arrhenius equation is crucial for success in JEE Main and for tackling complex challenges in areas like pharmaceuticals, environmental science, and materials science. Chemical kinetics, as explored in this chapter, stands as a cornerstone in the study of chemistry and its real-world applications.
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JEE Main 2025: Chemistry Chapter Chemical Kinetics
ShareFAQs on JEE Main 2025: Chemistry Chapter Chemical Kinetics
1. State brief history of Chemical Kinetics.
The law of mass action, which asserts that the speed of a chemical reaction is related to the number of reacting components, was first proposed by Peter Waage and Cato Guldberg in 1864, paving the way for the development of chemical kinetics.
Van 't Hoff was a chemist who published his famous "Études de dynamique chimique" in 1884. He received the Nobel Prize in Chemistry for the first time in 1901, "in honor of the tremendous contributions he has made by the discovery of the rules of chemical dynamics and osmotic pressure in solutions."
2. What are the factors that affect the reaction rate?
The rate of reaction varies based on the components involved. Acid/base reactions, salt production, and ion exchange are all examples of quick reactions.
The physical state of a reactant (solid, liquid, or gas) is also a significant determinant in the rate of change. Thermal motion brings reactants into touch when they are in the same phase, as in an aqueous solution.
Only the particles at the surface of a material can be engaged in a reaction.
Collisions of reactant species cause the reactions. The rate at which molecules or ions collide is determined by their concentrations.
The rate of a chemical reaction is frequently influenced by temperature. Molecules with more thermal energy are found at higher temperatures.
In a gaseous process, increasing the pressure increases the number of collisions between reactants, which speeds up the reaction.
3. What is the application of Chemical Kinetics?
Chemists and chemical engineers can use the mathematical models that explain chemical reaction kinetics to better comprehend and characterize chemical processes including food degradation, microorganism growth, stratospheric ozone decomposition, and biological system chemistry.
Kinetic models can be used to determine the temperature and pressure at which the largest yield of heavy hydrocarbons into gasoline occurs when performing catalytic cracking of heavy hydrocarbons into gasoline and light gas, for example.
4. What are some examples of chemical reactions in our daily life?
In plants, photosynthesis is a chemical reaction in which carbon dioxide and water are converted into food (glucose) and oxygen. It's a significant process since it creates oxygen and provides food for plants and animals.
Combustion reactions occur when you strike a match, ignite a candle, start a bonfire, or light a grill.
Digestion is a multi-step process with thousands of chemical reactions. When you put food in your mouth, water and the enzyme amylase break down sugar and other carbohydrates into simpler molecules.
5. What is the rate constant?
The proportionality constant, which explains the link between the molar concentration of the reactants and the pace of a chemical reaction, is known as the rate constant.
The rate constant, which is also called the response rate constant for the reaction rate coefficient, is shown by the letter k. It is influenced by the ambient temperature.
The following are two methods for calculating rate constant:
The Arrhenius equation
The order of the reaction and the molar concentrations of the reactants
6. Illustrate the five kinds of Chemical Reactions.
The five basic types of chemical reactions are
Combination
Decomposition
Single-replacement
Double-replacement
Combustion
The observer can place the chemical reaction into the given categories by analysing the reactants and products of a given reaction.
Many kinds of changes have happened during the time of chemical reactions such as a change in temperature, energy wasted via the process, and so on.
7. What is your opinion about the Rate of Reaction?
We can explain the rate of reaction as the amount at which the concentration is altering or the ratio of change in concentration and change in time.
It is given by: r = \[\frac{\Delta (c)}{\Delta t}\]
Where,
rate = r
change in concentration = \[\Delta (c)\]
change in time = \[ \Delta t \]
8. Mention a factor that affects the Rate Constant (k).
The factors that are affecting the rate constant are given below:
Increasing the temperature of a reaction generally speeds up the process because the rate constant increases according to the Arrhenius Equation.
With the rising value of T, the value of the exponential part of the equation becomes less negative. This results in an increased value of k.
9. What are the major reasons which cause problems for the Reaction Rate?
The major reasons which cause a problem for the reaction rate are
The concentration of reactants
Temperature
Phase and surface area of reactants
Effect of solvent
Catalyst
10. Define Order of Reaction For JEE Main?
The order of reaction in chemical kinetics refers to the exponent or power to which the concentration terms in the rate equation are raised. It quantifies how the rate of a chemical reaction is influenced by changes in the concentrations of reactants. There are three common types of reaction orders: zero order (rate is independent of concentration), first order (rate is directly proportional to the concentration of a single reactant), and second order (rate is directly proportional to the square of a single reactant's concentration or to the product of two reactants' concentrations). Determining the order of reaction is crucial for understanding reaction mechanisms and predicting reaction behaviour.
11. How to find an order of reaction in JEE Main?
Determining the order of reaction is a crucial aspect of JEE Main. Follow these steps to find the order of reaction:
Experimental Data: Collect experimental data on the reaction by measuring the rate of reaction at different concentrations of reactants.
Initial Rates: Use the initial rates of the reaction and corresponding concentrations. For a single reactant, the order is equal to the power to which the concentration is raised to give the rate.
Half-life Method: For first-order reactions, measure the half-life at different concentrations. The half-life is independent of initial concentration.
Graphical Analysis: Plot graphs of concentration vs. time or rate vs. concentration. The slope of the graph provides information about the order.
Method of Initial Rates: For reactions involving more than one reactant, the method of initial rates is used to determine the order concerning each reactant.
By employing these methods, you can successfully find the order of reaction, crucial for understanding reaction kinetics in JEE Main.
