CUET UG Chemistry Booster Test – 2 Basics of Chemical Kinetics

Thermodynamic calculations indicate that graphite is more stable than diamond under ordinary conditions. Therefore, the conversion of diamond into graphite is thermodynamically feasible. However, thermodynamics only predicts whether a reaction can occur; it does not indicate how fast the reaction will proceed. The conversion of diamond into graphite has an extremely high activation energy and therefore proceeds at an exceptionally slow rate. The rate is so small that no observable change occurs during a human lifetime. Consequently, diamonds appear permanent even though thermodynamics predicts their eventual conversion into graphite. This example clearly illustrates the difference between thermodynamic feasibility and reaction rate. A reaction may be feasible but still occur so slowly that it is practically unobservable.

Thermodynamics is concerned with the feasibility or spontaneity of a chemical reaction. It helps determine whether a reaction can occur under given conditions and therefore matches with reaction feasibility. Chemical kinetics, on the other hand, deals with the rate of chemical reactions and the factors affecting them. It helps explain how rapidly equilibrium is reached and therefore matches with the time taken to reach equilibrium. At the macroscopic level, chemists observe measurable quantities such as concentration, pressure, volume, and the amounts of reactants consumed or products formed. Hence, it matches with amounts reacted and rates of consumption. At the molecular level, reactions are explained in terms of molecular collisions, orientation, activation energy, and kinetic energy of molecules. Therefore, it matches with orientation and energy of molecules. Thus, the correct matching is: 1-d, 2-c, 3-b, 4-a This classification illustrates how chemistry connects observable experimental results with molecular-level explanations and theoretical predictions.

Thermodynamics is concerned with the feasibility and spontaneity of chemical reactions. It does not provide information regarding the reaction rate or how quickly equilibrium is achieved. Therefore Statement 2 is correct. Chemical kinetics studies the speed of chemical reactions and the factors affecting reaction rates. Hence Statement 3 is correct. Statement 1 is incorrect because altering reaction rates is the subject matter of chemical kinetics rather than thermodynamics. Statement 4 is also incorrect because equilibrium composition is determined by chemical equilibrium, not by kinetics. Thus, only Statements 2 and 3 are correct.

A negative Gibbs energy change signifies that a reaction is thermodynamically feasible under the given conditions. However, thermodynamics provides no information about how rapidly the reaction will occur. Many spontaneous reactions occur very slowly because they possess large activation energies. For example, the conversion of diamond into graphite is thermodynamically feasible but extremely slow. Therefore, even when ΔG is negative, the reaction rate must be determined separately through chemical kinetics. Kinetics studies activation energy, reaction mechanisms and factors influencing reaction speed. Hence, the only valid conclusion is that the reaction is feasible, while its rate requires kinetic analysis.

Chemical equilibrium refers to the state in which the rates of the forward and reverse reactions become equal and the concentrations of reactants and products remain constant. The equilibrium state provides information regarding how far a reaction proceeds before equilibrium is established. This is known as the extent of reaction. Thermodynamics predicts whether a reaction is feasible, while kinetics predicts how fast it occurs. Equilibrium specifically addresses the final position of the reaction and the relative amounts of reactants and products present. Therefore, equilibrium directly helps determine the extent to which a reaction proceeds.

The speed or rate of a chemical reaction indicates how rapidly reactants are converted into products. One practical way to understand reaction speed is by considering the time required for a reaction to approach or attain equilibrium. If equilibrium is reached within a very short time, the reaction is considered fast. Conversely, if equilibrium is attained only after a long duration, the reaction is regarded as slow. Chemical kinetics is the branch of chemistry that studies reaction rates and the factors affecting them. Thermodynamics tells us whether a reaction can occur, while chemical equilibrium tells us how far a reaction will proceed. The time required to reach equilibrium, however, is related to the speed of the reaction and therefore belongs to the domain of chemical kinetics.

For a reaction: the concentration of reactant R decreases as the reaction proceeds. Therefore: is a negative quantity. The rate of disappearance of reactant R is expressed as: The negative sign is introduced because reaction rate is conventionally expressed as a positive quantity. Since concentration decreases during the reaction, the concentration change is negative. Multiplying by –1 converts the value into a positive number without changing its physical meaning. This convention ensures consistency when comparing rates of disappearance of reactants and rates of appearance of products.

The average rate of a chemical reaction is defined as the change in concentration of a reactant or product divided by the corresponding time interval. Mathematically: For reactants: For products: Thus, the average rate depends on two quantities: • The amount by which concentration changes. • The time taken for that change. Neither the final concentration alone nor changes in system volume determine the reaction rate directly. Therefore, the correct definition involves both concentration change and time interval.

A rate law, also known as a rate expression, is a mathematical equation that relates the rate of a reaction to the concentrations of reactants. For a general reaction: the rate law may be written as: where: • k is the rate constant. • m and n are experimentally determined powers called reaction orders. The rate law provides important information regarding the dependence of reaction rate on reactant concentrations and forms the basis for most quantitative studies in chemical kinetics.

Chemical kinetics is concerned with the study of reaction rates and the factors that influence them. These factors include concentration, temperature, catalysts, surface area and reaction mechanisms. In designing dental filling materials, scientists need to control how rapidly the material hardens or sets. If the setting is too fast, handling becomes difficult; if too slow, treatment becomes inefficient. Chemical kinetics provides the tools required to understand and modify reaction rates. By studying kinetic parameters, manufacturers can design materials that set at the desired speed under specific conditions. Therefore, the branch of chemistry responsible for determining conditions that alter reaction rate is chemical kinetics.

Chemical reactions differ significantly in their rates. According to NCERT, precipitation of silver chloride occurs instantaneously when aqueous solutions of silver nitrate and sodium chloride are mixed. Since the reacting ions are already present in solution, product formation takes place almost immediately. Therefore, it is the fastest process among the given events. Food spoilage occurs through various chemical and biological processes such as oxidation, enzymatic action, and microbial decomposition. These reactions generally require several hours or days to produce noticeable changes. Hence, food spoilage occurs at a moderate rate. Rusting of iron is a slow oxidation process involving the reaction of iron with oxygen and moisture present in air. The process occurs gradually and may take weeks, months, or years before significant rust formation becomes visible. Therefore, rusting is the slowest reaction among the three. Thus, the correct decreasing order of reaction speed is: Precipitation of Silver Chloride → Spoilage of Food → Rusting of Iron or 3 → 1 → 2

Chemical kinetics is concerned with understanding the factors that influence the rate of chemical reactions. One of the most important real-life applications of this knowledge is the control of fuel combustion in automobile engines. The speed at which fuel burns affects power generation, fuel economy, engine efficiency, and pollutant emission. According to NCERT, reaction rates are influenced by factors such as temperature, concentration, pressure, catalysts, and the nature of reactants. Automobile engines are designed to optimize these factors so that fuel burns efficiently. Engineers carefully regulate fuel-air mixtures and operating temperatures to achieve controlled combustion. The question specifically focuses on controlling reaction speed rather than determining whether a reaction is possible. Therefore, the relevant kinetic concept is the study of factors that affect the rate of chemical reactions. This represents one of the most practical and significant applications of chemical kinetics in everyday life.

When aqueous solutions of silver nitrate and sodium chloride are mixed, silver ions react with chloride ions to form insoluble silver chloride. The reaction is represented as: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq) According to NCERT, this reaction occurs instantaneously. Since the reacting ions are already present in solution, they can collide and react immediately upon mixing. No complicated bond-breaking process is required before product formation occurs. The rapid appearance of the white silver chloride precipitate demonstrates the high rate of reaction. Such reactions are classified as fast ionic reactions. NCERT uses this example to illustrate the concept that some reactions occur almost instantaneously, whereas others may require hours, days, or even years. Therefore, the reaction is correctly classified as an instantaneous fast ionic reaction.

Rusting is a slow oxidation reaction in which iron reacts with oxygen and moisture present in the atmosphere. The product formed is hydrated iron(III) oxide, commonly known as rust. Unlike precipitation reactions that occur almost instantly, rusting takes place gradually over long periods. According to NCERT, rusting is a classic example of a very slow reaction. The process involves multiple steps, including oxidation of iron atoms and formation of oxide layers. The observable product does not appear immediately after exposure to air and moisture. Instead, visible rust develops slowly over days, weeks, months, or years. The slow rate of rusting illustrates the importance of chemical kinetics because even thermodynamically favorable reactions may proceed at very different speeds. Since the defining feature of rusting is the long time required for noticeable product formation, it is categorized as a very slow reaction.

At the macroscopic level, chemical kinetics deals with measurable quantities such as concentration, pressure, volume, and mass. According to NCERT, chemists are primarily interested in determining the amounts of reactants consumed, the amounts of products formed, and the rates at which these changes occur. When a researcher continuously measures concentration over time, the objective is to determine how rapidly reactants disappear or products appear. Such measurements form the basis for calculating reaction rates and establishing rate laws. These observations help chemists understand reaction behavior and compare the influence of various factors such as temperature and concentration. Macroscopic studies differ from molecular-level investigations. Concepts such as collision frequency, molecular orientation, and reaction intermediates belong to microscopic descriptions of reactions. Therefore, tracking concentration changes over time represents the study of amounts reacted or formed and their rates of consumption or formation.

According to NCERT, molecularity is defined as the number of reacting species, such as atoms, ions, or molecules, that must collide simultaneously in an elementary reaction to bring about a chemical change. Molecularity is always associated with an elementary step and provides insight into how a reaction occurs at the molecular level. Elementary reactions may be unimolecular, bimolecular, or termolecular depending on whether one, two, or three reacting species participate in the step. Molecularity is always a whole number and cannot be zero or fractional. Since molecularity describes the actual number of particles involved in a collision leading to reaction, it is an important concept in reaction mechanism studies. The question specifically asks about the number of species that must collide simultaneously in a single step. This directly corresponds to the definition of molecularity given in NCERT. Therefore, the correct answer is molecularity of the reaction.

Collision theory explains that chemical reactions occur when reacting particles collide with one another. However, not every collision results in product formation. According to NCERT, only effective collisions are capable of producing products. For a collision to be effective, the colliding molecules must possess sufficient kinetic energy known as threshold energy. In addition, the molecules must approach each other with a proper orientation that allows old bonds to break and new bonds to form. If either of these conditions is not satisfied, the collision becomes ineffective and no reaction occurs. Therefore, Statement 2 is correct because effective collisions require sufficient kinetic energy. Statement 3 is correct because proper orientation is essential. Statement 4 is also correct because ineffective collisions do not yield products. Statement 1 is incorrect because only a fraction of collisions are effective. Hence, Statements 2, 3, and 4 are correct.

The Arrhenius equation relates the rate constant of a reaction to temperature and activation energy. According to NCERT, activation energy is the minimum energy required by reacting molecules to undergo effective collisions and form products. Since activation energy represents energy per mole of reacting particles, its standard unit is joules per mole (J mol⁻¹). In many practical situations, activation energy may also be expressed in kilojoules per mole (kJ mol⁻¹), but the SI unit remains joules per mole. The unit can also be verified mathematically from the Arrhenius equation. Since the quantity Ea/RT must be dimensionless inside the exponential term, Ea and RT must possess identical units. Because R has units of J mol⁻¹ K⁻¹ and T has units of kelvin, the product RT has units of J mol⁻¹. Therefore, Ea must also have units of J mol⁻¹.

An elementary reaction is a reaction that occurs in a single step and cannot be further divided into simpler reaction stages. According to NCERT, reaction mechanisms consist of one or more elementary reactions whose combination gives the overall reaction. Elementary reactions are important because they describe the actual molecular events occurring during a chemical transformation. Each elementary reaction has a definite molecularity and represents a single collision event or molecular process. Complex reactions often occur through a sequence of elementary reactions involving intermediates. The question asks for the name given to reactions occurring in one step. This is precisely the definition of an elementary reaction. Understanding elementary reactions is essential for distinguishing between experimentally determined rate equations and the actual mechanism by which a reaction proceeds. Therefore, the correct answer is elementary reactions.

A zero-order reaction is one in which the rate of reaction remains independent of the concentration of reactants. According to NCERT, the rate law for a zero-order reaction is: Rate = k where k is the rate constant. Since no concentration term appears in the rate expression, changing the concentration of reactants does not influence the reaction rate. Zero-order behavior is commonly observed in reactions occurring on catalytic surfaces and in photochemical reactions under specific conditions. As the reaction proceeds, the rate remains constant until the reactant concentration becomes too low to maintain zero-order behavior. The term "zero order" does not mean that the reaction fails to occur. Instead, it means that the exponent of concentration in the rate equation is zero. Therefore, the correct interpretation is that the reaction rate is independent of reactant concentration.