UET UG Chemistry Booster Test – 2 Factors and Theories

The decomposition of N₂O₅ follows first-order kinetics. According to the Arrhenius equation, the rate constant increases with increase in temperature because a larger fraction of molecules possess energy greater than the activation energy. Therefore: At 0°C → Slowest reaction → Longest time for half decomposition At 25°C → Intermediate time At 50°C → Fastest reaction → Shortest time Hence, the decreasing order of time taken for half decomposition is: 3 > 2 > 1 Therefore, the correct answer is B. 3, 2, 1.

When temperature increases by 10°C, the Maxwell-Boltzmann distribution curve shifts toward higher energy values and becomes broader. Consequently, the fraction of molecules possessing energy greater than the activation energy increases significantly, often approximately doubling for many reactions. Statement 1 is correct because the peak shifts toward higher energy. Statement 2 is correct because the distribution broadens. Statement 3 is incorrect because the total area under the curve always remains equal to unity. Statement 4 is considered correct according to NCERT's empirical observation that reaction rate often doubles with a 10°C rise due to increased energetic molecules. Therefore, Statements 1, 2 and 4 are correct.

The Arrhenius equation is: k = Ae^(-Ea/RT) Taking natural logarithm: ln k = ln A − (Ea/R)(1/T) Comparing with the straight-line equation: y = c + mx we obtain: y = ln k x = 1/T Slope = -Ea/R Intercept = ln A Hence, the y-intercept is ln A. Therefore, the correct answer is D. ln A.

For a second order reaction: Rate = k[R]² Rate units: mol L⁻¹ s⁻¹ Therefore: k = (mol L⁻¹ s⁻¹)/(mol² L⁻²) = L mol⁻¹ s⁻¹ Hence, the unit of a second order rate constant is: L mol⁻¹ s⁻¹ Therefore, the correct answer is C.

In chemical kinetics: 1. Threshold energy is the minimum energy required for effective collision and equals activation energy plus energy already possessed by reactants. 2. Activation energy is the energy required to form the activated complex. 3. Reaction coordinate represents the profile of energy change during conversion of reactants to products. 4. Final enthalpy depends upon the nature of reactants and products. Therefore: 1 → d 2 → c 3 → b 4 → a Hence, the correct answer is A.

According to the Arrhenius equation: k = Ae^(-Ea/RT) where Ea is the activation energy. Since Ea appears in the exponent with a negative sign, decreasing Ea makes the exponent less negative. As a result, the value of e^(-Ea/RT) increases exponentially. Consequently, the rate constant k increases rapidly. This is the fundamental reason why catalysts increase reaction rates—they lower the activation energy barrier. Therefore, decreasing the activation energy causes an exponential increase in the rate constant. Hence, the correct answer is B. An exponential increase in the rate constant.

In a potential energy profile diagram, the highest point on the curve corresponds to the activated complex (transition state). This species possesses maximum potential energy and represents the stage where old bonds are partially broken and new bonds are partially formed. The activated complex is extremely unstable and exists only for a very short duration before decomposing into products. Therefore, the species present at the peak of the energy barrier is called the activated complex. Hence, the correct answer is B. Activated complex.

Statement 1 is correct because the enthalpy change of a reaction depends on the energies of reactants and products. Statement 2 is correct because after reaching the activated complex, the system moves toward products and releases energy. Statement 3 is incorrect because the activated complex is extremely unstable and exists only for a very short time. Therefore, Statements 1 and 2 are correct. Hence, the correct answer is A. 1 and 2 are correct.

According to the Maxwell-Boltzmann distribution, increasing temperature shifts the distribution toward higher energies and broadens the curve. As a result, a larger fraction of molecules possess energies significantly greater than the average energy. The area under the curve beyond the activation energy increases substantially. This increase in high-energy molecules is responsible for the increase in reaction rate with temperature. Therefore, the proportion of molecules with much higher energies becomes greater. Hence, the correct answer is D. It becomes greater.

The Maxwell-Boltzmann distribution describes how the kinetic energies of molecules are distributed in a large collection of particles. The graph plots the fraction (or number) of molecules against kinetic energy and helps explain the effect of temperature on reaction rates. It is widely used in chemical kinetics to determine the fraction of molecules possessing energy greater than the activation energy. Therefore, the correct answer is B. Maxwell-Boltzmann distribution.

A catalyst increases the rate of a reaction by providing an alternative pathway with lower activation energy. As a result, the rate constant and reaction rate change. However, thermodynamic quantities such as Gibbs free energy (ΔG), enthalpy (ΔH), and equilibrium constant remain unchanged. Since Gibbs energy is a state function depending only on the initial and final states of the system, a catalyst cannot modify its value. Therefore, the correct answer is C. Gibbs energy.

A catalyst lowers the activation energy of both forward and backward reactions equally. Therefore, it increases the rates of both reactions without changing the equilibrium composition. Statement 1 is correct because the catalyst accelerates both directions equally. Statement 2 is incorrect because the equilibrium constant depends only on temperature and is not altered by a catalyst. Statement 3 is correct because the catalyst helps the system attain equilibrium more rapidly. Therefore, Statements 1 and 3 are correct. Hence, the correct answer is A. 1 and 3 are correct.

The passage clearly states that the hydrolysis of ethyl acetate is actually a reaction involving both ethyl acetate and water. Therefore, the rate depends on the concentrations of both reactants. Since two reactants are involved in determining the rate, the true order of the reaction is second order. However, because water is present in large excess, its concentration remains nearly constant and the reaction appears to follow first order kinetics experimentally. Therefore, the correct answer is C. Second order.

In the hydrolysis of ethyl acetate, water is one of the reactants. However, water is taken in such a large excess that its concentration changes negligibly during the reaction. Since the concentration of water remains practically constant, it becomes incorporated into the rate constant. Consequently, the observed rate depends only on the concentration of ethyl acetate. This is the basis of pseudo first order kinetics. Therefore, the correct answer is D.

According to the intermediate complex theory, a catalyst forms temporary bonds with reactants, producing an intermediate complex. This intermediate exists only for a very short duration and subsequently decomposes to yield products and regenerate the catalyst. Because of its short lifetime, it is described as having a transitory existence. Therefore, the correct answer is B. Transitory existence.

An inhibitor is a substance that decreases the rate of a chemical reaction. Unlike catalysts, which increase reaction rates by lowering activation energy, inhibitors retard the reaction process. In chemical kinetics, inhibitors are often referred to as negative catalysts because their effect is opposite to that of catalysts. Therefore, a substance that reduces the reaction rate is properly termed an inhibitor. Hence, the correct answer is C. Inhibitor.

In collision theory, collision frequency (Z) represents the total number of molecular collisions occurring in a unit volume per unit time. Since chemical reactions occur through molecular collisions, collision frequency provides a measure of how frequently reactant molecules encounter one another. Therefore, collision frequency is expressed in terms of the number of collisions per unit time per unit volume. Hence, the correct answer is D. Volume.

Collision theory assumes that reactions occur when molecules collide with sufficient energy. However, for complex molecules, possessing sufficient energy alone is not enough. The reacting molecules must also collide with the correct orientation. Many collisions occur with improper orientation and therefore fail to produce products. This limitation leads to deviations between observed and predicted rate constants. To account for this, the steric factor (P) is introduced in the modified collision theory. Therefore, the correct answer is A. All collisions do not lead to the formation of products.

In the modified collision theory equation: k = PZAB e^(-Ea/RT) P represents the steric factor that accounts for proper orientation. ZAB represents collision frequency between reactant molecules. Ea denotes activation energy. e^(-Ea/RT) represents the fraction of molecules possessing sufficient energy to react. Therefore: 1 → d 2 → c 3 → b 4 → a Hence, the correct answer is D.

According to collision theory, successful reactions require both sufficient energy and proper molecular orientation. When bromoethane molecules collide with improper orientation, the required bond-breaking and bond-forming processes cannot occur. Consequently, the collision becomes ineffective. The molecules simply separate after collision without producing any products. Therefore, the correct answer is C. They simply bounce back and no products are formed.