Chemistry NCERT Exemplar Solutions Class 12th Chapter Four: Overview, Questions, Preparation

Ans: The reaction rate in collision theory is determined by two factors: energy and orientation factor. Certain reactions can be highly exothermic and energetically favoured, meaning the reactants have enough activation energy to collide effectively. However, they do not proceed at a fixed temperature, which is why the reactant molecules are not orientated properly during collisions, resulting in atoms of reactant molecules combining to produce products that do not face each other.

Ans:

As  illustrated in the graph, as the temperature rises, the peak pushes ahead, increasing probable kinetic energy while decreasing the number of molecules utilizing it, resulting in a faster rate of reaction.

3. Describe how the enthalpy of reaction remains unchanged when a catalyst is used in the reaction.

Ans: Catalyst action is a complicated concept. A catalyst aids in the formation of temporary bonds during a chemical reaction. The objective of a catalyst is to lowers the activation energy. A catalyst is a substance that increases the rate of a reaction without itself undergoing any permanent chemical change. The action of the catalyst can be explained by intermediate complex theory. According to this theory, a catalyst participates in a chemical reaction by forming temporary bonds with the reactants resulting in an intermediate complex. This has transitory existence and decomposes to yield products and the catalyst.

A small amount of the catalyst can catalyst a large number of reactants. A catalyst does not alter Gibbs energy, ΔG of a reaction. The difference in energy between reactants and products is constant.

It is also found that a catalyst does not change the equilibrium constant of a reaction rather, it helps in attaining the equilibrium faster, that is, it catalyses the forward as well as the backward reactions to the same extent so that the equilibrium state remains the same enthalpy of reaction means the difference in energy between reactant and product it will also remain unchanged. 

Ans: Average rate depends upon the change in concentration of reactants or products and the time taken for that change to occur. However, the average rate cannot be used to predict the rate of reaction at a particular instance as it would be constant for the time interval for which it is calculated. 

So, to express the rate at a particular moment of time we determine the instantaneous rate. Itis obtained when we consider the average rate at the smallest time interval say dt (i.e. when △t approaches zero). 

5. With the help of an example explain what is meant by pseudo first order reaction.

Ans: Tell us about the pseudo-first-order reaction. It's the reaction with the highest true rate law, yet it behaves like a first order reaction, but it's more specifically a second order reaction.

As an example, consider the hydrolysis of an ester.

CH₃COOC₂CH₅ + H₂O→CH₃COOH + C₂H₅OH

rate = k[CH₃COOC₂H₅][H₂O]…(constant)

k1[CH₃COOC₂H₅]

k = k[H₂O] 

The role of a catalyst is to change ______.

A. Gibbs energy of reaction

B. enthalpy of reaction

C. activation energy of reaction                                                         

D. equilibrium constant

Ans: Option C

Gibbs energy of reaction - Gibbs free energy is a single-valued combination of entropy and enthalpy. The direction of a chemical reaction at constant temperature and pressure is predicted by Gibbs free energy.

Enthalpy of reaction – When one mole of matter is converted by a chemical reaction under specified conditions, the enthalpy of reaction is the change that occurs in the system.

Activation energy of reaction – The lowest amount of energy necessary to activate molecules to a state where they can perform physical and chemical transformations is known as the activation energy of a process. The catalyst creates a lower-activation-energy pathway.

Equilibrium constant – The equilibrium constant is denoted by the letter "K." It is the equilibrium relationship between the products and reactants of a reaction in terms of a specified unit.

2. In the presence of a catalyst, the heat evolved or absorbed during the reaction _____.

A. increases

B. decreases

C. remains unchanged

D. may increase or decrease

Ans: Correct option:  C

A catalyst allows a chemical reaction to occur at a faster rate or under different conditions than it would otherwise. As a result, a catalyst affects the reaction's enthalpy change, or heat. As a result, in the presence of the reaction, the enthalpy does not vary, i.e., it remains constant, and no heat is produced or absorbed.

3. Activation energy of a chemical reaction can be determined by ________.

A. determining the rate constant at standard temperature

B. determining the rate constants at two temperatures

C. determining probability of collision

D. using catalyst

Ans: Correct option: B

The Arrhenius equation can be used to calculate the activation energy of a chemical process. At two temperatures, this determines the rate constants.

2.303log = $\frac{{\mathrm{K}}_2}{{\mathrm{K}}_1}$ = $\frac{{\mathrm{E}}_{\mathrm{a}}}{\mathrm{R}}$       $\frac{{\mathrm{T}}_2-{\mathrm{T}}_1}{{\mathrm{T}}_1{\mathrm{T}}_2}$

4. Consider Fig. 4.1 and mark the correct option.

A. Activation energy of forward reaction is E ₁+ E ₂and product is less stable than reactant.

B. Activation energy of forward reaction is E ₁+ E ₂and product is more stable than reactant.

C. Activation energy of both forward and backward reaction isE₁ + E₂ and reactant is more stable than product.

D. Activation energy of backward reaction is E₁and product is more stable than reactant.

Ans: Correct option A

Activation Energy → The amount of energy required to overcome the obstacle and generate a product

The activation energy of a forward reaction can be seen in the [E_af = E₁ + E₂] (This is an endothermic reaction.

Therefore, [E_af > E_ab]   

The energy of the product is high, while the energy of the reactant is low.

The lower the energy, the more stable and positive the situation becomes.

Δℎ= posistive

5. Consider a first order gas phase decomposition reaction given below:

A(g) → B(g) + C(g) 

The initial pressure of the system before decomposition of A waspt . After lapse of time ‘t’, total pressure of the system increased by x units and became ‘t’ the rate constant k for the reaction is given as _________.

Ans: Correct option B

Given:  

A(g) → B(g) + C(g) 

p_i= Initial pressure

(Time) t = 0

A(g) → B(g) + C(g) 

p_i        →0atm + 0atm

t,(p_i - x)atm

p_t = (p_i - x)atm + x + x = p_i + x

p_A = (p_i - x)

The value of x changes when it is substituted.

p_A = p_i - (p_t - p_i) = 2p_i - p_t

K =  $\frac{2.303}{\mathrm{t}}$ log $\frac{{\mathrm{p}}_{\mathrm{i}}}{2{\mathrm{p}}_{\mathrm{i}}-{\mathrm{p}}_{\mathrm{t}}}$

6. According to Arrhenius equation rate constant k is equal to A^e - . Which of the following options represents the graph of in k vs  .

Ans: Correct option A

Arrhenius equation Ae -   $\frac{\mathbf{}\mathbf{E}\mathbf{a}}{\mathbf{R}\mathbf{T}}$

K = rate constant

A= frequency factor

Ea= Activation Energy

R= gas constant

T= temperature

ln k = lnA - $\frac{\mathbf{E}\mathbf{a}}{\mathbf{R}\mathbf{T}}$

When the temperature rises, ln falls.

∴ ln kv/s $\frac{1}{\mathbf{T}}\mathrm{}$ , is a negative slope.

ln A is intercepted by k, and its magnitude decreases with time.

∴ Negative slope is obtained.

When compared to the other possibilities, T is increasing over time, which is incorrect.

ln k =  $\frac{-\mathbf{E}\mathbf{a}}{\mathbf{R}}$ $\frac{1}{\mathbf{T}}\mathbf{}$  + lnA

7. Consider the Arrhenius equation given below and mark the correct option.

k = Ae

A. Rate constant increases exponentially with increasing activation energy and decreasing  temperature.

B. Rate constant decreases exponentially with increasing activation energy and decreasing temperature.

C. Rate constant increases exponentially with decreasing activation energy and decreasing temperature.

Ans: Correct option D

k = Ae -    $\frac{\mathbf{}\mathbf{E}\mathbf{a}}{\mathbf{R}\mathbf{T}}$             

From this equation

Kα $\frac{1}{\mathrm{E}\mathrm{a}}$

When Activation energy Ea decreases, rate constant k increases.

Hence, Rate constant increases exponentially with decreasing activation energy and increasing temperature. 

8. A graph of volume of hydrogen released vs time for the reaction between zinc and dil. HCl is given in Fig. 4.2. Based on this mark the correct option   

(I) Average rate upto 40s is $\frac{{\mathrm{V}}_3-{\mathrm{V}}_2}{40}$

(II) Average rate upto 40 seconds is $\frac{{\mathrm{V}}_3-{\mathrm{V}}_2}{40-30}$

(III) Average rate upto 40 seconds is $\frac{{\mathrm{V}}_3}{40}$

(IV) Average rate upto 40 seconds is $\frac{{\mathrm{V}}_3-{\mathrm{V}}_1}{40-20}$

Ans: Correct option C

In the given graph V is the volume in the Y axis and time is in X axis

Zn + dil.HCl→ZnCl₂ + H₂↑

Average rate = 40s

Average rate of reaction = $\frac{\mathrm{C}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{g}\mathrm{e}\mathrm{}\mathrm{i}\mathrm{n}\mathrm{}\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{}\mathrm{o}\mathrm{f}\mathrm{}{\mathrm{H}}_2}{\mathrm{C}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{g}\mathrm{e}\mathrm{}\mathrm{i}\mathrm{n}\mathrm{}\mathrm{t}\mathrm{i}\mathrm{m}\mathrm{e}}$

$\frac{\mathrm{F}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{l}\mathrm{}\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}-\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{}\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}{\mathrm{F}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{l}\mathrm{}\mathrm{t}\mathrm{i}\mathrm{m}\mathrm{e}-\mathrm{}\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{}\mathrm{T}\mathrm{i}\mathrm{m}\mathrm{e}}$

Analyzing the graph line where time and volume intersect. 

$\frac{{\mathrm{V}}_3-0}{40-0}$

$\frac{{\mathrm{V}}_3}{40}$

9. Which of the following statements is not correct about order of a reaction

A. The order of a reaction can be a fractional number.

B. Order of a reaction is experimentally determined quantity.

C. The order of a reaction is always equal to the sum of the stoichiometric coefficients of reactants in the balanced chemical equation for a reaction.

D. The order of a reaction is the sum of the powers of molar concentration of the reactants in the rate law expression. 

Ans: Correct option C

Option (c) is the only one of the four assertions that is incorrect. In rate law expression, the order of reaction is equal to the sum of the power of concentration of the reactants.

xA + yB→zC

r = k(A)^x(B)^y 

Order x + y

The order of the reactions can also be a fraction. In a balanced chemical equation, the order of reaction may or may not be equal to the total of the stoichiometric coefficients of the reactants.

 

10. Consider the graph given in Fig. 4.2. Which of the following options does not show instantaneous rate of reaction at 40^th second?

I. $\frac{{\mathbf{V}}_5-{\mathbf{V}}_2}{50-30}$

II. $\frac{{\mathbf{V}}_4-{\mathbf{V}}_2}{50-30}$

III. $\frac{{\mathbf{V}}_3-{\mathbf{V}}_2}{40-30}$

IV. $\frac{{\mathbf{V}}_3-{\mathbf{V}}_1}{40-20}$

Ans: Correct option B

Reaction occurring at the smallest time interval is known as instantaneous rate of reaction. For e.g the instantaneous rate of reaction at 40s is the rate of reaction during a small interval of time close to 40s. Volume changes during a small-time interval close to the 40s.

Instantaneous rate can be determined graphically by drawing a tangent on the curve

Instantaneous reaction= $\frac{\mathrm{C}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{g}\mathrm{e}\mathrm{}\mathrm{i}\mathrm{n}\mathrm{}\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{}}{\mathrm{C}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{g}\mathrm{e}\mathrm{}\mathrm{i}\mathrm{n}\mathrm{}\mathrm{t}\mathrm{i}\mathrm{m}\mathrm{e}}$

Option B is incorrect since the line travels through the graph but does not link to any of the Y axis lines. As a result, option B is incorrect.

11. Which of the following statements is correct?

A. The rate of a reaction decreases with passage of time as the concentration of reactants decreases.

B. The rate of a reaction is same at any time during the reaction.

C. The rate of a reaction is independent of temperature change.

D. The rate of a reaction decreases with increase in concentration of reactants.

Ans: Correct option  A

Because reactant concentrations fall as reactants are transformed to products, reaction rates decrease over time. When reactant concentrations are raised, reaction rates generally increase.

12. Which of the following expressions is correct for the rate of reaction given below?

5Br - (aq) + BrO₃ - (aq) + 6H + (aq)→3Br₂(aq) + 3H₂O(l)

A. $\frac{\mathbf{\Delta }\left[\mathbf{B}\mathbf{r}\right]}{\mathbf{\Delta }}$ = 5 $\frac{\mathbf{\Delta }\left[{\mathbf{H}}^{+}\right]}{\mathbf{\Delta }\mathbf{t}}$

B. $\frac{\mathbf{\Delta }\left[\mathbf{B}\mathbf{r}\right]}{\mathbf{\Delta }}$ = $\frac{6}{5}$ $\frac{\mathbf{\Delta }\left[{\mathbf{H}}^{+}\right]}{\mathbf{\Delta }\mathbf{t}}$

C. $\frac{\mathbf{\Delta }\left[\mathbf{B}\mathbf{r}\right]}{\mathbf{\Delta }}$ = $\frac{5}{6}$ $\frac{\mathbf{\Delta }\left[{\mathbf{H}}^{+}\right]}{\mathbf{\Delta }\mathbf{t}}$

D. $\frac{\mathbf{\Delta }\left[\mathbf{B}\mathbf{r}\right]}{\mathbf{\Delta }}$ = 6 $\frac{\mathbf{\Delta }\left[{\mathbf{H}}^{+}\right]}{\mathbf{\Delta }\mathbf{t}}$

Ans: Correct option C

Rate of Disappearance = $\frac{-1}{5}$ $\frac{\mathbf{\Delta }\left[\mathbf{B}\mathbf{r}\right]}{\mathbf{\Delta }}$   = - $\frac{\mathbf{\Delta }\left[\mathbf{B}\mathbf{r}{\mathbf{O}}_3\right]}{\mathbf{\Delta }\mathbf{t}}$ =  $\frac{-1}{6}$ $\frac{\mathbf{\Delta }\left[{\mathbf{H}}^{+}\right]}{\mathbf{\Delta }\mathbf{t}}$

$\frac{\mathbf{\Delta }\left[\mathbf{B}\mathbf{r}\right]}{\mathbf{\Delta }}$ = $\frac{5}{6}$ $\frac{\mathbf{\Delta }\left[{\mathbf{H}}^{+}\right]}{\mathbf{\Delta }\mathbf{t}}$

13. Which of the following graphs represents exothermic reaction?

A. (a) only

B. (b) only

C. (c) only

D. (a) and (b) only

Ans: Correct option A

Exothermic Reaction: Exothermic reaction is a chemical reaction in which the enthalpy change is negative, and energy is released in the form of light and heat.

As we all know, ΔH is negative.

Activation energy (forward reaction) – (backward reaction)

(Forward reaction) - E_a(backward reaction) 

The product's exothermic energy exceeds the activation energy of the reactants, therefore the correct answer is A, or graph 1, which depicts an exothermic reaction.

14. Rate law for the reaction A + 2B→C is found to be  

Rate = k[A][B]          

Concentration of reactant ‘B’ is doubled, keeping the concentration of ‘A’ constant, the value of rate constant will be______.

A. the same

B. doubled

C. quadrupled

D. halved

Ans: Correct option A

The rate concentration of a reaction does not depend upon concentrations of the reactions. Hence, it will remain the same. Even if the equation shows the double concentration level even the rate concentration doubles so it’s the same throughout.

Following with the equation 

A + 2B→C

If rate considered as (1)

Rat_e1 = k[A][B]

If rate considered as 2

Then, Rate₁ = k[A][2B]      

Rate₂ = 2Rate₁     

Option A is correct answer

15. Which of the following statements is incorrect about the collision theory of chemical reaction?

A. It considers reacting molecules or atoms to be hard spheres and ignores their structural features.

B. Number of effective collisions determines the rate of reaction.

C. Collision of atoms or molecules possessing sufficient threshold energy results into the product formation.

D. Molecules should collide with sufficient threshold energy and proper orientation for the collision to be effective.

Ans: Correct option C

In the case of gases, collision theory is used to predict the speeds of chemical processes. According to this idea, the reacting chemicals must collide with enough energy and in the right direction. The molecules travel quicker and the frequency increases as the temperature rises.

This proves that all three other possibilities, A, B, and D, are correct. Option C does not correspond to the correct answer.

16. A first order reaction is 50% completed in 1.26 ×10¹⁴s. How much time would it take for 100% completion?

A. 1.26 ×10¹⁵s

B. 2.52 ×10¹⁴s

C. 2.52 ×10²⁸s

D. infinite

Ans: Correct option D

Note. It is not possible to perform 100% of the reaction because is half-life

Basically, it is called half-line period of the reaction. So, the time taken for 100% completion of the reaction is infinite.

17. Compounds ‘A’ and ‘B’ react according to the following chemical equation.

A[g] + 2B[g]→2C

Concentration of either ‘A’ or ‘B’ were changed keeping the concentrations of one of the reactants constant and rates were measured as a function of initial concentration. Following results were obtained. Choose the correct option for the rate equations for this reaction.

A. Rate = k[A] ² [B]

B. Rate = k[A][B] ²

C. Rate = k[A][B]

D. Rate = k[A] ²[B] ⁰

Ans: Correct option B

Let order of A and B be x and y.

r = k[A]^x[B]^y

0.1 = k(0.3)^x(0.3)^y_______1

0.4 = 0.1 = k(0.3)^x(0.6)^y_______2

0.2 = 0.1 = k(0.6)^x(0.3)^y______3

Divide 2 by 1

$\frac{0.4}{0.1}$  = $\frac{{\left(0.6\right)}^{\mathrm{y}}}{{\left(0.3\right)}^{\mathrm{y}}}$

Divide 3 by 1

$\frac{0.2}{0.1}$  = $\frac{{\left(0.6\right)}^{\mathrm{x}}}{{\left(0.3\right)}^{\mathrm{x}}}$

Hence Rate law is

r = k[A]¹[B]²

18. Which of the following statements is not correct for the catalyst?\

A. It catalyses the forward and backward reaction to the same extent.

B. It alters ∆G of the reaction.

C. It is a substance that does not change the equilibrium constant of a reaction.

D. It provides an alternate mechanism by reducing activation energy between reactants and products

Ans: Correct option C    

In the presence of a catalyst, the value of ΔG cannot be changed for any reaction.

ΔG=−RtlnQ

Where Q is the Reaction Quotient, which is determined by the product and reactant concentrations. As a result, ΔG has no connection to the catalyst. Only when the reaction is spontaneous, which must be negative, is it checked. As a result, ΔG cannot be changed.

19. The value of rate constant of a pseudo first order reaction ____________.

A. depends on the concentration of reactants present in small amount

B. depends on the concentration of reactants present in excess.

C. is independent of the concentration of reactants.

D. depends only on temperature. 

Ans: Correct option C

Let's start with what a pseudo-first-order response is.

Although the pseudo-first-order reaction looks to be an order, it belongs to another order. It's a second-order reaction because it involves two reactants.

Let's have a look at a reaction.

CH₃Br + OH→CH₃OH + Br^- 

So, the rate law for the reaction is

Rate = k[OH][CH₃Br]

Rate = k[OH^- ][CH₃Br] = k(constant)[CH₃Br] = K'[CH₃Br]

Only the concentration of CH₃Br will change during the reaction, and the rate will be determined by the reaction's modifications.

20. Consider the reaction A⇌B. The concentration of both the reactants and the products varies exponentially with time. Which of the following figures correctly describes the change in concentration of reactants and products with time?

Ans: Correct option B

If A→B then the concentration of both reactants and the products vary exponentially with time. But, in option B graph the reactant concentration decreases exponentially and the product concentration increases.