12th Chemistry Exam questions of Chapter 4

Table of Contents

12th chemistry Chapter 4: Chemical Kinetics

Welcome, 12th chemistry exam Important question based on CBSE board curriculum and corresponding to the recent 12 class Chemistry syllabus in text questions.
By rehearsing these Class 12 significant inquiries, understudies will actually want to rapidly survey every one of the thoughts shrouded in the part and plan for the Class 12 Yearly assessments as well as other selection tests like NEET and JEE.

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Check our 12th Chemistry exam solution of chapter 2 click here.

Check our 12th Chemistry exam solution of chapter 3 click here.

State a condition under which a bimolecular reaction is kinetically a first order reaction.(Chemistry book)

This could happen if a reactant is consumed in such a large excess that the reaction results in a very slight change in its concentration. The process is a first-order pseudo-reaction. An ester’s hydrolysis in an acidic medium, for instance.

For which type of reactions, order and molecularity have the same value?(Chemistry book)

For simple reactions that occur in a single step, order and molecularity have the same value.

For a zero order reaction will the molecularity be equal to zero? Explain.(Chemistry book)

No,the molecularity for the reaction can neither be zero nor fractional

For a reaction A + B → Products, the rate law is – Rate = k [A][B]3/2. Can the reaction be an elementary reaction? Explain.(Chemistry book)

 Molecularity in an elementary reaction refers to the quantity of atoms or ions coming into contact to undergo a reaction. The order of reaction should have been 1 if the reaction had been simple (a single step reaction); however, it is 3/2, or fractional. This demonstrates that the reaction is not elementary but rather complex in nature.

For a certain reaction a large fraction of molecules has energy more than the threshold energy, yet the rate of reaction is very slow. Why?(Chemistry book)

The orientation factor and the energy factor are the two main factors that affect the reaction rate. In addition to having energy at threshold levels. When a collision occurs, the reacting species needs to be oriented correctly. The orientation effect is incorrect in the reaction that is being observed. Effective collisions are therefore not as common as one might think. This means that the reaction is slow.

Write the rate equation for the reaction 2A + B → C if the order of the reaction is zero.(Chemistry book)

The rate equation for the reaction 2A + B → C in the zero order reaction is

   Rate = k [A]0 [B]0 = k

Solution of chapter 4 sum:

12th chemistry

How can you determine the rate law of the following reaction?(Chemistry book)

          2NO(g) + O₂(g) → 2NO₂ (g)

It is possible to ascertain the reaction’s rate law experimentally. First, the concentration of one of the reactants—let’s say O2—is measured greatly above its concentration, and the rate of reaction is calculated in relation to NO (g). Next, a significant excess of the other reactant, NO, is added, and the rate of reaction is calculated in relation to O2. The reaction’s rate law expression can be written as

Rate = k [NO]x [O2]y

Where x and y are actual coefficients of the reaction.

In a reaction, if the concentration of reactant A is tripled, the rate of reaction becomes twenty-seven times. What is the order of the reaction?(Chemistry book)

For the reaction, rate (r) = k [A]n ……..(1)

According to the given date rate (27r) = k [3A]n ………(2)

Divided equation (2) by (1) , (27r)/(r) = k [3A]n/ k [A]⇒ 27 = 3n ⇒ n = 3

The order of the reaction is 3.

The reaction between H₂(g) and O₂(g) is highly feasible yet allowing the gasses to stand at room temperature in the same vessel does not lead to the formation of water. Explain.(Chemistry book)

Although the reaction between H2(g) and O2(g) is highly feasible, leaving the gasses at room temperature in the same vessel does not result in the formation of water because the activation energy for the reactants is very high at room temperature and not readily available.

Why does the rate of a reaction increase with a rise in temperature?(Chemistry book)

The energy that the reacting species possesses increases with temperature. More species are therefore able to pass through the activation barrier. As a result, when temperature rises, so does the reaction rate.

Oxygen is available in plenty in air yet fuels do not burn by themselves at room temperature. Explain.(Chemistry book)

In general, fuel combustion reactions have a very high activation energy. You cannot get this at room temperature. At room temperature, the fuels as such cannot burn or undergo combustion.

Why is the probability of reaction with molecularity higher than three very rare?(Chemistry book)

The quantity of reacting molecules or species that collide at the same time is known as the molecularity of a reaction. The likelihood of them colliding at the same time decreases as the number rises. This indicates that the likelihood of more than three reacting molecules colliding at the same time is extremely low. Stated differently, there is very little chance of reactions having more than three molecules.

Why does the rate of any reaction generally decrease during the course of the reaction?(Chemistry book)

Any reaction’s rate is directly correlated with the reactant concentration. Both the concentration and the reaction rate gradually drop as the reaction proceeds.

Thermodynamic feasibility of the reaction alone cannot decide the rate of the reaction. Explain with the help of one example.(Chemistry book)

The enthalpy of reaction (ΔrH) is negative because many chemical reactions are thermodynamically feasible. They still don’t happen. In actuality, there are other factors besides aactivation energy that affect reaction velocity. If the molecules undergoing the reaction do not possess enough activation energy, their collisions will be ineffective and produce no products.

Why in the redox titration of KMnO4 vs oxalic acid, we heat oxalic acid solution before starting the titration?(Chemistry book)

That is, at room temperature, the reaction is very slow. The reaction rate rises as the temperature rises. In this reaction, oxalic acid and KMnO4 acidified with diluted H2SO4 typically occur at a temperature of roughly 50°C.

Why is molecularity applicable only for elementary reactions and order is applicable for elementary as well as complex reactions?(Chemistry book)

While complex reactions often involve several stages, elementary reactions are just one. Each step in a multiple step response has its own molecularity; these steps’ molecularities do not add up. Complex reactions lack molecularity as an outcome. In contrast, the order observed for the slowest step in a complex reaction—also known as the step that determines the reaction’s rate—represents the order of the reaction as a whole. Therefore, we can deduce that order applies to simple as well as complicated reactions, while molecularity is only important in single-step reactions.

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