Chemistry 12 Chapter 4 Exemplar Solutions Chemical Kinetics Part 6

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III. Short Answer Type

Question 33:

State a condition under which a bimolecular reaction is kinetically first order reaction.


Bimolecular reaction becomes kinetically first order when one of the reactants is in excess.

Question 34:

Write the rate equation for the reaction if the order of the reaction is zero.


Question 35:

How can you determine the rate law of the following reaction?


See page no. 99 of NCERT textbook for Class XII.

Question 36:

For which type of reactions, order and molecularity have the same value?


If the reaction is an elementary reaction, order is same as molecularity.

Question 37:

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?


Three, because

Question 38:

Derive an expression to calculate time required for completion of zero order reaction.


For completion

Question 39:

For a reaction Products, the rate law is — Can the reaction be an elementary reaction? Explain.


During an elementary reaction, the number of atoms or ions colliding to react is referred to as molecularity. Had this been an elementary reaction the order of reaction with respect to B would have been 1, but in the given rate law it is . This indicates that the reaction is not an elementary reaction.

Question 40:

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


Apart from the energy considerations, the colliding molecules should also have proper orientation for effective collision. This condition might not be getting fulfilled in the reaction.

Question 41:

For a zero order reaction will the molecularity be equal to zero? Explain.


No, the molecularity can never be zero or a fractional number.

Question 42:

For a general reaction, plot of concentration of A vs time is given in Fig. 4.3. Answer the following question on the basis of this graph.



(i) What is the order of the reaction?

(ii) What is the slope of the curve?

(iii) What are the units of rate constant?


(i) Zero



Question 43:

The reaction between and is highly feasible yet allowing the gases to stand at room temperature in the same vessel does not lead to the formation of water. Explain.


This is because activation energy for the reaction is very high at room temperature.

Question 44:

Why does the rate of a reaction increase with rise in temperature?


At higher temperatures, larger fraction of colliding particles can cross the energy barrier (i.e. the activation energy), which leads to faster rate.

Question 45:

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


The activation energy for combustion reactions of fuels is very high at room temperature therefore they do not burn by themselves.

Question 46:

Why is the probability of reaction with molecularity higher than three very rare?


The probability of more than three molecules colliding simultaneously is very small. Hence possibility of molecularity being three is very low.

Question 47:

Why does the rate of any reaction generally decreases during the course of the reaction?


The rate of a reaction depends on the concentration of reactants. As the reaction progresses, reactants start getting converted to products so the concentration of reactants decreases hence the rate decreases.

Question 48:

Thermodynamic feasibility of the reaction alone cannot decide the rate of the reaction. Explain with the help of one example.


Thermodynamically the conversion of diamond to graphite is highly feasible but this reaction is very slow because its activation energy is high.

Question 49:

Why in the redox titration of vs oxalic acid, we heat oxalic acid solution before starting the titration?


The reaction between and oxalic acid is very slow. By raising the temperature we can enhance the rate of reaction.

Question 50:

Why can’t molecularity of any reaction be equal to zero?


Molecularity is the number of molecules taking part in an elementary step. For this we require at least a single molecule leading to the value of minimum molecularity of one.

Question 51:

Why molecularity is applicable only for elementary reactions and order is applicable for elementary as well as complex reactions?


A complex reaction proceeds through several elementary reactions. Numbers of molecules involved in each elementary reaction may be different i.e., the molecularity of each step may be different. Therefore, discussion of molecularity of overall complex reaction is meaningless. On the other hand, order of a complex reaction is determined by the slowest step in its mechanism and is not meaningless even in the case of complex reactions.

Question 52:

Why can we not determine the order of a reaction by taking into consideration the balanced chemical equation?


Balanced chemical equation often leads to incorrect order or rate law. For example the following reaction seems to be a tenth order reaction.

This is actually a second order reaction. Actually the reaction is complex and occurs in several steps. The order of such reaction is determined by the slowest step in the reaction mechanism. Order is determined experimentally and is confined to the dependence of observed rate of reaction on the concentration of reactants.

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