Here in this post, you can find Chemical Kinetics Questions with Answers from the various examinations.
Table of Contents
Most Impotent Chemical Kinetics Questions with Answer
[1]. Mention the factors which affect the rate of a chemical reaction.
Sol: The rates of chemical reactions are influenced by a number of factors. These are :
(i) Concentration of reactants. The rate of a chemical reaction is proportional to the concentration of the reacting species taking part in the reaction. It is maximum to start with and slowly decreases since the concentration of the reacting species decreases accordingly.
Concentration of Reactant ↑ ∞ Number of reactant ↑∞ Fraction of molecules having energy equal to or greater than activation energy ↑ ∞ Dentistry of reactant ↑∞ Number of collusion between reactant ↑∞ Formation of Product Molecules ↑ ∞ Rare of Reaction ↑
- In case of reversible chemical reactions, the rate of chemical reaction can be studied separately for both the forward and backward reactions.
- In case of gaseous reactions, the increase in pressure increases the reaction rate.
(ii) Temperature. In general, the increase in temperature increases the reaction rate (there are a few exceptions as well). Actually, the energy of the reactant species increases with the increase in temperature and so will be number of collisions.
Temperature of Reaction medium ↑ ∞ Internal Energy of Reactant molecules ↑∞ Fraction of molecules having energy equal to or greater than activation energy ↑ ∞ Dentistry of reactant ↑∞ Number of collusion between reactant ↑∞ Formation of Product Molecules ↑ ∞ Rare of Reaction ↑
- It has been observed that in most of the cases, about 10° increase in temperature makes reaction rate double.
- The effect of temperature is quite independent of the concentration of the reactant species.
(iii) Presence of catalyst. In many chemical reactions, the reaction rate can be enhanced by certain foreign substances called catalysts. These are actually not consumed in the reactions and also do not undergo any change in chemical characteristics. However, their physical states such as colour, particle size etc., might change.
(iv) Nature of reactants. The nature of the reacting species may also influence the reaction rate. For example, the combustion of nitric oxide (N=O) is faster as compared to that of carbon monoxide (C≡O)
(v) Surface area. An increase in the surface area provides more opportunity for the reactants to come in contact or collide resulting in increased reaction rate.
(vi) Exposure to radiations. Many chemical reactions known as photochemical reactions are carried in the presence of sunlight. For example,
In these reactions, the photons of light are the source of energy which helps in breaking the bonds in the reacting molecules so that may react and form molecules of products.
Order of the Reaction
[1. For a reaction, A + B → Products, the rate law is given by: r = k [A]1/2[B]2. What is the order of reaction?
Ans. Rate law(r) = k [A]1/2[B]2
order of reaction = 1/2+2=2.5
[2]. The conversion of molecules X to Y follows second order kinetics. If the concentration of X is increased three times how will it affect the rate of formation of Y?
Ans. The reaction is: X—>Y
According to rate law,
rate = k[X]2
If [X] is increased to 3 times, then the new rate is
rate’ = k[3X]2
rate’ = 9 k [X]2 = 9 rate
Thus, the rate of reaction becomes 9 times and hence the rate of formation of Y increases 9 times.
[3]. A reaction is first order in A and second order in B.
(i) Write the differential rate equation.
(ii) How is the rate affected on increasing the concentration of B three times?
(iii) How is the rate affected when the concentrations of both A and B is doubled?
Answer: (i) Rate = k[A][B]2
(ii) Rate = kab2
when [B] three times, rate = ka (3b)2
= 9 kab2 = 9 times.
(iii) When both [A] and [B] are double, rate = k2a(2b)2 = 8 kab2 = 8 times.
[4] In a reaction between A and B, the initial rate of reaction (r0 ) was measured for different initial concentrations of A and B as given below:
| A ( mol L-1) | 0.20 | 0.20 | 0.40 |
| B ( mol L-1) | 0.30 | 0.10 | 0.05 |
| r0 ( mol L-1 s-1) | 5.07 x 10-5 | 5.07 x 10-5 | 1.43 x 10-4 |
What is the order of the reaction with respect to A and B?
Answer:
Zero Order Reaction
[1]. The decomposition of NH3 on platinum surface is zero order reaction. What are the rates of production of N2 and H2 if rate constant(k)=2.5 x 10-4 mol-1 L s-1
Answer:
[2].
First Order Reaction
- A first order reaction has a rate constant 1.15 x 10-3 s-1. How long will 5 g of this reactant take to reduce to 3 g?
Answer:
2. Time required to decompose SO2Cl2 to half of its initial amount is 60 minutes. If the decomposition is a first order reaction, calculate the rate constant of the reaction.
Ans. For 1st order reaction, k=0.693/t1/2 = 0.693/60 = 1.155×10-2 min-1 = 1.925×10-4 s-1
3.
Second Order Reaction
Arrhenius equation
1. What will be the effect of temperature on rate constant?
Ans. In general, the rate constant for a reaction nearly becomes double with about every 10° rise in temperature because of the fact that the effective collisions become almost double. The exact dependence of the reaction rate on temperature is given by Arrhenius equation; k=Ae−Ea/Rt.
Where A is the Arrhenius factor or the frequency factor. It is also called pre exponential factor. R is gas constant and Ea is activation energy.
2. The rate of the chemical reaction doubles for and increase of 10 K in absolute temperature from 298 K. Calculate Ea.
Ans. In general,
3. The activation energy for the reaction, 2 HI(g) —-> H2+I2 (g) is 209.5 k J mol-1 at 581 K.Calculate the fraction of molecules of reactants having energy equal to or greater than activation energy?
Answer. Fraction of molecules having energy equal to or greater than activation energy is given by:
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