11th Class Chemistry Chapter 11 REACTION KINETICS Short Question Answers

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11th Class Chemistry Chapter 11 REACTION KINETICS Short Question Answers Below

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1.The units of “rate constant “of second order reaction reaction is dm3, mol-1, sec-1, but the unit of “rate of reaction” is mol. Dm-3 sec-1. Justify it?
The rate of a reaction is the rate of change of concentration with respect to time:
Since   Rate Δ[E]/Δt    =          mole dm-3/s    = moles dm-3s-1
In case of second order reaction is
Rate     =  K [A] [B]
k           =  Rate/[A] [B]  =   mole dm-3s-1/mole dm-3 mole dm-3
K           = dm3 mol-1 s
2.The rate of a chemical reaction is an ever changing parameter under the given conditions. Comment upon the statement?
When the reaction progress, it is very fast at the beginning, slow somewhere in middle and very concentration according to law of mass action. The concentration decrease every moment, so rate decreases every moment.
3.The reaction rate decreases every moment but the rate constant ‘K’ of the reaction is a constant quantity, under the given conditions. Justify it?
Rate constant is the rate of chemical reaction when the concentration of reactants hypothetical reaction
A + B → C+D
Rate = K [A][B]
The concentration of A and B decrease with the passage of time, so the rate decrease. But the rate ‘K’ remains the same for the reaction, throughout its progress under the given conditions.
4.What is order of a reaction? When the reactions become zero order?
The sum of exponents in the rate of a chemical reaction is called order of reaction. Those chemical reactions whose rates are independent of the concentration of reactions, and are mostly influenced by a suitable catalyst or light are zero order reactions. The units for rate constant for such reactions are equal to the units of rates of reactions.
5.50% of a hypothetical first order reaction completes in one hour. The remaining 50% needs more than one hour to convert itself into products?
When the half is one hour it means that 50% of reaction is completed in one hour and 50% of reat of 50% again requires one hour. In other words 75% is completed in two hours and 87.5% in 3 hours. So, It will take many hours to get completed.
6.The radioactive decay is always a first order reaction. How?
Radioactive substance have a signal species at a moment, whose nucleus is being broken up without the help of any external agency. So, only one reactant is present and it flows the first order mechanism.
7.Differentiate between average and instantaneous rate of reaction?
The rate of the chemical reaction at a specific time is called instantaneous rate of reaction. The rate of reaction between two specific time intervals is called average rate of reaction. These two rates are equal for only one instant at any time interval. At the beginning of reaction, the instantaneous rate is higher, than the average rate. The average rate will be equal instantaneous rate when time interval approaches zero.
8.The sum of the coefficient of a balanced chemical equation is not necessarily important to give the order of reaction. Justify it?
There are many reactions in which the coefficients of overall balanced equation don’t become the powers in the rate expression if the reaction. In such reactions there are more than one steps. The coefficients of that slowest step determines the order of reaction.
9.What type of information is obtained for the order of reaction from half life period of that reaction?
Half life period depends upon the initial concentration of reactant. Actually, half life period is inversely proportional to the initial concentration of reactants raised to the power one less than the order of reaction. So from the initial concentration and half life period order of reaction can be calculated.
(t1/2)n   1/ an– 1
10.Define order of reaction with the help of an example?
Order the reaction is the sum of the exponents in the rate expression of a chemical reaction.
2Hl (g)                   gold surface
→                            H2(g) + l2(g)
(zero order reaction)
Rate of reaction = K[Hl]0   (zero order reaction).
11.The order of a reaction is obtained from the rate expression of a reaction and the rate expression is obtained from the experiments?
There are many reactions which the consisted of more than one steps. One of the steps may be slowest which controls the overall rate of reaction and the order of the reaction. That step is determined experimentally, so order of reaction is an experimental quantity.
12.How the half life periods of the chemical reactions are related with the initial concentrations of the reactants for first, second and third order reactions?

The kinetic equations for reactions of different orders are different. When we put the conditions of the half life periods, then we get the expressions for half periods of various reactions having various orders.

(i). For zero order reaction (t1/2)0  = a/2
(ii). For first order reaction (t1/2)1 = 0.693/Ka0
(iii). For third order reaction (t1/2)3 = 1/ Ka2

13. How the reactions of fractional order can be studied by the method of half life periods?
Half life period of any order reaction is inversely proportional of the initial concentration raised to the power one less than the order of reaction.
log t1/t2
(t1/2)n    1/ an-1                so, n    ―1
log a2/a1
t1 and t2 are the half life periods, while a1and a2  are the initial concentration for a particular reaction. If the reaction has a fractional order, then we can calculate its order with the help of this equation.
14. How the half life periods and initial concentrations are related to give the order of reaction?
The formula is an under
log t1/t2  +1
log [a2/a1]
‘a1’ and a2’ are initial concentrations while ‘t1’ and ‘t2’ are the half life periods.
15. How the mechanism of a chemical reaction can help to point out the rate determining step?
When the reactions happens in more than one steps, than we have to write down a series of steps. In this way, we come to know about the nature of intermediates. This total information about the chemical reaction is called the mechanism of reaction. Out of these steps, one of the steps is slowest. This slowest step is the rate determining.
16. What is the effect of temperature on the activation energy of a reaction?
Activation energy of the chemical reactions is a constant parameter and does not depend upon the temperature. Anyhow, by increasing the temperature greater number of molecules get a chance to cross the activation energy barrier.
17. What is temperature coefficient of a reaction?
It has been observed that by increasing the temperature of a reaction by 10 K, the rate constant doubles. This is called temperature coefficient of a reaction.
18. How does the increase of temperature increases the rate of the chemical reactions?
The increase of temperature increases the number of effective collisions. Greater number of molecules find the chances to cross the barrier of activation energy and so the rate increases.
19. How the temperature is related with the rate constant?
Arrhenius has given quantitative relationship between rate constant, energy of activation and temperature
k      = Ae-Ea/RT
or, log k     = Ea/2.3o3RT + log A
20. How the energy of activation can be calculated from the Arrhenius plots?
when a graph is plotted between inverse of temperature on x-axis and log k on the y- axis, then a straight line with the negative slope is obtained. The slope of this straight line gives us the energy og activation.
21. Activation energy is the minimum amount of energy more than the average energy which is just sufficient is convert reactants into products?
Activated complex is a collection of molecules of reactants at the time of collision having greater energy than the reactants and products. At this stage old bonds become weak and the possibilities for the new bonds are there.
22. The reactions happen due to collisions among the molecules, but all the collisions are not fruitful. Justify it?
In order to chemical reaction, a minimum amount of energy more than the average energy is required. Hence, only those collisions will prove to be fruitful whose energies are more than or equal to energy of activation. Moreover, proper orientation is also required. Due to these facts the reaction does not happen in a single moment, but it takes some time.
23. Why the reactions having lower energies of activation have faster rates?
Those reactions whose energies of activation are less have greater number of molecules to cross the energy barrier under the given conditions. It means energy of activation is one of the important parameters to control the rate of chemical reaction.
24. How does a catalyst chemically remain the same at the end of the chemical reaction?
Whenever a catalyst is added to the chemical reaction its chemical composition does not change. Anyhow, its physical state changes.

For example, oxygen is obtained from KCLO3 in the presence of MnO2. First of all the MnO2 is used in the form of granules. It is converted into fine power at the end of the reaction.

25. A particular catalyst is suitable for a particular chemical reaction. Give examples?
When the nature of the catalyst changes, then the nature of the products also changes for the same reaction
(i). HCOOH    Al2O3      H2O  + CO

HCOOH  Cu       H2 + CO2
(ii). 4NH3 + 5O2         pt        4NO + 6H2O

4NH3  + 3O2       Cu        2N2 + 6H2O

26. How does a catalyst effect the reversible chemical reactions and their energy of activation?
A catalyst does not change its Kc  value. It helps to established equilibrium stage earlier. It lowers the energy of reaction by increasing the rates of forward and backward steps. Actually, is gives a new path to the reaction and alters the mechanism.
27. What are the controlling factors on the activity of the enzyme?
The activity of the enzyme is controlled by the pH and the temperature. All the enzymes have greater efficiency when the temperature is 450C, but these enzymes and destroyed at 1000C. Moreover, pepsin in the stomach acts when the pH of the stomach is 1.4.
28. What is catalytic poisoning? Give two examples of catalytic poisoning.
When the foreign substance is present is heterogeneous catalysis, then it makes the catalyst ineffective. This may be temporary for permanent. In permanent poisoning the poison chemically reacts. Arsenic present in contact process for the manufacture of sulphuric acids makes the platinum inert. The presence of CO in Haber’s process makes the catalyst inert.
29. What is promoter or activator?
That substance which increases the activity of the catalyst is called activator. In the manufacture of the ammonia by Haber’s process, small amounts of Al2O3  or Cr2O3 increase the efficiency of iron catalyst.
30. How the enzymes act as catalyst?
Enzymes have cavities which act as active centers. The molecules of the substrate undergoing the reaction fit into these cavities as the key fits into the lock. A complex is produces which decomposes to give the product. (i). Urea is hydrolysed into NH3 and CO2in the presence of urease. (II). Sugar is converted to glucose and fructose by invertize.
31. A finally divided catalyst may prove to be more effective. Give reasons?
In a finally divided solid material, the bare surface or the area of contact increases. Greater number of molecules of reactants find the greater chance to be catalysed.
32. What is the effect of light on the rate of reaction?
There are many photochemical of reactants, greater the chances of direct collisions and greater the rates of reactions.
33. How the rate of reaction is affected by surface area?
Greater the surface area of molecules of reactants, greater the chances of direct collisions and greater the rates of reaction.
34. What do you mean by autocatalysis?
That substance which is formed in the course of reaction some time acts as catalyst.
(a). The hydrolysis of ethyl acetate is catalysed by acetic acid which is formed during the reaction and the acts as autocatalyst.
(b). Copper reacts with nitric acid but the reaction is slow in the beginning. The reaction rate increases due to the formation of nitrous acid, during the progress of reaction.
(c). The reaction of oxalic acid in acidified KMnO4 is slow at the beginning, but after sometime, the MnSO4 produced by reaction makes it faster.

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