Chemistry Cha 9 Chemical Kinetics Class 11 Notes KPK

Chemistry for Class 11 Khyber Pakhtunkhwa Board Class 11 Chemistry Notes 2021.

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Chemical Kinetics Class 11 Notes KPK

Q.2 i) Determine the overall orders from the following rate equations.

a. rate = k [NO]2 [O2]
b. rate = k [NO]2

Answer:
Overall order of reaction:
Overall order of reaction is the sum of all exponents of the concentration terms in the rate equation. Overall orders of the given reactions are calculated as follows,
a) rate = k [NO]2 [O2]
The order is 2 with respect to NO and 1 with respect to O2. Hence the overall order of reaction will be 2 + 1 = 3 (3rdorder).
b) rate = k [NO]2
As single reactant (NO) is involved in this reaction and order is 2 with respect to NO. So the overall reaction will also be 2 (2nd order).


Q.2 ii) Explain why a molecular collision can be sufficiently energetic to cause a reaction.

Answer:
When two molecules approach each other for collision, they experience a force of repulsion between them. If the colliding molecules possess sufficient kinetic energy, they overcome these repulsive forces and cause the atoms in the molecules to vibrate violently.
Thus bonds holding atoms together are broken by these violent vibrations, and new bonds are formed. So a molecular collision can be sufficiently energetic to cause a reaction depending upon the energies of colliding molecules. The reaction rate also depends upon the number of collisions.
If we increase the number of molecules in the desired medium, ultimately the number of collisions also increases and it has a huge impact on the rate of reaction.

Read more: Chemistry Cha 8 Acids Bases & Salts 11th Notes for kpk


Q.2 iii) Name the four factors that increase the rate of reaction.

Answer:
Factors Affecting Rate of Reaction:
Following are the four factors which increase the rate of reaction.

  1. (Increased) Concentration of reactants
  2. (Reduced) Particle size of Solid reactant
  3. (Increased) Temperature of reaction mixture
  4. Presence of Catalyst

Q.2 iv) In terms of colliding molecules give two reasons why increasing temperature of a reacting system increases its rate of reaction.

Answer:
Effect of Temperature on Reaction Rate:
By increasing temperature, the rate of reaction increases due to the following two reasons.

  • Say yes more. Yes opens doors. No closes them. Yes pushes us. No keeps us safe at home. Imagine all the opportunities waiting for a yes.
  • Increase in temperature increases the kinetic energy of the molecules and hence speeds up the molecular motion. This results in more collisions between molecules in a given time and chance of the reaction increases.
  • At high temperature, more molecules of reactants have energy higher than the activation energy of the reaction. That is why the number of effective collisions are increased which increases the reaction rate.

Q.2 v) Consider two gases A and B in a container at room temperature. What effect would the following changes have on the rate of the reaction between these gases?

a. The pressure is doubled.
b. The number of molecules of gas A is doubled.
c. The temperature is decreased.

Answer:
a)
By doubling the pressure, the molecules of gas are pushed in to a smaller volume. The distance between the molecules becomes smaller and number of collisions are roughly doubled. It would almost double the rate of reaction.
b)
Increase in number of moles of gas ‘A’ increases its concentration. The molecules become crowded and number of collisions increase. It would causes an increase in the rate of reaction.
c)
Decrease in temperature decreases the kinetic energy of molecules. Chances of the effective collision are decreased which would result in the decrease in reaction rate.


Q.2 vi) The rate constant for the reaction


CO + NO2 → CO2 + NO
At 400 °C is 0.50 liter/mol sec, and the reaction is first order with respect to both CO and NO2.
a. What is the overall order of the reaction?
b. What is the rate of the reaction at 400°C when the concentration of CO is 0.025 mol/L and that of NO2 is 0.040 mol/L?

Answer:
CO + NO2 → CO2 + NO
Rate constant = k = 0.50 liter/mol sec
Temperature = 400oC
Conc. of CO = [CO] = 0.025 mol/L
Conc. of NO2 = [NO2] = 0.040 mol/L
a)
As the reaction is first order with respect to both CO and NO2, the overall order of the reaction will be the sum of these individual orders, that is 1+1 = 2 (2nd order).
b)
The rate equation for the given reaction considering the given orders is written as follows,
Rate = k[CO][NO2]
Putting the values in above equation, we have:
Rate = k[CO][NO2]=(0.50)(0.025)(0.040)
Rate = 5 x 10-4 mol litre-1 sec-1

Read more: Chemistry Chapter 7 Chemical Equilibrium Class 11 Notes


Explain briefly why all collisions between reactant molecules do not lead to reaction?

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Answer:
All collisions between reactant molecules do not lead to reaction due to the following two reasons:
The colliding molecules must possess a certain amount of energy, the activation energy for the effective collision. The molecules colliding with the energies less than the activation energy of the reaction do not lead to a reaction.
Only those collisions between reactant molecules are effective which take place with proper orientation or arrangement of the reactant molecules.


Chemistry Notes for Class 11 Long Question 2021

Answer:
All collisions between reactant molecules do not lead to reaction due to the following two reasons:

  • The colliding molecules must possess a certain amount of energy, the activation energy for the effective collision. The molecules colliding with the energies less than the activation energy of the reaction do not lead to a reaction.
  • Only those collisions between reactant molecules are effective which take place with proper orientation or arrangement of the reactant molecules.

ons Chemistry Class 11 Notes for kpk 2021

Q.3) Define the following terms.

(a) Rate of the reaction
(b) Rate constant.
(c) Order of the reaction.
(d) Rate law or rate equation.

Answer:
a) Rate of Reaction:

“Rate of reaction is defined as the change of concentration of reactant or product in unit time”. 

For example, if we consider the reaction between Carbon monoxide (CO) and nitrogen dioxide (NO2) to form carbon dioxide (CO2) and nitric oxide (NO), the rate in terms of product CO2 can be expressed as,
CO + NO2 → CO2 + NO

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b) Rate Constant (k):

“Rate constant is defined as the rate of the reaction when the concentration of each reactant is taken as unity”. 

The rate constant is also known as specific reaction rate. It depends upon temperature and it has different units for different reactions.
c) Order of Reaction:

“The sum of the exponents of the concentration terms in the rate expression of the reaction is known as order of reaction”.

For example, the given reaction is second order because both the exponents of concentrations are unity and their sum is two.
CO + NO2 → CO2 + NO
Rate = k[CO]1[NO2]1
d) Rate Law:

“The rate of chemical reaction is proportional to the molar concentration of the reacting substances raised to appropriate power”.

For example the rate law for the reaction below is written as,
CO + NO2 → CO2 + NO
Rate ∝ [CO][NO2]
Rate = k[CO][NO2]


Q.4 a) Define the activation energy. What role does it play in chemical reaction?

Answer:
Activation Energy:

“The minimum energy of reactant molecules required for effective collision to undergo a certain reaction is called Activation energy”.

Role of Ea in Chemical Reactions:
For a reaction to occur between molecules, a certain amount of energy must be absorbed to weaken the bonds holding the reactant molecules together. The quantity “Ea” represents the minimum energy required to bring the reactants to a state where they can rearrange to form products.


Two molecules to react must collide with each other but energy of collision may not result into reaction. Any molecule in motion possesses kinetic energy; the faster it moves, the greater is the kinetic energy. A fast moving molecule must collide with another molecule.


When molecules collide, part of their kinetic energy is converted to potential energy (e.g. as vibrational energy). If the kinetic energies are large, then the colliding molecules will vibrate so strongly as to break some of the chemical bonds. If the kinetic energies are small, the molecules will just touch each other with certain force.
Thus, there is some minimum collision energy below which no reaction occurs. This energy is called ‘activation energy’. It is the minimum amount of energy required to initiate a chemical reaction.

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The species temporarily formed by the reactant molecules as a result of the collision before they form the product is called the activated complex. It is extremely unstable and has a short life span. The diagram shows the form of the reaction profile for the collision of reactant molecules. On the left, the horizontal line represents the energy of the two stationary reactant molecules that are far apart from one another. The potential energy rises from this value only when the separation of the molecules is so small that the two molecules are in contact.
It rises as bonds start to break. The potential energy reaches at a peak value when the two molecules are highly distorted. Then it starts to decrease as new bonds are formed. At the right side of the maxima, the potential energy rapidly falls to low value as the product molecules form. For the reaction to be successful, therefore, the molecules must approach with sufficient kinetic energy along their line of approach to carry the over the activation barrier, the peak in the reaction profile.


Q.4 b) Household gas (methane) bums in the presence of oxygen and gives energy for our daily use. If a mixture of methane and oxygen is kept in a container for indefinite period, no reaction takes place, explain?

Answer:
For a reaction to occur it has to overcome its activation energy, Ea. A mixture of methane and oxygen does not react itself until flame is provided. Flame helps to overcome the activation energy of the reaction and initiates the combustion process. But once the reaction is started, heat is produced continuously in this exothermic reaction and flame is no more required.
So a mixture of methane and oxygen if kept in a container for infinite period does not react itself because it lacks the necessary energy of activation.


Q.5) Explain how does a catalyst increase the rate of a reaction? Compare a catalyzed and uncatalyzed reaction on a potential energy diagram.

Answer:
Effect of Catalyst on Rate of Reaction:
Catalyst increases the rate of reaction by changing the reaction path via a lower energy activated complex requiring lower energy than the uncatalyzed reaction. As catalyst decreases the activation energy of the reaction, more molecules of reactants become able to cross this energy barrier and chances of reaction increase. As a result the rate of reaction increases.
Comparison of Catalyzed & Uncatalyzed Reaction:
The following figure gives the comparison for reaction rates of catalyzed and uncatalyzed reactions.

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The figure compares the reaction paths for catalyzed and uncatalyzed reactions. Ea is the activation energy in the presence of a catalyst. Many molecules, that do not have kinetic energy to overcome the Ea potential energy barrier, are able to cross the lower Ecat barrier thus making the reaction faster.


Q.6) Explain the relationship between reactant concentration and rate of the reaction.

Answer:
Reactant Concentration and Rate of reaction:
The relationship between reactant concentration and the rate of reaction is given by the rate law. The rate law states as follows:

“The rate of chemical reaction is proportional to the product of molar concentration of the reacting substances raised to appropriate power”.

Explanation:
The rate of reaction is directly related to the concentration of the reactant. At the start of reaction, the rate is high because concentration of the reactants is maximum. As the reaction proceeds the concentration of reactants falls and the rate of reaction also decreases. For example, the reaction between CO and NO2 gases occurs as follows,
CO + NO2 → CO2 + NO
The rate for this reaction is proportional to the concentration of CO as well as NO2. So we can write,
Rate ∝ [CO]
Rate ∝ [NO2]
Combining the above two equations,
Rate ∝ [CO] [NO2]
Rate = k[CO] [NO2]
Where k is proportionality constant which is known as rate constant of the reaction.

Q.7) Discuss why, (according to collision theory of chemical reactions) some molecular collisions result in a chemical reaction while others don’t. Draw and explain a reaction energy diagram (reaction profile) for


a. An exothermic reaction.
b. An endothermic reaction.
Also show on the diagram
a. Position of the reactants energy.
b. Position of the product energy.
c. Activation energy.
d. ΔH of the reaction.

Answer:
Molecular Collisions & Chemical Reaction:
For a reaction to happen, the reactant molecules must collide with each other effectively. Some collision cause reaction while in others the colliding molecules just bounce back. The reasons for effective and ineffective collisions are discussed below.

  • If the molecules collide with certain kinetic energy which is equal or greater than the activation energy of the reaction, they can result in a chemical reaction. The colliding molecules use that energy to overcome the repulsive forces and cause the atoms in the molecules to vibrate violently.
    Thus, bonds holding atoms together are broken by these violent vibrations, and new bonds are formed. If the colliding molecules is less than the energy of reaction, the molecules just collide and bounce back resulting in no chemical reaction.
  • All the collisions between molecules possessing the required energy of activation do not lead to reaction. The manner in which the molecules collide is also important.
    Only those molecules result in a reaction which collide in a specific orientation. If the orientation of colliding molecules is not proper, it results no reaction.

Reaction Profiles for Exothermic and Endothermic Reactions:
Exothermic reaction occurs with the release of energy and hence the potential energy of products is less than those of reactant (as a part of energy is released in the form of heat). The energy released is shown with red in the diagram and it depends upon the energy difference between reactants and products.
On the other hand, endothermic reaction occurs with the absorption of heat and the potential energy of products becomes greater than that of the reactant. Energy absorbed in this case is labelled with red in the diagram and depends upon the difference of enthalpies between products and reactants.

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Position of the reactants and products energy, activation energy and ΔH of reaction (heat released or absorbed) is shown in both cases in the above diagram.


Q.8) Discuss the transition state theory of reaction rate, in which two conditions must be fulfilled if a molecular collision results in reaction.

Answer:
Transition State Theory (TST):
According to this theory, when reactant molecules interact with each other, the existing bonds weaken and the bond length increases, while the new bonds start to form. Finally the reacting molecules form some kind of hypothetical structure (cluster of atoms) known as the transition state.
Explanation:
Consider the following general reaction to explain transition state theory.

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Reactant molecules AB interact and form a transition complex. The transition state does not represent a real molecule, it is impossible to isolate a transition state. However, it is assumed to possess properties common to real molecule, such as molecular weight, intermediate distances, a definite enthalpy, definite composition but loses structure and the ability to rotate and vibrate.
The transition state may either return to the initial reactant or it proceeds to form products. In going from the transition state to products, the A—A and B—-B bond distance decreases while the A—-B bond distance increases further.
The energy changes of this reaction are shown in the figure below.

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E(transition) is always greater than E(reactants). As the transition state splits into products molecules, the A-A atoms and the B-B atoms attract each other very strongly. Hence energy is evolved. E(product) is, therefore, always lower than E(transition). The transition state always lies at the highest point of the reaction energy diagram.
Conditions for Reaction during Collision:
When molecules collide, they should fulfil the following conditions so that the collision may result into reaction.

  • The reacting molecules must possess kinetic energy equal to or greater than the activation energy of the reaction.
  • The molecules must collide in proper orientation in order to make effective collision which results into reaction.
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