CLASS 12 chemistry ch 4

Class 12 chemistry chapter 4

Chemical Kinetics Class 12 Notes Chemistry Chapter 4

1. Chemical kinetics is the branch of chemistry which deals with the study of rates (or fastness) of chemical reactions, the factors affecting it and the mechanism by which the reactions proceed.

2. Rate of reaction is the change in concentration of reactants or products per unit time.

For a general reaction, A+B –> C

The rate of reaction

The negative sign indicates that the concentration is decreasing with time.

Unit for reaction rate is mol L-1s-1.

3. The rate of reaction is not a constant quantity (except for zero order reactions). It decreases as the reaction proceeds in the forward direction.

4. A rate law expresses a mathematical relationship between the reaction rate and the molar concentration of one or more reactants.

Where m and n are determined experimentally and represent the order of reaction with respect to A and B respectively, m + n represents the overall order of reaction.

5. Rate constant is the rate of reaction when the concentration of each of reacting species is unity. It is represented by ‘k’ It is also called specific reaction rate or velocity constant of reaction.

6. Order of reaction is defined as the sum of the exponents to which the concentration terms are raised in the rate equation (or rate law) of the reaction. It can be fraction, zero or any whole number.

7. Modularity of reaction is defined as the number of reacting particles (atoms or molecules or any other species), which collides simultaneously to bring about the chemical change.

It is a theoretical concept. Its value is always a whole number. It is never more than three. It cannot be zero.

8. First order reaction: A reaction is said to be first order if its reaction rate is determined by the variation of one concentration term only.

9. The integrated rate equation expresses the concentration of reactants as a function of time.

10. The integrated rate equation for a first order reaction is given as

11. Second order reaction: The reaction in which sum of powers of concentration terms in rate law equation is two.

12. Zero order reaction: Those reactions in which rate of reaction does not change with concentration of the reactants.

Rate law for such a reaction is expressed as. Rate = k [A]°[B]°

13. Half life period: It is the time required for the initial concentration of the reactant to be reduced to half its value.

12. Zero order reaction.  Reactions in which the reaction rate does not change with the concentration of the reactants.

 Evaluate the law for such a reaction.  Rate = k [A] ° [B] °:

 13. Half-life.  The time required for the initial concentration of the reagent is reduced by half.

 14. It has been found that for a chemical reaction with a temperature rise of 10 ° C, the speed stability almost doubles.

 15. Reaction temperature coefficient Ratio of the stability of the reaction rate at two temperatures which differ from each other by 10: C. Usually, the two temperatures taken are 35 ° C and 25 ° C.

 16. The evolution of temperature stability as a function of temperature can be represented by the equation "Arrenius".

 K = Ae-Ea / Rt:

  where A is stable, called the frequency factor, and Ea is called the activation energy.

 The stability of the stability at two different temperatures from the above equation is related as follows:

 17. There are two important theories about reaction rates.

 (i) collision theory և

 (ii) Theory of the state of transition.

Chemical Kinetics

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• Chemical kinetics: It is the branch of chemistry that deals with the study of reaction rates and their mechanisms.
• Rate of reaction: It is the change in concentration of reactant (or product) in unit time.
• The unit of rate of reaction is mol L-1s-1.
• A + B → C + D
  Rate of disappearance of A=−d[A]dt$A=\frac{-d[A]}{dt}$
  where d[A] is small change in conc. of ‘A’ and dt is small interval of time
  Rate of disappearance of B=−d[B]dt$B=\frac{-d[B]}{dt}$
  Where d[B] is small change in conc. of ‘B’ and dt is small interval of time
  Rate of appearance of C=+d[C]dt$C=\frac{+d[C]}{dt}$
  Where d[C] is small change in conc. of ‘C’ and dt is small interval of time
  Rate of appearance of D=+d[D]dt$D=\frac{+d[D]}{dt}$
  Where d[D] is small change in conc. of ‘D’ and dt is small interval of time
  Rate =−d[A]dt=−d[B]dt=+d[C]dt=+d[D]dt$=\frac{-d[A]}{dt}=\frac{-d[B]}{dt}=\frac{+d[C]}{dt}=\frac{+d[D]}{dt}$
• Rate law or rate equation: It is the expression which relates the rate of reaction with concentration of the reactants. The constant of proportionality ‘k’ is known as rate constant.
• Average rate: It is the rate of reaction measured over a long time interval.
• Average rate =ΔxΔt$=\frac{\mathrm{\Delta }x}{\mathrm{\Delta }t}$
  where is Δx change in concentration and Δt is large interval of time.
• Instantaneous rate: It is the rate of reaction when the average rate is taken over a particular moment of time. Instantaneous rate =dxdt$=\frac{dx}{dt}$.
  where dx is small change in conc. and dt is the smallest interval of time.
  It is the expression which relates the rate of reaction with concentration of the reactants.
• Rate constant: When the concentration of reactants is unity, then the rate of reaction is known as rate constant. It is also called specific reaction rate.
  The constant of proportionality ‘k’ is known as rate constant.
• Molecularity of a reaction: The total number of atoms, ions or molecules of the reactants involved in the reaction is termed as its molecularity. It is always in whole number and is never more than three. It cannot be zero.
• Order of a reaction: The sum of the exponents (power) of the concentration of reactants in the rate law is termed as order of the reaction. It can be in fraction. It can be zero also.
  If rate law expression for a reaction is Rate = k [A]x [B]y
  Then its order of reaction = x + y
• Order cannot be determined with a given balanced chemical equation. It can be experimentally determined.
• Integrated rate law for zero order reaction: R → P
  dxdt=k[R]0$\frac{dx}{dt}=k[R{]}^0$
  k=[R0]−[R]t$k=\frac{[R_0]-[R]}{t}$
  If we plot a graph between concentration of R vs time t, the graph is a straight line with slope equal to -k and intercept is equal to [Ro].
• Half- life of a reaction: The time taken for a reaction, when half of the starting material has reacted is called half- life of a reaction.
• For zero order reaction, the half-life time is t1/2=[R0]2k$t_{1/2}=\frac{[R_0]}{2k}$
• For first order reaction, the half-life time is t1/2=0.693k$t_{1/2}=\frac{0.693}{k}$, where ‘k’ is rate constant.
  It is independent of initial concentration for first order reaction.
• Rate law for first order reaction: R→$\to$P
  k=2.303tlog[R0][R]$k=\frac{2.303}{t}\log \frac{[R_0]}{[R]}$
  where ‘k’ is rate constant or specific reaction rate, [Ro] is initial molar conc., [R] is final molar conc. after time ‘t’.
  k=2.303tlogaa−x$k=\frac{2.303}{t}\log \frac{a}{a-x}$
  where ‘a’ is initial conc. reacted in time ‘t’ final conc., after time ‘t’ is (a – x).
• If we plot a graph between ln[R] with time, we get a straight line whose slope = - k and intercept ln[Ro].
• To calculate rate constant for first order gas phase reaction of the type
  A(g)→$\to$B(g) + C(g)
  k=2.303tlogp1(2pi−pt)$k=\frac{2.303}{t}\log \frac{p_1}{(2p_i-p_t)}$
  Where pi is initial pressure of A, pt is total pressure of gaseous mixture containing A , B, C
  p$p$t = p$p$A + p$p$B + p$p$C
• Pseudo first order reaction: The reaction which is bimolecular but order is one is called pseudo first order reaction. This happens when one of the reactants is in large excess.Example - Acidic hydrolysis of ester (ethyl acetate).
  CH3COOC2H5+H2O$C{H}_{3}COO{C}_2{H}_5+H_{2}O$−→−H+CH3COOH+C2H5OH$\stackrel{H^{+}}{\to }C{H}_{3}COOH+C_2{H}_{5}OH$
• Activation energy (Ea): It is extra energy which must be possessed by reactant molecules so that collision between reactant molecules is effective and leads to the formation of product molecules.
• Arrhenius equation of reaction rate: It gives the relation between rate of reaction and temperature.
  k=Ae−Ea/RT$k=A{e}^{-E_a/RT}$
  lnk=lnA−Ea/RT$\ln k=\ln A-E_a/RT$
  logk=logA−Ea2.303RT$\log k=\log A-\frac{E_a}{2.303RT}$
  where k = rate constant, A = frequency factor, Ea = energy of activation R = gas constant, T = temperature in Kelvin,
  logk2k1=Ea2.303R(T2−T1T1T2)$\log \frac{k_2}{k_1}=\frac{E_a}{2.303R}\left(\frac{T_2-T_1}{T_1{T}_2}\right)$
• Probability factor or Steric factor
  Rate=PZAB.e−EaRT$Rate=P{Z}_{AB}.e^{\frac{-E_a}{RT}}$
  Where ZAB represents the collision frequency of reactants, A and B, e−EaRT$e^{\frac{-E_a}{RT}}$represents the fraction of molecules with energies equal to or greater than Ea and P is called the probability or steric factor.
• Mechanism of reaction: It is the sequence of elementary processes leading to the overall stoichiometry of a chemical reaction.
• Activated complex: It is an unstable intermediate formed between reacting molecules. Since, it is highly unstable and it readily changes into product.
• Rate determining step: It is the slowest step in the reaction mechanism.
• The number of collisions per second per unit volume of the reaction mixture is known as collision frequency (Z).
• FAQ (Frequently Asked Questions)

  Q1: What are the concepts students are helped with revision in class 12 chemistry notes chapter 4 chemical kinetics?

  A1: The various concepts, topics, and subtopics that students can revise from the  class 12 chemistry notes chapter 4 chemical kinetics are as mentioned below:

  4.1 The rate of a Chemical Reaction

• Q2: What do you mean by chemical kinetics?

  A2: Chemical kinetics also known as reaction kinetics refers to the study of various types of reactions and the different factors influencing the nature and the rate of these chemical reactions. It also sids to collect and investigate information regarding the different mechanisms and properties of the reaction along with defining the characteristics of the reaction, its reactants and products.

  Q3: What are the factors affecting the reaction rate?

  A3: The factors affecting the rate of reaction are as follows: 

  • Concentration of Reactants

  Collision theory states that the particles of the reactants colloid with each other to form the respective products. Therefore higher the concentration of the reactants, greater the speed of the reaction. 

  • Temperature

  A rise in temperature increases the energy of the molecules resulting in an increasing number of collisions between the reactant molecules per unit time. Therefore this results in an increasing rate of reaction. However, there are two types of reaction that occur based on the temperature - endothermic and exothermic. Taking into consideration the kind of reaction that might occur, the temperature must be varied according to the requirement. Also remember, in a system where more than one reaction is possible, the same reactants can produce different products under different temperature conditions.

  • Phase And Surface Area of Reactants

  A chemical reaction often involves more than two reactants. When two or more reactants are in the fluid state, then their particles are more fragile rather than when they are in solid-state. This results in a higher rate of collision amongst them. In a heterogeneous medium, the collision between the particles occurs at an interface between phases. Compared to the homogeneous case, the number of collisions between reactants per unit time is significantly reduced, and so is the reaction rate. When two or more reactants are in the same phase of fluid, their particles collide more often than when either or both are in solids phase or when they are in a heterogeneous mixture.

  • Effect of Solvent

  The nature of the solvent also depends on the reaction rate of the solute particles. Some reactants tend to perform better in the solvent state rather than in solid-state. 

  • Catalyst

  Catalysts are popular chemicals that are known to alter the rate of the reaction by changing the reaction mechanism. There are two types of catalysts namely, promoters and poisons. Promoters are known to increase the rate of reaction while demoters tend to decrease the rate of reaction.