CHEMICAL KINETICSCLASS- XII

VINAY KUMARPGT CHEMISTRY

KV NTPC KAHALGAONPATNA REGION

Or Rate of reaction = -R] = +P] t t

• INSTANTANIOUS RATE OF REACTION:- it is the rate of the reaction at the particular moment of time and measured as a very small concentration change over a very small time interval. t 0 for a reaction R P

Instantaneous rate = -dR] = +dP] dt dt

• r inst. = -dR] = - slope of R

dt• r inst. = +dP] = + slope of P

dt• FACTORS AFFECTING THE RATE OF A CHEMICAL REACTION-(I) Nature of reactant(II) Concentration of reactant(III)Temperature(IV)Surface area of reactant(V) Radiation

• GENERAL EXPRESSION FOR RATE OF REACTION:-

For a general chemical reaction

aA + bB cC + dD

Rav. = -1 A] = -1 B] = 1 C] = 1 D]

a t b t c t d t

Rinst. = -1 dA] = -1 dB] = 1 dC] = 1 dD]

a dt b dt c dt d dt

• RATE LAW:- It is experimentally determined expression which relates the rate of reaction with the concentration of reactants.

For a hypothetical reaction A + B Products

Rate A]m B]n Rate = k A]m B]n Where k is the rate constant .If A] = B] = 1 Mol L-1 than Rate = k

Thus rate constant is the rate of reaction when concentration of each reactant in the reaction is unity.

• ORDER OF REACTION:- It may be defined as the sum of the power of the concentration of reactants in the rate law expression. Order of chemical reaction can be 1,2 or 3 and even may be fractional.

• MOLECULARITY OF REACTION:- The total number of reacting species( molecules, atoms or ions) taking part in an elementary chemical reaction. The molecularity of a reaction may not be fractional.

• INTEGRATED RATE LAW FOR ZERO ORDER CHEMICAL REACTION:-

Consider a general zero order reaction R P

1

1

2

• Comparing eq-2 with strait line equation y = m x + c , if we plot [R] against t we get a strait line with slope= -k and intercept equal to [R]0.

• Further simplify equation 2 we can get the rate constant k

2

• Half life for zero order reaction-

• INTEGRATED RATE LAW FOR FIRST ORDER CHEMICAL REACTION:-

1

4

5

5 4

12

3

6

7

2 t

3

• Half life for the first order of reaction:-

• PSEUDO FIRST ORDER REACTION:-

• DETERMINATION OF ORDER OF REACTION:-1. Graphical Method:- This method is applicable to those reactions wherein only one reactant is involved.

2. Initial rate Method:-

This method is used for those reactions where more than one reactant is involved.In this method we carried out some series of experiments.

• We change the one reactant’s concentration and determine the rate of reactions by keeping the constant concentration of each other reactants and compare the rate from initial concentration rate.

• Similarly, we repeat the experiments for all other reactants and compare the rate from initial concentration rate and finally determine the overall rate of reaction.

3. Integrated rate law Method:-• In this method we put the data of the reaction

under investigation in all the integrated rate equation one by one .

• The expression which gives a constant value of rate constant decide the order of reaction.

• Temperature dependence of a rate of a reaction:-

Most of the chemical reactions are accelerated by increase in temperature. For example, in decomposition of N2O5, the time taken for half of the original amount of material to decompose is 12 min at 50oC, 5 h at25oC and 10 days at 0oC. We also know that in a mixture of potassiumpermanganate (KMnO4) and oxalic acid (H2C2O4), potassium permanganate gets decolourised faster at a higher temperature than that at a lower temperature.

• It has been found that for a chemical reaction with rise in temperature by 10°, the rate constant is nearly doubled.

• The temperature dependence of the rate of a chemical reaction can be accurately explained by Arrhenius equation.

• It was first proposed by Dutch chemist, J.H. van’t Hoff but Swedish chemist, Arrhenius provided its physical justification and interpretation.

k = A e -Ea /RT 1

• where A is the Arrhenius factor or the frequency factor. It is also called pre-exponential factor. It is a constant specific to a particular reaction. R is gas constant and Ea is activation energy measured in joules/mole (J mol –1).

• It can be understood clearly using the following simple reaction

2H2(g) + I2(g)→ 2HI(g)

• According to Arrhenius, this reaction can take place only when a molecule of hydrogen and a molecule of iodine collide to form an unstable intermediate.

• It exists for a very short time and then breaks up to form two molecules of hydrogen iodide. According to Arrhenius, this reaction can take place only when a molecule of hydrogen and a molecule of iodine collide to form an unstable intermediate.

• It exists for a very short time and then breaks up to form two molecules of hydrogen iodide.

• The energy required to form this intermediate, called activated complex (C), is known as activation energy (Ea). Reaction coordinate represents the profile of energy change when reactants change into products. Some energy is released when the complex decomposes to form products. So, the final enthalpy of the reaction depends upon the nature of reactants and products.

• Ludwig Boltzmann and James Clark Maxwell used statistics to predict the behaviour of large number of molecules. According to them, the distribution of kinetic energy may be described by plotting the fraction of molecules (NE/NT) with a given kinetic energy (E) vs kinetic energy. Here, NE is the number of molecules with energy E and NT is total number of molecules.

• The peak of the curve corresponds to the most probable kinetic energy, i.e., kinetic energy of maximum fraction of molecules.

• There aredecreasing number of molecules with energies higher or lower than this value. When the temperature is raised, the maximum of the curve moves to the higher energy value and the curve broadens out, i.e., spreads to the right such that there is a greater proportion of molecules with much higher energies.

• The plot of ln k vs 1/T gives a straight line. Thus, it has been found from Arrhenius equation that increasing the temperature or decreasing the activation energy will result in an increase in the rate of the reaction and an exponentialincrease in the rate constant.

• slope = – Ea/ R and intercept = ln A. So we can calculate Ea and A using these values. At temperature T1, equation (1) is

ln k1 = – Ea/RT1 + ln A (2)

• At temperature T2 eq.(1) is(3)

• A is the constant for this particular reaction.• K1 and k2 are the rate constant for the

temperatures T1 and T2 respectively.• Substracting eq(2) from eq(3)

(4)

• Effect of Catalyst on the rate of a chemical reaction:-

• A catalyst is a substance which alters the rate of a reaction without itself undergoing any chemical change at the end of the chemical reaction. For example MnO2 increases the rate of decomposition of potassium chlorate to a great extent.

• According to intermediate complex theory a catalyst participate in a chemical reaction by forming temporary bonds with the reactants resulting in a intermediate complex.

• This has a transitary existence and decompose to yield products and the catalyst.

• Collision Theory of a Chemical Reaction:-• According to this theory the molecules of

reactants are having sufficient kinetic energy so they may collide with each other and make product molecules.

• The number of collisions per second per unit volume of the reaction mixture is known as collision frequency (Z).

A + B →Products• rate of reaction can be expressed as• Rate = P ZAB e –Ea/RT

• Where ZAB = the collision frequency of reactants A & B.

• P= Probability or steric factor.• e –Ea/RT = fractions of molecules with energies

equal to greater than Ea.