Notes

Chapter#09                       Chemical Kinetic
Topic	SLOs
Chemical kinetics	9.1.1	Chemical Kinetic:  The branch of physical chemistry which deals with the rate of reactions, their mechanism, conditions and the factors influencing the rate of reactions.
Rate and Order of reaction	9.2.1	Rate of Reaction:                                                                 The rate of reaction is defined as change in concentration of reactant or product per unit time. Rate = Change in Conc. of reactant of product Time taken Let us consider a simple reaction. A                 B                 The concentration of A decreases and that of “B” increases as time passes and hence the rate of reaction may be equal to the rate of disappearance of A which is equal to rate of appearance of B.                 The rate of a chemical reaction is not uniform and as the reaction going on, the rate of formation of product become slow therefore overall rate of reaction is the average of whole rate of reaction. Unit of Rate of Reaction: Rate   =   Dx   =   mole dm-3   =   mole dm-3 sec-1                                        Dt             Sec
	9.2.2	Rate Law: According to Law of “Mass Action”. “The rate of reaction is directly proportional to the active mass of the reactant.” Rate   µ   Active Mass. Rate    =   K (Active Mass) Active mass mean molar concentration having exponent as degree of activity or “order of reactant”. If order is represented by “n” than Rate Law is given by. Rate  =  K[R]n Consider a rate reaction. aA  +  bB                                  Product Rate   µ    [A]a [B]b Rate   =   K[A]a [B]b Where a & b are the order of reactant A & B respectively. Order of  reaction  =  n  =  a + b Rate Constant: “It is the rate of reaction when concentration of reactants are unit at a given temperature”. Rate   =   K [R]n K   =   Rate          [R]n If concentration of reactants is unit.  K  =  Rate “Unit of Rate Constant” K = Rate       [R]       (a)  If Zero order (n = 0) K = Rate         [R]o K = Rate Where rate ml dm-3, sec-1 K = mole dm-3 s-1       (b) If  1st order( n = 1) K = Rate        [R]                                                             K = moledm-3 sec-1       moledm-3 K = Sec-1       (c)  If 2nd order (n = 2) K = Rate        [R]2 K = mole dm-3 sec-1                      (mole dm-3) (mole dm-3) K = dm3 mole-1 sec-1       (d) If 3rd order (n = 3) K = Rate        [R]3 K = mole dm3 sec-1      (mole dm-3)3 K = mole dm3 sec-1                       (mole dm-3) mole2 dm-6 K = dm6 mole-2 sec-1
	9.2.3
	9.2.4
	9.2.5
Collision  Theory ,Transition state and Activation energy	9.3.1	Collision Theory:                         The frequent collision among the molecules of reactants may breaks the existing bond and initiate the chemical reaction, this is called as effective collision of molecules.             All the collision of molecules are not effective only a fraction of molecule which have energy greater than the average energy and effective symmetry of the molecules. Effective Collision: Ineffective Collision: X2   Y2                                            Y2     Energy Of Activation:             “The excess energy which is required for the reaction in addition to the average energy of the molecules is called Energy of Activation”. It is denoted by Ea and is measured in K.J/mole.             According to the “collision theory” chemical reaction can only be possible when the effective collision among the molecules takes place and it happens only when the molecules of reactants it happens only when the molecules of reactants acquire threshold energy.             “The minimum energy which required for the reaction is called Threshold Energy”. It is denoted by El.
	9.3.2
	9.3.3
	9.3.4	FACTORS INFLUENCING THE RATE OFCHEMICAL REACTIONS: Following factors effect the rate of reaction. 1.                  Concentration 2.                  Temperature 3.                  Catalyst 4.                  Pressure 5.                  Surface Area 6.                  Radiation 1.                 Concentration: According Guldberg Law “The rate of reaction is directly proportional to the “Active Mass” of the reactant” The greater the conc. greater the active mass and higher the rate of the reaction.             Consider the rate equation.                         A  +  B                        Product             Here. ·                  If conc. of A or B are double the rate of reaction will also be doubled. ·                  If conc. of both A and B are double the rate of will be four times increased. The increase in the concentration of reacting molecules cause more crowding and more collision of molecules which results in an increase in the rate of formation of products. 2.      Temperature: The increase in temperature increase the kinetic energy of the molecules which decrease the energy of the activation and increase the fraction of the molecules possessing threshold energy. In general the rate of a reaction doubles for every 10 K rise in temperature it is due to increase in the frequency of collisions. The graph of Kinetic Energy verses fractions of molecules at different temperature is given below.              The shaded area of the curve of T2 is double than T1 shows that a rise of temperate increase the rate of reaction. 3.      Catalyst: Catalyst is a substance which provide new path for the reaction by changing the threshold energy. The catalyst which decrease the threshold energy by increasing the effective collision per unit time are known as positive catalyst. The catalyst which increase the threshold energy and energy of activation by decreasing the effective and energy of activation by decreasing the effective collision per unit time are known as negative catalyst also known as “Inhabitor” 4.                  Pressure: The reactions in which the reactants are gases are influence by the pressure. The increases in the pressure decrease the volume of the reacting mixture. The molecules come close each other which increase in the collision of the reacting molecules and increase the rate of formation of product. 5.      Surface Area of Reactant: In a heterogeneous reaction in which one or more reacting species are in different phase such as solid, the surface area of solid reactant effect the rate of reaction. Greater the surface area the more is the frequency of collision of molecules, and the higher the rate of reaction. Consider the reaction. CaCO3(s) + 2HCl(l)                  CaCl2(g) + H2O(g) + CO2(g)                         If we take solid chunk of marble CaCO3, the reaction rate is slow. But finely divided marble reacts vigorously because it offers greater surface area for HCl to act upon. 6.      Radiation: Some chemical reactions proceed only under the influence of light. These are called photochemical reactions. The rate of these reaction does not depend upon rate of reaction therefore these reaction are also called “Zero order reaction” e.g.      CH4  +  Cl2         hv              CH3Cl  +  HCl The rate of these reactions are depend upon the intensity of light or no. of photon absorb.
Catalyst	9.4.1
	9.4.2
	9.4.3