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Kinetics IV
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Order and molecularity of a reaction

Shaillee Kaushal is teaching live on Unacademy Plus

Shaillee Kaushal
Faculty in Chemistry with a teaching experience of 14 years. Specialised in teaching for boards and competitions.

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  2. ..Reaction path without catalyst Presence of a positive catalyst The function of positive catalyst is to lower down the activation energy. Reaction path with catalyst activation energy Catalyst present - -- - - - Energy of reactior The effect of a catalyst is to lower the energy of activation Catalyst absent Collision number Collision number

  3. Molecularity of a reaction The molecularity of a reaction is the number of reactant molecules taking part in a single step of the reaction PC15 PC13 + Cl2 (Unimolecular) 2HIH2 12 (Biomolecular) 2s02 022S03(Trimolecular) The minimum number of reacting particles (molecules, atoms or ions) that come together or collide in a rate determining step to form product or products is called molecularity of a reaction. Molecularity is a theoretical concept Molecularity cannot be zero, -ve, fractional, infinite and imaginary Molecularity can never be greater than three. There are some chemical reactions whose molecularity appears to be more than three 4HBr + O2 2H2O + 2Br2(molecularity = 5) 2Mnoit 16H+ + 5C2O2- 2Mn2 + + 10CO2 + 8H20(Molecularity-23) Such reactions involve two or more steps each step has its own molecularity not greater than three.

  4. Order of reaction: Lets consider the following reaction: aA bB ccProducts According to law of Mass action: Rate-k[A]"[B]"[C]c According to Rate law: Rate k[A]P[B] [C] order- p+q+r order is experimentally determined quantity. It may be equal to zero, positive, negative fractional and greater than three. However infinite and imaginary values are not possible

  5. Examples: (i) 2H202 2H2O + 02 Step 1 H2O2 H2O + [0] slow step Step 2 [0]02 fast step Step 1 Is the rate determining step. Rate = k[H2O2] st order reaction (i) 2NO2 +F2 2NO2F Step 1 N02 F22NO2F (slow step) Step2. N02+ F N02F-(fast step) Rate RINO2J[F2] Order 2 (iii) CH3CHO CH4 + CO Rate R[CH3CHo115 Order 1.5

  6. Zero order ist order | 2nd order t | 3rd order AI (b) Plots from integrated rate equations: 1 Zero ord st order2nd order 3rd order (Conc.)2 onc.

  7. Pseudo-order reaction: Reactions whose actual order is different from that expected using rate law expression are called pseudo-order reactions For example:- RCI + H2O-> ROH + HCl Rate - k[RCI] Order of the above reaction is '1' although the expected order is '2' this is because water is taken in excess therfore its concentration is taken as constant. The reaction is therfore pseudo first order

  8. Molecularity Order of reaction is sum of power of molar concentration of the reacting species in the rate equation of the reaction. 2.It may be a whole number, zero, fractional, positive or negative. is experimentally determined 1. It is the number of reacting species which bring the chemical change 2. It is always a whole number 3. It is a theoratical concept