PHYSICS-15-10- 11th (PQRS) SOLUTION

https://docs.google.com/document/d/10647P__qsX-i2qunusHfkbiEgfFztsBc/edit?usp=sharing&ouid=109474854956598892099&rtpof=true&sd=true concentration, temperature, pressure and catalyst; elementary and complex reactions, order and molecularity of reactions, rate law, rate constant and its units, differential and integral forms of zero and first order re actions, their characteristics and half - l ives, effect of temperature on rate of reactions - Arrhenius theory, activation energy and its calculation, collision theory of bimolecular gaseous reactions (no derivation). RATE OF REACTION Rate of reaction is the rate of change of concentration of either reactant or product with time. Let a reaction be represented as Reactant (A) ⎯⎯→ Product (P) If dx mole of reactant is transferred towards the product side in small time dt then the rate of reaction is dx/dt. For reactant rate of reaction = − d[A] dt (– means decrease in concentration of the reactant) For product rate of reaction = + d[P] dt (+ means increase in concentration of the product) FACTORS AFFECTING RATE OF REACTION Nature of the reactant : Different amount of energies are required for breaking of different bonds and different amount of energies are released in the formation of different bonds. Concentration of the reactant : Greater the concentration of reactants, faster is the reaction. Surface area of the reactant : Rate of reaction increases with increase in surface area. Presence of light : Some reactions donot take place in dark but takes place in presence of light. Such reactions are called photochemical reactions. Temperature : The rate of reaction increases with an increase in temperature. For every 10°C rise in temperature the rate of reaction becomes twice or thrice for a homogeneous reaction. The temperature coefficient is defined as the ratio of the specific reaction rates of a reaction at two temperatures differing by 10°C. CHAPTER INCLUDES Rate of reaction Factors affecting rate of reaction Concept of activation energy Order of reaction Zero order and its characteristics First order and its characteristics Rate c onsta nt for differe nt order of reaction Photochemical reactions Temp. coefficient = K t +10 Kt = 2 or 3. The rate constant (K) and temperature are related by Arrhenius equation given by Arrhenius equation, k = Ea Ae RT i.e., log k = log A – Ea/RT. logk By plotting graph log k vs 1 activation energy can be determined. T 1/T Presence of catalyst : The positive catalyst lowers down the activation energy. The greater the decrease in activation energy higher will be the reaction rate. CONCEPT OF ACTIVATION ENERGY The minimum amount of energy required by reactant molecules to participate in a reaction is called activation energy Activation energy = Threshold energy – average kinetic energy of reacting molecules Threshold energy = Initial potential energy of reactant molecules + activation energy. Activation Energy Energy Progress of reaction Progress of reaction ETH = Threshold Energy, HR = Enthalpy or Energy or Potential of reactants. HP = Enthalpy or Energy or Potential of product, (Ea)f = Activation energy for forward reaction. (Ea)b = Activation energy for backward reaction. ORDER OF REACTION Order of reaction is the number of variable concentration terms which determine the rate of reaction. Order is experimental quantity which may be a whole number, zero, fractional, positive or negative. For a complex reaction the order is given by the slowest step (rate determining step) present in the sequence of the reaction steps. In a number of reactions the order is different from molecularity. This is due to the fact that one of the reactants is present in large excess. The sum of the powers of concentration terms which determine rate of a reaction is also called order of reaction. Zero order reaction : A reaction is said to be of zero order if the rate of reaction is independent of concentration of the reactant. H2(g) + Cl2 (g) ⎯⎯h⎯v → 2HCl(g) 2NH3 ⎯⎯M⎯o →N2 + 3H2 HI ⎯⎯Δ⎯⎯→ 1 H + 1 I Au surface 2 2 2 2 N O ⎯⎯Δ⎯⎯→N 1 O 2 Au surface 2 2 2 Rate Expression : Let A ⎯⎯→ Product ⇒ dx ∝[A] 0 dt ⇒ dx = Kdt On integration ⇒ dx = K[A]0 = K dt [K is a constant called Rate Constant] x = Kt + C [where C is the integration constant] when t = 0, x = 0 ∴ C = 0 ∴ x = Kt ∴ Characteristics : Unit of K = mol L–1 time–1 Half life period : Time required for the completion of half of the reaction. Half life for zero order reaction is directly proportional to initial concentration of the reactant The concentration of reactant decreases linearly with time. First Order Reaction : A first order reaction is one whose rate is determined by the variation of one concentration term only. All radioactive disintegration reactions are of the first order. H2O2 ⎯⎯Δ →H2O + ½O2 CH3COOH + CH3OH ⎯→ CH3COOCH3 + H2O C H O H O H C H O C H O 12 22 11 2 ⎯ ⎯→ 6 12 6 6 12 6 NH4NO2 ⎯⎯Δ →N2 + 2H2O Rate Expression Let A ⎯⎯→ Product initially a 0 after time t a–x x ⇒ dx ∝(a − x) dt ⇒ dx = K(a − x) dt dx = Kdt a − x On integration, – ln (a–x) = Kt + C [where C is the integration constant] When t = 0, x = 0 ∴ C = – ln a ⇒ – ln (a – x) + ln a = Kt ⇒ ln a a − x = Kt ⇒ K = l ln t ⇒ a a − x Characteristics Unit of K = time–1 Half life period : The half life period is independent of initial concentration of the reactant. On plotting a graph between log of concentration and time we get log C The slope of this line gives the value of −K 2.303 from which K can be calculated. General expression of unit of rate constant K = mole1–n litren–1 time–1. Where n is the order of reaction RATE CONSTANTS FOR DIFFERENT ORDER OF REACTION Reaction Order Rate law eqn. Rate constant A → product 0 Rate = k k = 1([A t 0 ] − [A]) A → product 1 Rate = k[A] k = 2.303 log [A0 ] t [A] 2A → product 2 Rate = k[A]2 1 ⎡ 1 1 ⎤ k = ⎢ − ⎥ t ⎣[A] [A 0 ] ⎦ PHOTOCHEMICAL REACTIONS A reaction which takes place in the presence of light (hν) is known as a photochemical reaction Ex. Ex. H2 (g) + Cl2 (g) ⎯⎯h⎯ν → 2HCl(g) O3 ⎯⎯h⎯ν →O2 + O A photochemical reaction proceeds via free radical (homolytic fission of bond) mechanism consisting three steps Initiation Propagation Termination Ex. H2 + Cl2 ⎯⎯h⎯ν → 2HCl Initiation Cl2 ⎯⎯h⎯ν → 2Cl∙ } primaryprocess Propagation ⎫ ⎪ Cl∙ + H2 → HCl + H∙ ⎪ ⎬ secondary process H∙ + Cl → HCl + Cl∙ ⎪ ⎪ Termination ⎪ Cl∙ Cl∙ → Cl2 ⎪ ⎭ ❑ ❑ ❑