question_answer

  Express clearly what you understand by 'rate expression' and 'rate constant' of a reaction?                         [2] (b) Nitrogen pent oxide decomposes according to the equation This first order reaction was allowed to proceed at 40°C and the data given below were collected.

	Time (min)
0.400 0.289 0.209 0.151 0.109	0.00 20.00 40.00 60.00 80.00

  (i) Calculate the rate constant for the reaction. Include units with your answer. (ii) Calculate the initial rate of reaction. (iii) After how many minutes will  be equal to 0.350 M?                                                                                  [3] Or (a) Define (i)Order of a reaction                                     (ii) Elementary step in a reaction.                                                          [2] (b) A first order reaction has a rate constant value of 0.005 . If we begin with 0.10 M concentration of the reactant, how much of the reactant will remain after 3.0 hours?                                                                   [3]  

Answer:

  Rate expression is the rate law giving rate in terms of molar concentration of the reactants. Rate of disappearance of Rate of disappearance of Complete rate is given by rate law                               ?(i) [1] where [A] and [B] are the molar concentration of the reactants A and B respectively. a: order wrt A b : order wrt B k : is called rate constant If [A] = [B] =1  at a given time then  by Eq. (i) Thus, rate constant is equal to the rate expression when molar concentration of each reactant is.  (b) (i) For first-order reaction  

Set	(a)	(a-x)	Time	K
(i)	0.400 M	0.289	20 min	0.01625
(ii)	0.400 M	0.209	40 min	0.01623
(iii)	0.400 M	0.151	60 min	0.01624
(iv)	0.400	0.109	80 min	0.01625

Average value of (ii)Initial rate                                           [1] (iii) Let time (concentration of  be 0.350                                                                 [1] Or (a) (i) Order of reaction The number of reactants whose concentration change and which are involved in rate-determining step (slow step) is called order of reaction. product                                 a = order wrt A b = order wrt B We assume that concentration of A and B changes and  both  the  reactants  are  involved  in rate-determining step. Case I Concentration of A does not change and slow step  product thenorder = 2 wrt B Case II Cone of B does not change and slow step  product such that Then order = 1 wrt A                                  [1] Order can be whole number fractional and even negative. (ii) Elementary step in a reaction An elementary reaction is a reaction that occurs in a single step, with no experimentally detectable reaction intermediates. If no such intermediate can be detected or need to be postulated to interpret the behaviour, a reaction is assumed tentatively to be elementary. Initially it was assumed to be elementary. But this takes place in part by a mechanism involving following elementary steps.                         [1]  (b) For first order reaction                                  Initial concentration =0.10 M time =3.0 h =180 min Concentration after 180 min =(a-x)            Antilog of 0.3908                       [1]