InterviewSolution
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InterviewSolution
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(a)Explain briefly the collision theory of bimolecular reactions . [Or] [b]Discuss about the hydrolysis of salf of weak acid and week base and derived pH value for the solution. |
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Answer» Solution :(a) Collision theory on the kinetic theory is based on the kinetic theory of GASES. According to this theory.chemicalreaction occur as a RESULT of collisions between the reacting molecules Let us understand this If a consider the following reaction `A_(2(g))+B_(2(g))rarr2AB_((g))` If we consider that , the reaction between `A_2 and B_2`molecules proceeds through collision between them , then the would be proportional to the number of collisions per second . The number of collisions is directly proportional to the concentration of both `A_2 and B_2` . Collision rate `prop [A_2][B_2]`Collision rate `= Z[A_2][B_2]`where , Z is a constant . The Collision rate in gas can be CALCULATED from kinetic theory of gases. For a gas at room temperature (298K) and 1 atm pressure, each molecule undergoes approximately `10^9`collisions per second i.e I collision in `10 ^(-9)`second This if every collision resulted in reaction, the reaction would be complete in `10^(-9)`second In actual practice does not happen . It implies that all collisions are not effective to lead to the reaction. In order to react, the Colliding molecules must posses a minimum energy calledactivation energy . The molecules that collide with less energy that activation energy will remain intact and no reaction occurs. Fraction of effective collisions (f) is given by the expression , `f -=e^((-E_a)/(RT))` . Fraction of collisions is further reduced dut to orientation factor i.e., even if the reactant collide with sufficient energy , then will not unless the orientation of the reactant molecules is suitable for the formation of the transition state . The fraction of effective collisions (f) having proper orientation is given by the steric factor P. Rate `=pxxfxx` collision rate Rate `Pxxe^.(-E_a)/(RT)xxZ[A_2][B_2]""......(1)` As per the rate law , Rate `=k[A_2][B_2]""...(2)` Where k is the rate constant . On comparing equation (1) and (2) , the rate constant k is , `k=PZe^(((-E_a)/(RT))` [OR] (b) i. Consider the hydrolysis of ammonium acetate `CH_3COONH_(4(aq))rarrCH_3COO_((aq))^(-)+NH_(4(aq))^+` ii. In this case both the cation `(NH_4^+) and (CH_3COO^-)` anion have the tendency to react with water . `CH_3COO^(-)+H_2^OhArrCH_3COOH+OH^(-)` `NH_4^(+)H_2OhArrNH_4OH+H^+` iii. The nature of the solution depends on the strength of acid (or) base i.e. , if `K_a gt K_b` , then the solution is acidic and `pH lt 7` if `K_a lt K_b` then hte solution is basic and `pH gt 7`. IV. The relation between the DISSOCIATION constant `K_a , K_b` and hydrolysis constant is given by the following expression. `K_a. K_b.K_h=K_w` v. pH of the solution `pH = 7 + 1//2 pK_a - 1//2 pK_b` |
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