Chemistry (Chapter - 3 & 4) - Chemical Kinetics, D & F Block Elements

CHEMISTRY (CH - 3 & 4) - CHEMICAL KINETICS, D & F BLOCK ELEMENTS 1. (a) On what ground can you say that scandium (Z = 21) is a transition element but zinc (Z = 30) is not? (b) Silver atom has completely filled d orbitals (4d10) in its ground state. How can you say that it is a transition element? 2. The decomposition of N2O5 in CCl4 at 318K has been studied by monitoring the concentration of N2O5 in the solution. Initially the concentration of N2O5 is 2.33 mol.L–1 and after 184 minutes, it is reduced to 2.08 mol L–1. The reaction takes place according to the equation 2N2O5 (g) → 4 NO2 (g) + O2 (g) Calculate the average rate of this reaction in terms of hours, minutes and seconds. What is the rate of production of NO2 during this period? 3. For the reaction R → P, the concentration of a reactant changes from 0.03M to 0.02M in 25 minutes. Calculate the average rate of reaction using units of time both in minutes and seconds. 4. (a) Why do the transition elements exhibit higher enthalpies of atomisation? (b) In the series Sc (Z = 21) to Zn (Z = 30), the enthalpy of atomisation of zinc is the lowest, i.e., 126 kJ mol–1. Why? (c) Name a transition element which does not exhibit variable oxidation states. 5. From the rate expression for the following reactions, determine their order of reaction and the dimensions of the rate constants. (i) 3NO(g) → N2O (g) Rate = k[NO]2 (ii) H2O2 (aq) + 3I– (aq) + 2H+ → 2H2O (l) + 3 I − Rate = k[H2O2 ][I- ] (iii) CH3CHO (g) → CH4 (g) + CO(g) Rate = k [CH3CHO]3/2 (iv) C2H5Cl (g) → C2H4 (g) + HCl (g) Rate = k [C2H5Cl] 6. In a reaction, 2A → Products, the concentration of A decreases from 0.5 mol L–1 to 0.4 mol L–1 in 10 minutes. Calculate the rate during this interval? 7. Calculate the overall order of a reaction which has the rate expression (a) Rate = k [A]1/2 [B]3/2 (b) Rate = k [A]3/2 [B]–1 8. (a) Why is Cr2+ reducing and Mn3+ oxidising when both have d4 configuration? (b) How would you account for the increasing oxidising power in the series VO2+ < Cr2O72– < MnO4– ? (c) How would you account for the irregular variation of ionisation enthalpies (first and second) in the first series of the transition elements? 9. Identify the reaction order from each of the following rate constants. (i) k = 2.3 × 10–5 L mol–1 s–1 (ii) k = 3 × 10–4 s–1 10. For the reaction: 2A + B → A2B the rate = k[A][B]2 with k = 2.0 × 10–6 mol–2 L2 s–1. Calculate the initial rate of the reaction when [A] = 0.1mol L–1, [B] = 0.2 mol L–1. Calculate the rate of reaction after [A] is reduced to 0.06 mol L–1 . 11. Describe the oxidising action of potassium dichromate and write the ionic equations for its reaction with: (i) iodide (ii) iron(II) solution (iii) H2S 12. For a reaction, A + B → Product; the rate law is given by, r = k [A]1/2 [B]2 . What is the order of the reaction? 13. (a) Why is the E- value for the Mn3+ /Mn2+ couple much more positive than that for Cr3+ /Cr2+ or Fe3+/Fe2+? Explain (b) Why is the highest oxidation state of a metal exhibited in its oxide or fluoride only? (c) Which is a stronger reducing agent Cr2+ or Fe2+ and why ? 14. The conversion of molecules X to Y follows second order kinetics. If concentration of X is increased to three times how will it affect the rate of formation of Y? 15. Describe the preparation of potassium permanganate. How does the acidified permanganate solution react with (i) iron(II) ions (ii) SO2 and (iii) oxalic acid? Write the ionic equations for the reactions. 16. The decomposition of NH3 on platinum surface is zero order reaction. What are the rates of production of N2 and H2 if k = 2.5 × 10–4 mol–1 L s –1? 17. (a) Calculate the magnetic moment of a divalent ion in aqueous solution if its atomic number is 25. (b) Actinoid contraction is greater from element to element than lanthanoid contraction. Why? 18. (a) Calculate the ‘spin only’ magnetic moment of M2+ (aq) ion (Z = 27) (b) Explain why Cu+ ion is not stable in aqueous solutions? 19. The rate constant for the decomposition of hydrocarbons is 2.418 × 10–5s –1 at 546 K. If the energy of activation is 179.9 kJ/mol, what will be the value of pre-exponential factor. 20. The following data were obtained during the first order thermal decomposition of SO2Cl2 at a constant volume. SO2Cl2 (g) → SO2 (g) + Cl2 (g) Calculate the rate of the reaction when total pressure is 0.65 atm. 21. The initial concentration of N2O5 in the following first order reaction N2O5 (g) → 2 NO2 (g) + 1/2O2 (g) was 1.24 × 10–2 mol L–1 at 318 K. The concentration of N2O5 after 60 minutes was 0.20 × 10–2 mol L–1. Calculate the rate constant of the reaction at 318 K. 22. The decomposition of dimethyl ether leads to the formation of CH4 , H2 and CO and the reaction rate is given by Rate = k [CH3OCH3 ]3/2. The rate of reaction is followed by increase in pressure in a closed vessel, so the rate can also be expressed in terms of the partial pressure of dimethyl ether, i.e., Rate = k(pCH3OCH3)3/2 If the pressure is measured in bar and time in minutes, then what are the units of rate and rate constants? 23. For M2+/M and M3+/M2+ systems the E- values for some metals are as follows: Use this data to comment upon: (i) the stability of Fe3+ in acid solution as compared to that of Cr3+ or Mn3+ (ii) the ease with which iron can be oxidised as compared to a similar process for either chromium or manganese metal. 24. In a reaction between A and B, the initial rate of reaction (r0 ) was measured for different initial concentrations of A and B as given below: What is the order of the reaction with respect to A and B? 25. Write down the electronic configuration of: (i) Cr3+ (ii) Cu+ (iii) Co2+ (iv) Mn2+ (v) Pm3+ (vi) Ce4+ (vii) Lu2+ (viii) Th4+ 26. Consider a certain reaction A → Products with k = 2.0 × 10 –2s –1. Calculate the concentration of A remaining after 100s if the initial concentration of A is 1.0mol.L–1. 27. Sucrose decomposes in acid solution into glucose and fructose according to the first order rate law, with t1/2 = 3.00 hours. What fraction of sample of sucrose remains after 8 hours? 28. (a) Predict which of the following will be coloured in aqueous solution? Ti3+, V3+ , Cu+, Sc3+, Mn2+, Fe3+ and Co2+. Give reasons for each. (b) Compare the stability of +2 oxidation state for the elements of the first transition series. 29. A first order reaction is found to have a rate constant, k = 5.5 × 10-14 s-1 . Find the halflife of the reaction. 30. The following data were obtained during the first order thermal decomposition of N2O5 (g) at constant volume. Calculate the rate constant. 31. A first order reaction has a rate constant 1.15 × 10-3 s-1. How long will 5 g of this reactant take to reduce to 3 g? 32. (a) Why are Mn2+ compounds more stable than Fe2+ towards oxidation to their +3 state? (b) Explain briefly how +2 state becomes more and more stable in the first half of the first row transition elements with increasing atomic number? 33. The decomposition of hydrocarbon follows the equation k = (4.5 × 1011s –1) e-28000K/T Calculate Ea . 34. The half-life for radioactive decay of 14C is 5730 years. An archaeological artifact containing wood had only 80% of the 14C found in a living tree. Estimate the age of the sample. 35. To what extent do the electronic configurations decide the stability of oxidation states in the first series of the transition elements? Illustrate your answer with examples. 36. The rate constant for the decomposition of N2O5 at various temperatures is given below: Draw a graph between ln k and 1/T and calculate the values of A and Ea . Predict the rate constant at 30° and 50°C. 37. A reaction is second order with respect to a reactant. How is the rate of reaction affected if the concentration of the reactant is (i) doubled (ii) reduced to half ? 38. What may be the stable oxidation state of the transition element with the following d electron configurations in the ground state of their atoms : 3d3 , 3d5 , 3d8 and 3d4 ? 39. Time required to decompose SO2Cl2 to half of its initial amount is 60 minutes. If the decomposition is a first order reaction, calculate the rate constant of the reaction. 40. The following results have been obtained during the kinetic studies of the reaction: 2A +B→ C+D Determine the rate law and the rate constant for the reaction.