MI Prelim 2008 H2 Chemistry Paper 2

Class Adm No Candidate Name: This question paper consists of 13 printed pages. [Turn over Preliminary Examinations 2008 Pre-university 3 H2 CHEMISTRY 9746 / 02 Tuesday 9 Sep 2008 1h 30 min Additional materials: Data Booklet READ THESE INSTRUCTIONS FIRST 1. Do not turn over this question paper until you are told to do so. 2. Write your name, class and index number in the spaces provided at the top of this page. 3. Write in dark blue or black pen in the spaces provided on the Question Paper. 4. You may use a soft pencil for any diagrams or graphs. 5. DO NOT use paper clips, highlighters, glue or correction fluid or tape. 6. Answer ALL the questions. 7. Give non-exact numerical answers correct to 3 significant figures , or 1 decimal place in the case of Mr and Ar, unless a different level of accuracy is specified in the question. 8. The number of marks is given in brackets [ ] at the end of each question or part question. 9. You are reminded of the need for clear presentation in your answers and to show all working in calculations. 10. The use of a calculator is expected, where appropriate. Paper 2 Question No 1 2 3 4 5 6 7 Marks Obtained 60

1. Electrical generating plants transport large amounts of hot water through copper pipes, and oxygen dissolved in the water can cause a major corrosion problem, Hydrazine (N2H4) is added to avoid the problem by reacting with the oxygen: N2H4(aq) + O2(g) → N2(g) + 2 H2O(l) (a) Draw dot-and-cross diagram for hydrazine and determine the bond angle with respect to a nitrogen atom. [2] Bond angle: ……………………… (b) Explain the data below. [3] Compound Melting point / oC N2H4 1 P2H4 -134 CH3CH2NH2 -81 ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. ………………………………………………………………………………………………. 2

(c) With the use of the Data Booklet, draw an energy level diagram for the formation of N2H4 and determine the standard enthalpy change of formation of N2H4. [3] (d) An alternative method suggested to prevent the copper pipes from corroding is to coat the copper pipes with a layer of aluminium oxide. (i) Complete the diagram below, by filling in correct labels into the empty boxes, to illustrate the process. [1] (ii) Write the half equations for reactions which occur at the anode and cathode. [ 2 ] Cathode: ……………………………………………………………………… Anode: ……………………………………………………………………… ……………………………………………………………………… [Total: 11m] 3

2. The following reaction is sometimes used to produce H 2 needed for production of ammonia in the Haber process. CH4(g) + CO2(g) 2CO(g) + 2H2(g) Temperature / K Kp 1200 3.75 1300 7.74 (a) (i) Write an expression for K p for the reaction. Determine the units for Kp, given that the partial pressure is measured in atm. [2] (ii) What is the percentage yield of H 2 when an equimolar mixture of methane and carbon dioxide with a total pressure of 20.0 atm reaches equilibrium at 1200K? [2] (b) Van’t Hoff equation shown below can be used to determine the enthalpy change of the reaction. ln 1 2 K K )1 1( 1 2T T R H rxn −Δ− = θ (i) Calculate the standard enthalpy change of reaction. [1] 4

(ii) Calculate the standard change in entropy for the reaction at equilibrium. [2] (c) The rate equation for the reaction is found to Rate = 1.2 × 10-18 [CH4][CO2] The backward reaction will have the rate equation as: Rate = 2.6 × 10-18 [CO]2[H2]2 What is the equilibrium constant, Kc, for the formation of carbon monoxide and hydrogen from methane and carbon dioxide? [2] [Total: 9m] 3. (a) A student carried out the following experiment involving silver chloride, silver iodide and copper (II) ions. Step 1: To the solid mixture of silver chloride and silver iodide, add excess aqueous ammonia and shake. Step 2: The solution is filtered and the residue is washed with distilled water. The filtrate is collected and labelled as Solution X. Step 3: Solution X is gradually added to a diluted solution containing copper (II) ions. 5

Predict and explain the observation for Step 2 and 3. Include chemical equations where possible. [6] Step 2 Observation: Yellow residue remains. Explanation: AgCl forms soluble Ag(NH 3)2 + with excess ammonia. AgCl(s) Ag+(aq) + Cl-(aq) Ag+(aq) + 2NH3(aq) [Ag(NH3)2]+ AgI remains as its Ksp is very low and hence cannot contribute sufficient amount of Ag+ for the formation of the soluble complex. Step 3 Observation: Pale blue ppt formed dissolved to give a deep blue solution. Explanation: The OH- from ammonia reacts with Cu 2+ initially to form a pale blue ppt of Cu(OH)2. NH3 + H2ONH4 + + OH- Cu2+(aq) + 2OH-(aq) Cu(OH)2(s) In excess ammonia, the ammonia acts as a competing ligand to combine with Cu2+ to form complex of [Cu(NH3)4]2+ which is a deep blue solution. Cu(OH)2(s) Cu2+(aq) + 2OH-(aq) --- equation 1 Cu2+(aq) + 4NH3 (aq) [Cu(NH3)4]2+(aq) With the formation of the complex, [Cu2+] decreases. This results in the equilibrium position to shift to the right in equation 1. Therefore, the pale blue ppt dissolves. [Total: 6m] 6

4. (a) The curve below shows how the volume of oxygen evolved varies with time when 50 cm 3 of a 2.0 moldm–3 solution of hydrogen peroxide, H2O2, decomposes at 298K. (i) State how you could use the curve to find the rate of reaction at point A. [1] ………………………………………………………………………………………….. ………………………………………………………………………………………….. ………………………………………………………………………………………….. (ii) Using the same axes, sketch and label the following curves: Curve X: 100 cm 3 of a 1.0 mol dm–3 solution of H2O2 at 298 K Curve Y: 25 cm 3 of a 2.0 mol dm–3 solution of H2O2 in the presence of a catalyst at 298 K [ 2 ] (iii) Hydrogen peroxide decomposes more rapidly in the presence of aqueous Fe 3+. Write equations to shown how Fe3+ is able to catalyse the decomposition of hydrogen peroxide. [2] ………………………………………………………………………………………….. ………………………………………………………………………………………….. ………………………………………………………………………………………….. 7

(v) Explain, using the Maxwell Boltzmann distribution curve, how the presence of a catalyst leads to an increase in the rate of the reaction. [2] ………………………………………………………………………………………….. ………………………………………………………………………………………….. ………………………………………………………………………………………….. ………………………………………………………………………………………….. [ T o t a l : 7 m ] 5. Laughing gas, N 2O, can be prepared from H2 and NO: H2(g) + 2NO(g) → N2O(g) + H2O(g) A study of initial concentration versus initial rate at a certain temperature yields the following data for this reaction: [H 2] / mol dm-3 [NO] / mol dm-3 initial rate / mol dm-3 0.1000 0.5000 2.560 x 10-6 0.2000 0.3000 1.843 x 10-6 0.2000 0.6000 7.373 x 10-6 8

(a) Determine the order of reaction with respect to H 2 and NO. [2] (b) Hence determine the rate constant, k. Include the units of k in your answer. [2] (c) Comment on why this rate determining step is unusual. [1] ………………………………………………………………………………………………. ………………………………………………………………………………………………. [Total: 5m] 6. (a) (i) Suggest a 2-step procedure to form compound P from but–1–ene. [2] CC O HO O C H H x CH3y Compound P Step 1: Reagents: ……………………………………………………….. Conditions: ……………………………………………………… Step 2: Reagents: ……………………………………………………….. Conditions: ……………………………………………………… 9

(i