2019 SAJC H2 Chem Prelim P3 ANS

1 NAME Class ST ANDREW’S JUNIOR COLLEGE JC2 Preliminary Examination H2 Chemistry (9729) Paper 3 Free Response 18 September 2019 2 hours Additional Materials: Data Booklet, Answer Booklet READ THESE INSTRUCTIONS: Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all questions. Marks [60] Section B Answer one question. Marks [20] A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 14 printed pages.

2 Section A Answer all the questions in this section. 1 (a) Use of the Data Booklet is relevant to this question. The Hoffmann voltameter is used to electrolyse concentrated iron ( III) chloride solution. The setup of the Hoffmann voltameter is shown below. (i) Describe the observations at the cathode and anode. Explain your answer with the aid of suitable ion-electron equations. [2] Anode (+): 2Cl–  Cl2 + 2e Greenish-yellow gas was evolved at the anode. Cathode (-): Fe3+ + e  Fe2+ The yellow solution turned green (ii) State the difference in the observations at the anode when dilute aqueous iron ( III) chloride solution is us ed instead. Explain your answer using relevant E0 data. [2] Anode (+): 2H2O  O2 + 4H+ + 4e (equation not required) Colourless gas is evolved the E°(O2/H2O) is +1.23V which is more negative (less positive) / preferentially oxidised compared to E°(Cl2/ Cl–) = +1.36V. (b) The electrolysis of a hot aqueous solution of Na X yielded a reddish brown gas, X2, at the anode and hydrogen gas at the cathode. The red litmus paper turned blue when dipped into the solution at the cathode. (i) Suggest the identity of the reddish brown gas X2. [1] Pt electrode Pt electrode + – electrolyte

3 Br2 (ii) Write an ion-electron equation for the reaction at the cathode. [1] Cathode: 2H2O + 2e  H2 + 2OH– Comments:  Many students chose the SHE equation. This is not correct as H+ was not mentioned to be present. (iii) Some of the X2 dissolved in the alkaline medium to form X– and XO3– ions. Write a balanced equation to represent the reaction of X2 in alkaline medium. [1] 3Br2 + 6OH–  5Br– + BrO3– + 3H2O (iv) Describe a chemical test, which does not involve silver nitrate, that could distinguish between sodium chloride and sodium iodide. [2] Add Br2 (aq). For KCl, orange Br2 is not decolourised. For KI, orange solution of Br2 turns brown. 2I– + Br2  I2 + 2Br– (c) Describe and explain how the thermal stability of the hydrogen halides varies down Group 17. [2] Down the group, size of halogen increases , leading to less effective orbital overlap between H and X. Bond energy of H – X decreases / H-X bond is longer and weaker and is easily broken. Thus, thermal stability of hydrogen halides decreases down the group. (d) When methylcyclopentane is treated with a small quantity of chlorine in the presence of ultraviolet light, 4 mono -chlorinated constitutional isomers are produced. Draw the structures of the isomers and state the ratio in which they are formed. [3]

4 3 : 1 : 4 : 4 (e) SiCl4 reacts readily with water to form a solution of pH 2. However, SiO2 does not react with water and only reacts with hot concentrated alkali such as NaOH. (i) Explain why SiCl4 reacts readily with water, but SiO 2 only reacts with hot concentrated NaOH. Use relevant equations to explain your answer. [3] SiCl4 has energetically available vacant orbitals to accept lone pair of electrons from H2O and undergo complete hydrolysis. SiCl4(l) + 2H2O(l) SiO2(s) + 4HCl(aq) SiO2 is an acidic oxide and reacts with basic/ionic NaOH. SiO2(s)+ 2NaOH (conc.)  Na2SiO3(aq) + H2O(l) SiO2 is a giant covalent structure with strong covalent bonds , requiring a lot of energy to overcome/break , and hence does not reacts with / not soluble in water. (ii) Using Cl2(g) as an example, define the term bond energy. [1] Bond energy is the enthalpy change to break one mole of Cl-Cl bonds in gaseous state. OR Cl2(g)  2Cl(g) (iii) Construct an energy level diagram to calculate the enthalpy change of formation of SiCl4. Your diagram should include relevant data from the Data Booklet together with the following data: [3]

5 Enthalpy change of atomisation of Si = +338 kJmol-1 Enthalpy change of vapourisation of SiCl4 = +29 kJmol-1 ΔHf = +338 + 2(244) – 4(359) – (29) = -639 kJmol-1 [Total: 21 marks] 2 (a) Phosphorus-containing compounds can react in various ways: as an acid, as a base, as an electrophile, as a nucleophile, as an oxidising agent and as a reducing agent. Study the following reactions and decide in which way the phosphorus-containing compound is reacting in each case. Explain your answers fully. (i) H3PO4 + H2O → H2PO4− + H3O+ [1] Acid . donate proton (or H+) to the water molecule. (ii) Reducing agent. It was oxidised due to the addition of O or Oxidation state of P decreased from +3 to +5 OR [1] ΔHf

6 (CH3)3NO was reduced since there was a loss of O or Oxidation state of N decreased from -1 to 3 (iii) [1] Nucleophile. The lone pair of electrons on the P attacks the electron deficient C of CH3Br via nucleophilic substitution. (b) 2-chloropropanoyl chloride, W, can be obtained from propanoic acid by heating it with Cl2 and PCl5. Reaction 1 : CH3CH2CO2H(l) + Cl2(g) + PCl5(s) → CH3CHClCOCl(l) + 2HCl(g) + POCl3(l) W H > 0 (i) Explain the different reactivities of the two chlorine atoms in W towards water. [2] The carbonyl carbon of acyl chloride is bonded to 2 electronegative O and Cl making it highly electron deficient, while the carbon of the alkyl chloride is only bonded to Cl. Hence, acyl chloride is more susceptible to the attack of nucleophiles (water). (ii) Explain fully why the pH of an aqueous solution of 2 -chloropropanoyl chloride is 1.0 while that of an aqueous solution of propanoic acid of the same concentration is greater than 1.0. [2] Aqueous 2 -chloropropanoyl chloride is more acidic than aqueous propanoic acid. 2-chloropropanoyl chloride hydrolyses fully in water to form a carboxylic acid and HCl which is a strong acid which dissociates fully in water. Propanoic acid dissociates partially in water. OR

7 CH3CHClCOCl + H 2O CH3CHClCOOH + H + + C l– CH3CH2COOH + H2O CH3CH2COO– + H3O+ (iii) W can be used to produce sweet -smelling Z, C 6H8O4, via compounds X and Y, as shown in Fig. 2.1 Fig. 2.1 Suggest structures for compounds X, Y and Z in Fig. 2.1. [3] X: CH3CH(OH)CO2−Na+ Y: CH3CH(OH)CO2H Z: (iv) W can undergo the following reaction scheme. State the reagents and conditions for Steps 1 and 2. [2] Step 1: (excess) CH3NH2, heat Step 2: LiAlH4, dry ether (v) V is an isomer of W. V rotates the plane of polarised light. It gives a positive test with 2,4 - dinitrophenylhydrazine. Draw the displayed formula of V, identifying the chiral carbon. [1]

8 (vi) Use the table of characteristic values for infra -red absorption frequencies in the Data Booklet to answer this question. Infra-red adsorption freque