VJC H2 CHEM P3 Prelim

 VJC 2011 9647/03/PRELIM/11 [Turn over 2 1 Use of the Data Booklet is relevant to this question. Cyanidin is a natural pH indicator that can be found in many red berries, including grapes, blackberry and cherry. Aqueous solutions of cyanidin change colour with pH, appearing red at pH < 3, violet at pH 7-8, and blue at pH > 11. In addition, cyanidins are antioxidants that can scavenge free radicals and counteract ageing caused by oxidative damage in our body tissues and cells. Cyanidin Cyanidin (CyH) is a weak monoprotic acid. In one experiment, 25.0 cm 3 of cyanidin was titrated against 0.10 mol dm –3 of dilute NaOH at 25°C. The following titration curve was obtained using a data logger. (a)(i) Explain what is meant by a weak monoprotic acid, with reference to CyH. (ii) Calculate the molar concentration of the cyanidin solution. (iii) Given that the solution of cyanidin is only 0.8% dissociated into ions, calculate the value of K a for cyanidin, stating clearly its units. (iv) Explain, with the aid of an appropria te equation, why the pH at end-point B is greater than 7. × 26.20 Volume of NaOH /cm3 A B pH

 VJC 2011 9647/03/PRELIM/11 [Turn over 3 (v) Calculate the pH value at end-point B. Hence, state the colour change observed at the end-point. [10] (b) An aqueous solution of cyanidin CyH and Cy- can act as a buffer. (i) Copy the titration curve onto your answer script and label I – the buffer region II – the point corresponding to maximum buffer capacity by indicating the pH and volume of NaOH used. (ii) Explain, with the help of equations , how an aqueous mixt ure of CyH and Cy - can control pH when relatively sm all amounts of acid or base are added to the solution. (iii) In a sample of cherry juice at 25°C, the concentration ratio of Cy - to CyH was found to be 3 to 1. Using the Ka value determined above, calculate the pH value of the cherry juice. (iv) Sam tried to dilute the same cherry juice sample by adding 100 cm 3 of deionized water at 25°C. Predict the pH of the resulting sample. [6] (c) Cyanidin can be synthesized via a number of steps from either of the two molecules shown below: Molecule P Molecule Q (i) Which molecule P or Q will have a higher boiling point? Explain your answer. (ii) Draw the structur es of all the possibl e organic products when P and Q react separately with chlorine in trichloromethane. [4] [Total: 20]

 VJC 2011 9647/03/PRELIM/11 [Turn over 4 2 This question is about elements and their compounds in Period 3, Group IV and Group VII. (a) The elements in Period 3 range from metals on the left of the Periodic Table to non-metals on the right. By describing one physical property of the element and one chemical property of its chloride, explain why phosphorus can be regarded as a non-metal. Write equation(s) where relevant. [2] (b) (i) The element aluminium and its compounds have some properties characteristic of metals, and some of non-metals. Aluminium hydroxide, for example, is known to be amphoteric. Explain the meaning of the word in italics. Aluminium sulfate and calcium oxide ar e sometimes added to water supplies to co-precipitate suspended solids and bacteri a. A small amount of aluminium- containing ions remains in solution and its presence in drinking water may contribute to the mental illness known as Alzheimer’s disease. (ii) Write a balanced equation for the reaction that o ccurs when aluminium sulfate and calcium oxide are added to water, given that aluminium hydroxide is one of the products formed. (iii) Explain why adding too much calcium ox ide would increase the probability of contracting Alzheimer’ s disease. Write equations for all reactions that occur. [4] (c) Explain each of the followin g as fully as you can. Justify your answers with relevant data from the Data Booklet. Write balanced equations, including state symbols, for any reaction that occurs. (i) HCl  can be prepared by adding concentrated sulfuric acid to solid sodium chloride. However the yield of H I is very low when concentrated sulfuric acid is added to solid sodium iodide. (ii) When a hot glass rod is plunged into a gas jar of gaseous H I, some violet vapour is seen. On repeating the experiment with HBr, no change is observed. [ 5 ]

 VJC 2011 9647/03/PRELIM/11 [Turn over 5 (d) Tetraethyl-lead (IV), Pb(C2H5)4, is an example of an organometallic compound. It can be used as an antiknock agent in petro l. At the high tem perature in a car engine, the Pb-C bond in Pb(C 2H5)4, breaks to yield lead atoms and ethyl radicals. (i) By using the VSEPR theory, predict the shape of the Pb(C2H5)4 molecule. (ii) Using only the data provided below, draw an energy level diagram to calculate ∆H1, the enthalpy change of the following reaction: 2C(g) + 5H(g)  CH3CH2(g) Data: (iii) Hence, with the aid of the data in (ii) and additional data given below, draw an energy cycle to calculate the Pb-C bond energy in Pb(C 2H5)4. Data: [7] (e) The ethyl radical formed in (d) reacts readily with chlorine gas to give chloroethane. Another possible product is 1, 2-dichloroethane. Outline the steps in the mechanism to show t he formation of 1, 2-dichloroethane, starting from chlorine gas and ethyl radical. [2] [Total: 20] Enthalpy change of atomization of C(s), ∆H 2 +717 kJ mol1 Enthalpy change of atomization of H2(g), ∆H3 +218 kJ mol1 Enthalpy change of formation of CH3CH3(g), ∆H4 85 kJ mol1 Bond dissociation energy of the first C-H bond in CH3CH3(g), ∆H5 +435 kJ mol1 Enthalpy change of atomization of Pb(s), ∆H6 +195 kJ mol -1 Enthalpy change of formation of Pb(C2H5)4(g), ∆H7 +264 kJ mol -1

 VJC 2011 9647/03/PRELIM/11 [Turn over 6 3 This question is about transition elements and their compounds. (a) The graph below shows the second to fourth ionisation energies for the first row dblock elements scandium to zinc. (i) Explain what is meant by the term transition element. (ii) Give an equation each to represent the second ionisation energy of chromium and the fourth ionisation energy of iron respectively. (iii)Explain briefly why the second ionisation energy of chromium is higher than that of manganese. (iv)Explain briefly why the fourth ionisation energy of cobalt is lower than that of i r o n . [6] Ionisation Energy / kJ mol1 5980 Proton number Elements 1240 1310 1370 1590 1510 1560 1640 1750 1960 1730 2390 2720 2870 2990 3250 2960 3230 3390 3350 3238 7110 4170 4600 4700 5190 5400 5100 5400 5690 0 1000 2000 3000 4000 5000 6000 7000 8000 20 22 24 26 28 30 2nd IE 3rd IE 4th IE Sc Ti V Cr Mn Fe Co Ni Cu Zn

 VJC 2011 9647/03/PRELIM/11 [Turn over 7 (b) The use of the Data Booklet is relevant to this question. In one experiment, when aqueous iron( III) sulphate is boiled with an excess of potassium cyanide, a red solution A is obtained. In another experiment, when aqueous iron(III) sulphate is added to aqueous pota ssium iodide, a brown solution B is obtained. Solution B is found to catalyse the reaction between iodide and peroxodisulphate, S 2O8 2, whereas solution A does not affect the rate of this reaction. When aqueous sodium thiosulf ate is added to solution B followed by aqueous sodium hydroxide, the br own colour is discharged and a gr