NJC H2 CHEM P3 Question Paper Prelim

1 [Turn Over NATIONAL JUNIOR COLLEGE PRELIMINARY EXAMINATIONS Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 3 Free response 9647/03 Wed 17 Sep 2014 2 hours READ THESE INSTRUCTIONS FIRST Answer any four questions. Start your answer to each question on a fresh piece of paper. A Data Booklet is provided. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely behind the cover page. This paper consists of 11 printed pages and 1 cover page.

2 [Turn Over 1 Iodine monochloride, ICl, is an interhalogen compound that is formed when chlorine gas is passed through iodine crystals. Generally, interhalogen compounds have chemical properties similar to that of halogens. (a) (i) Predict the physical state of ICl at room temperature. (ii) By considering the partial charges in ICl, explain how water reacts with ICl. (iii) Hence construct an equation for the reaction of ICl with water. [3] (b) Methylbenzene reacts with ICl in the presence of FeCl3 to form compound A. (i) Draw the structure of A. (ii) Describe the mechanism of this reaction, clearly indicating the role of FeCl3. [4] (c) FeCl3 and FeF3 are both formed from a metal and a non-metal. However, in molten state, only FeF3 is able to conduct electricity and FeCl3 does not. (i) Suggest the structures of FeC l3 and FeF 3 and explain why iron forms two different types of compounds when reacted with the different halogens – fluorine and chlorine. (ii) Using the information given below as well as relevant data from the Data Booklet, construct an energy level diagram for the formation of FeF 3 from its elements to determine its lattice energy. enthalpy change of atomisation of Fe +340 kJ mol –1 first electron affinity of F –328 kJ mol –1 standard enthalpy change of formation of FeF3 –989 kJ mol –1 [7] Suggest an equation for the f C

3 [Turn Over (d) Iron is a transition metal, a d-block element that varies from s-block metals in terms of physical properties. The table below gives data about some physical properties of the elements – iron and calcium. property calcium iron relative atomic mass 40.1 55.8 atomic radius/ nm 0.197 0.126 density/ g cm–3 1.54 7.86 electrical conductivity Good Good (i) Write down the full electronic configurations of calcium and iron. (ii) Explain why the atomic radius of iron is smaller than that of calcium. (iii) Suggest why iron has a higher density than calcium. (iv) Explain which of the 2 metals would be expected to have a higher electrical conductivity. [6] [Total:20]

4 [Turn Over 2 (a) In the qualitative analysis of organic compounds, the presence of a carbonyl functional group can be confirmed by using 2,4dinitrophenylhydrazine. When 2 cm 3 of 2,4 dinitrophenylhydrazine solution is added to separate test tubes containing 2 cm 3 propanone and 2 cm 3 propanal respectively, a bright orange precipitate is observed in both test tubes. Write a balanced equation for the reaction of 2,4 dinitrophenylhydrazine with propanal. Hence, identify the type of reaction. [2] (b) Fehling's solution, an alkaline copper( II) tartrate complex, can be used to distinguish between propanal and propanone. A positive test would show the formation of a brick red solid, Cu2O. (i) Identify which of the two organic compounds above would give a positive test with the Fehling’s solution. Give the structure of the organic product for this reaction. The brick red solid, Cu 2O, is isolated through filtration. It is soluble in excess ammonia to give a colourless solution B. When left exposed to air, the colourless solution B turns into a deep blue solution, containing the complex ion [Cu(NH3)4]2+. (ii) Explain why the solution containing the complex ion [Cu(NH 3)4]2+ is deep blue in colour. (iii) Identify the oxidation state of copper in the copper containing species in the colourless solution B, and write its electronic configuration. Hence explain why solution B is colourless. [6] (c) [Cu(NH3)4]2+ can also be prepared by adding excess aqueous NH 3 to aqueous copper(II) sulfate. Three reactions involving aqueous copper( II) sulfate are illustrated below. I (aq) Excess conc HCl NH3 (aq) Excess NH3 (aq) Pale blue ppt C Yellow solution D [Cu(NH3)4]2+ CuSO4 (aq) White ppt E in brown solution Suggest an equation for the f C

5 [Turn Over (c) (i) Explain fully, with the aid of equations, how the pale blue precipitate C is formed when NH3 (aq) is added to CuSO4 (aq). (ii) Write an equation for the formation of the yellow solution D. Hence, explain what will be observed when water is added to solution D. When hexane is added to the brown solution containing the white precipitate E and shaken, a purple hexane layer is formed above a colourless aqueous layer containing the white precipitate E. (iii) By referring to the types of intermolecular interactions, explain why the aqueous solution turns from brown to colourless upon mixing of hexane. (iv) Construct an ionic equation for the formation of the white precipitate E when I–(aq) is added to CuSO4 (aq). [6] (d) Ethane−1,2−diamine, NH2CH2CH2NH2 (abbreviated as en when it acts as a ligand), can undergo ligand exchange reaction with [Cu(NH3)4]2+ to form [Cu(en)2]2+. The standard entropy change for this reaction has a value of 88 J mol1 K1. (i) Write an equation for this reaction and hence suggest the sign of the entropy change. (ii) Draw the structure of the complex ion [Cu(en)2]2+ and identify its shape. [4] (e) Ethane–1,2–diamine, NH 2CH2CH2NH2, can be synthesised by reacting 1,2dichloroethane with ammonia under suitable conditions. ClCH 2CH2Cl NH 2CH2CH2NH2 (i) State the type of reaction occurring in the above synthesis. (ii) A student attempted to carry out the above experiment. One of the side products obtained has the molecular formula C4H10N2. Suggest the structure of this side product. [2] [Total:20] NH3

6 [Turn Over 3 Dental erosion is the irreversible loss of tooth structure due to chemical dissolution by acids. Our dental enamel is composed primarily of hydroxyapatite, Ca10(PO4)6(OH)2. Ca10(PO4)6(OH)2 (s) + aq 10Ca 2+ (aq) + 6PO4 3 (aq) + 2OH (aq) When a tooth is placed in distilled water of pH 7, a small amount of it will slowly dissolve. In contrast, our saliva fluid contains calcium, phosphate and hydroxide ions that prevent the dissolution of our dental enamel. Acidic foods and drinks, such as Coca Cola which contains phosphoric acid, may damage our teeth when it is frequently consumed. The solubility of our enamel increases about 10 – fold for each unit decrease in pH. (a) (i) Phosphoric(V) acid, H 3PO4, is a triprotic acid. The magnitude of the second acid dissociation, Ka2, is much smaller than the first acid dissociation Ka1. Suggest an explanation for this observation using relevant equations. (ii) Identify two reasons for the increased solubility of enamel at low pH under constant temperature. Explain your answer. [5] (b) One other important function of our saliva is its buffering ability. There are three possible buffer systems in saliva:  the protein buffer,  the carbonic acid/ hydrogencarbonate buffer,  the phosphate buffer. The carbonic acid/ hydrogencarbonate buffer maintains the oral cavity at about pH 6.3 to reduce the risk of den