MJC 2008 H1 Chem Prelim Paper 2 (Questions)

Class Reg Number Candidate Name ___________________________________ Meridian Junior College JC2 Preliminary Examinations 2008 H1 Chemistry 8872 Paper 2 11 September 2008 2 hours INSTRUCTION TO CANDIDATES Write your name, class and register number in the spaces at the top of this page. This booklet contains Section A and B of your paper. Section A (Pg 1-11) Answer ALL questions in Section A in the spaces provided on the question paper. You are advised to spend about 1 hour on Section A. Section B (Pg 12-17) Answer two out of three questions in Section B on the foolscap paper. You are advised to spend about 1 hour on Section B. Begin each question on a fresh page of writing paper. Fasten your answers for Section B behind Section A. You are advised to spend about 30 min per question. INFORMATION FOR CANDIDATES The number of marks is given in brackets [ ] at the end of each question or part question. This question paper consists of 17 printed pages. Examiner’s Use Paper 1 MCQ / 30 Q1 / 8 Q2 / 4 Q3 / 11 Q4 / 7 Paper 2 Section A Q5 / 10 Q6 / 20 Q7 / 20 Paper 2 Section B Q8 / 20 Total / 110 1

SECTION A (40 marks) Answer all questions in the space provided. 1(a) The graph below represents the first i onization energies from beryllium to strontium. Explain the general trend presented by the graph. [2] Energy Element Be Mg Ca Sr (b) Write the electronic configurations of chromium ion, Cr 3+. [1] 2

(c) Between Cr3+ and F -, state and explain which ion would be deflected the most when they are passed through an electric field? [2] (d) The first ionization energies of ber yllium, boron, nitrogen and oxygen are presented in the graph below. Energy Element Be B N O Explain the decrease in ionization energy from: (i) Be to B 3

(ii) N to O [3] [Total: 8] 2 Certain enthalpy changes such as ent halpy change of formation cannot be found by direct experiment. Instead it can be found by applying Hess’ Law to simple energy cycles. (a) 500 cm 3 of water was heated in a calorimeter by burning a 1.30 g of butane, C4H10. The temperature rise of the water recorded was 30 °C. Calculate the enthalpy change of combustion of butane. [2] 4

(b) The energy cycle for the complete combustion of butane is shown below. C4H10 (g) + 13/2 O2(g) 4CO2(g) + 5H2O (l) ΔHc (C4H10) 4C(s) + 5H 2(g) Using your answer in part (a) and the following enthalpy changes, calculate the enthalpy change of formation of butane. Δ H c (C) = - 393.5 kJ mol-1 Δ Hc (H2) = - 285.8 kJ mol-1 [1] (c) Chlorofluorocarbons, CFCs, are small alkane molecules where a few of the hydrogen atoms have been replaced by chlo rine atoms and fluorine atoms. The use of CFCs in daily life products have been banned in many countries because CFCs are environmentally hazardous. (i) State one use of CFCs. (ii) Why is CFCs an environmental concern? [1] [Total: 4] 5

3(a) Explain what is meant by the lattice energy of an ionic compound. [1] (b) Based on their lattice energies, explain why MgO is preferably used as refractory lining for furnaces rather than MgCl 2, [2] (c) MgCl 2 is highly soluble in water and MgO is only sparingly soluble in water. Give an explanation for the observation. [2] 6

(d) MgCl 2 and MgO are added to separate aqueous solutions, each containing a few drops of phenolphthalein. Predict, with reasoning, the expected observations. Support your answers with any relevant equations. [3] (e) Unlike magnesium chloride, aluminium chlo ride is a covalent compound. In the gaseous state, AlCl 3 has an apparent Mr of 267.0. Draw a dot-and-cross diagram to illustrate the bonding of (i) Magnesium chloride (ii) Aluminium chloride in the gaseous state [2] (f) Unlike all other Group II oxides, beryllium oxide, BeO, is amphoteric in nature, like aluminium oxide, Al 2O3. Suggest a reason for their similarity in chemical properties. [1] [Total: 11] 7

4(a) Certain chlorides and oxides of period 3 elements dissolve in water. Elements K, L and M are in Period 3. Chlorides of K and L dissolve in water to form a neutral solution (pH = 7) and an acidic solution (pH ≈ 3) respectively. Oxide of element M is found to be insoluble in water, whereas its chloride forms a strongly acidic solution (pH ≈ 2). (i) Suggest the identity of elements K and L and with the aid of equations, explain the above observations as fully as you can. [5] (ii) Identify element M and with the aid of an equation, explain why the chloride of M forms a strongly acidic solution. [2] [Total: 7] 8

5 Calcium hydroxide is a white solid which is only sparingly soluble in water. 1.85 g of Ca(OH)2 will dissolve in 100 cm 3 of water. Ammonia is a pungent gas which is very soluble, 89.9 g of NH 3 dissolves in 100 cm 3 of water. The pH of saturated aqueous Ca(OH)2 and NH3 solution are 13.7 and 13.0 respectively. Both compounds are useful for agricultu ral purposes. Calcium hydroxide can be used to control the pH level in soil while ammonia is reacted with nitric acid to produce ammonium nitrate for fertilizing uses. In gardening, the pH level of the soil is very important. Soil pH is an important consideration for farmers and gardeners fo r several reasons, including the fact that many plants and soil life forms prefer either alkaline or acidic conditions. However, some diseases tend to thrive when the soil is too alkaline or acidic, and that the pH can affect the availability of nutrients in the soil. Certain plants such as hydrangea will form fl owers of different colours according to the pH of the soil. To encourage bl ue hydrangea flowers, the soil must have a pH of 5.0–5.2 and pH of 6.0–6.2 for pink blooms. To raise the pH level of the soil, the gardener can adopt t he liming method. The two common choice of lime is garden lime containing CaCO 3 and quicklime, CaO. Quicklime is caustic and cannot be applied directly to the soil, instead it is spread around the land in heaps to absorb rain and form slaked lime (Ca(OH) 2), which is then spread on the soil. Their use is prohibited by the org anic standards and while fast acting, the effect is short lived in comparison to the garden lime. (a) Define the term pH. [1] (b) (i) Calculate the concentration in mol dm -3 of I saturated calcium hydroxide solution 9

II saturated ammonia solution [2] (ii) Hence, explain whether calcium hy droxide is a strong or weak base. [2] (b) A blue hydrangea is planted in a plot of so il. The pH of the soil is 5.1 and the volume of water in the soil is 500 cm 3. (i) Calculate the minimum concentration of H + ions required for the blue hydrangea to change to pink color. 10