MJC H1 Chem Prelim Paper 2 (Answers)

Answers for MJC 2008 H1 Chem Prelim Paper 2 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 Down the group, from Be to Mg, Nuclear charge increases as proton number increases. Valence electrons are added to a next outer shell ⇒ valence electrons are furt her away from the nucleus. Significant increase in shielding effect outweighs the increase in nuclear charge. Effective nuclear charge decreases Weaker electrostatic force of attraction between nucleus and valence electrons ⇒ Ionization energy decreases (b) Write the electronic configurations of chromium ion, Cr 3+. [1] 1s 2 2s22p6 3s23p63d3 1

(c) Between Cr3+ and F -, state and explain which ion would be deflected the most when they are passed through an electric field? [2] Extent of deflection ∝ q m q m ratio for Cr3+ = 3 52 = 0.0577 q m ratio for F- = 1 19 = 0.0526 Therefore Cr 3+ will deflect the most (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 Be: 1s2 2s2 B: 1s22s22p1 ƒ The 2p electron in B is at a higher energy level than the 2s electron in Be ƒ Hence, less energy is required to re move the 2p electron from B compared to Be. 2

(ii) N to O [3] N: 1s2 2s2 2p3 O: 1s 22s22p4 ƒ There is inter-electron repulsion between the paired el ectrons in the 2p orbital of O. ƒ Hence, less energy is required to re move the 2p electron from O compared to N [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] Heat absorbed, Q = mc∆T = 500 x 4.18 x 30 J = 62700 J No of moles of butane burnt = 1.30/58 = 2.24 x 10 -2 Heat absorbed by water, Q = Heat released by combustion of butane, Q’ Q’ = ΔHc (C4H10) x n ΔHc (C4H10) = – -2 62700 2.24 x 10 =–2799 kJ mol-1 3

(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 Hc (C) = - 393.5 kJ mol-1 Δ Hc (H2) = - 285.8 kJ mol-1 [1] By Hess’s Law, Δ Hf (C4H10) = 4ΔHc (C) + 5ΔHc (H2) - ΔHc (C4H10) = 4(-393.5) + 5(-285.8) – (–2799) = -204 kJ mol-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. CFCs commonly used as refrigerants or aerosols (ii) Why is CFCs an environmental concern? [1] CFCs causes the destruction/depletion of the ozone layer. [Total: 4] 4

3(a) Explain what is meant by the lattice energy of an ionic compound. [1] The energy released when one mole of the ionic compound is formed from its constituent gaseous ions at 298K and 1 atm. (b) Based on their lattice energies, explain why MgO is preferably used as refractory lining for furnaces rather than MgCl2, [2] radius of anion, r- : O2- < Cl- charge of anion, r - : O2- > Cl- | ΔHlatt | : MgO > MgCl2 Mg2+ and O2- are held together by stronger electrostatic forces of attraction. Hence MgO has a higher melting which makes it a better choice as a refractory lining. (c) MgCl2 is highly soluble in water and MgO is only sparingly soluble in water. Give an explanation for the observation. [2] Both MgCl 2 and MgO have giant ionic lattice structure. MgCl 2 is highly soluble because the formation of ion-dipole bon ds release energy which causes the dissociation of ions from the lattice structure. MgO is only sparingly soluble because of its highly exothermic lattice energy. 5

(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] For MgCl2, the phenolphthalein solution will remain colourless as the Mg2+ ions will polarised water to such an extent that hydrolysis occurs to give a slightly acidic solution (pH ≈ 6.5) MgCl2 (s) + aq → [Mg(H2O)6]2+ (aq) + 2Cl- (aq) [Mg(H2O)6]2+(aq) [Mg(H2O)5(OH)]+(aq) + H+(aq) For MgO, the phenolphthalei n solution will turn pink as MgO dissolves slightly in water to give a weakly alkaline solution (pH ≈ 9) MgO (s) + H2O(l) Mg2+ (aq) + 2OH-(aq) (e) Unlike magnesium chloride, aluminium ch loride is a covalent compound. In the gaseous state, AlCl3 has an apparent Mr of 267.0. Draw a dot-and-cross diagram to illustrate the bonding of (i) Magnesium chloride 6 2 xx Clx x - x . xx 2+ Mg (ii) Aluminium chloride in the gaseous state [2] C . Al C Cl Cl Al Cl Cl . . x . x . x . x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x l x x x l (f) Unlike all other Group II oxides, beryllium oxide, BeO, is am photeric in nature, like aluminium oxide, Al2O3. Suggest a reason for their similarity in chemical properties. [1] The similarity is due to the diagonal relationship. Al and Be have similar charge density. [Total: 11]

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] Element K is Na. Na+ ion has low charge density. Hence, only hydration takes place with no hydrolysis, hence NaCl (aq) is neutral with pH 7. NaCl (s) +aq → Na+ (aq) + Cl- (aq) Element L is A l. Al2Cl6 undergoes both hydration and hydrolysis in water. Al 3+ has very high charge density. Hence, Al 3+ ion polarises its surrounding water molecules, weakening the O-H bond such that H + ions are released into solution. The solution is acidic (pH≈3). Al2Cl6(s) + 12H2O(l) → 2[Al(H2O)6]3+(aq) + 6Cl-(aq) [Al(H2O)6]3+(aq) → [Al(H2O)5(OH)]2+(aq) + H+(aq) (ii) Identify element M and with the aid of an equation, explain why the chloride of M forms a strongly acidic solution. [2] Element M is Si. SiCl4 undergo complete hydrolysis in water to give strongly acidic solution (pH ≈ 2). SiCl4 + 2H2O → SiO2 + 4HCl [Total: 7] 7

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