MJC H2 CHEM P3 ANS

[Turn over 1 MJC Preliminary Exam 2011 H2 Chemistry Paper 3 – Mark Scheme 1(a) Lattice energies are often used as a measure of the energetic stability of ionic compounds. Usually ionic compounds with the same c rystal structure can be compared based on the lattice energy value. Lattice energies need to be determined experimental ly using thermochemical data. However based on electrostatic consideration s (i.e. attraction and repulsion of ions), theoretical values of the latti ce energies can be calculated using equations such as the Kapustinskii equation: −+ −+ + ×××−=∆ rr zzH latt υ1 .107 kJ mol -1 where υ is the number of ions in the empirical formula −+ zz , is the charge of the cation and anion respectively −+ rr , is the ionic radius (in nm) of the cation and anion respectively The table below shows the numerical values of latti ce energies for a series of magnesium and other related compounds. Compound LiC l MgC l2 MgBr 2 Mg I2 MgO Experimental value / kJ mol -1 - 853 - 2526 - 2440 - 2327 - 3933 (i) Define, with the aid of an equation, the lattice e nergy of magnesium oxide. Lattice energy of magnesium oxide is the energy released when one mole of magnesium oxide is formed from its constitu ent gaseous ions at 298K and 1 atm . Mg 2+(g) + O2-(g) → MgO (s) (ii) By quoting appropriate data from the Data Booklet , explain why the lattice energies of the magnesium halides decrease from MgC l2 to Mg I2. From the Data Booklet , ionic radii in nm of Cl-: 0.181, Br - : 0.195, I- : 0.216 Since ∆ Hlatt  ∝ −+ −+ + rr qq , but the cation and charges of the ions are the same, lattice energy would decrease as ionic radii of the halide increases down the group.

[Turn over 2 (iii) By using appropriate data from the Data Booklet , calculate the theoretical lattice energy of magnesium chloride. −+ −+ + ×××−=∆ rr zzH latt υ1 .107 -1 mol kJ 2612 −=∆ latt H (iv) The lithium and magnesium ion have similar ionic r adii. Based on this assumption, use the value calculated in (a)(iii) to estimate the theoretical lattice energy of lithium chloride. 2MgCl for 3 1LiCl for latt latt HH ∆×=∆ -1 mol kJ 871 LiCl for −=∆ latt H (v) Explain the differences between the theoretical and experimental values for lithium chloride and magnesium chloride respectively, and comment whether the assumption made in (a)(iv) is a valid one. [10] Compound LiC l MgC l2 Theoretical value / kJ mol -1 - 871 - 2612 Experimental value / kJ mol -1 - 853 - 2526 There is a discrepancy between the experimental and theoretical lattice energies for ionic compounds with partial covalent character. Hence for LiC l, there is very little covalent character as there is a much smaller difference between the theoretical and expe rimental values. Hence, the ionic radius of Li + is likely to be as that predicted by the assumption. Conversely the charge density of Mg 2+ is relatively high, resulting in greater polarization of the Cl - electron cloud, hence the larger degree of covalent character in MgCl 2.

[Turn over 3 (b) Describe, and suggest an explanation for, the tren d observed in the thermal decomposition of nitrates of the Group II elements from magnesium to barium. Write a general equation for the reaction that occurs on heating. [3] Down the group, • radius of cations increases • charge density of cations decreases • polarising power of cations decreases • cation is less able to distort electron cloud of N O 3 2- anion • Thermal decomposition of nitrates decreases • Thermal stability of nitrates increases and higher temperature required to bring about decomposition. M(NO 3)2(s) → MO(s) + 2NO 2(g) + ½ O 2(g) (c) In an experiment, water is added to a test tube con taining the solid remaining after magnesium carbonate has been heated. Dilute sulfuric acid is then added to the test tube. The procedure is repeated to the solid remaining after barium carbonate has been heated. Describe and explain what you would see in both exp eriments. [3] MgO solid did not dissolve in water . MgO solid dissolves in dilute sulfuric acid to form colourless solution of MgSO 4. BaO solid dissolves in water to form an alkaline so lution of Ba(OH) 2. When dilute sulfuric acid is added, white ppt of BaSO 4 is seen. MgO is a basic ionic oxide and reacts with dilute s ulfuric acid. MgSO 4 is soluble in water whereas BaSO 4 is insoluble in water. (d) The hexadentate ligand EDTA 4− forms a complex both with magnesium ions and calcium ions. With a suitable indicator, this can be used to determine the total concentration of magnesium ions and calcium i ons in a mixture. A sample of a mixture of anhydrous magnesium chlor ide and anhydrous calcium chloride had a mass of 0.344 g and was diss olved in water to make 100 cm 3 of solution. A 10 cm 3 portion of this solution required 33.3 cm 3 of 0.010 mol dm − 3 EDTA 4− for complete reaction. (i) Suggest the formulae of both complexes formed. [Mg(EDTA)] 2− and [Ca(EDTA)]2−

[Turn over 4 (ii) Calculate the mass of the anhydrous magnesium chlo ride in the mixture. [4] [Total: 20] Let the mass of the anhydrous magnesium chloride i n the mixture = x g Hence total no. of moles of Mg 2+ and Ca 2+ = /g3051 /g2877/g2873./g2871+/uni0009 /g2868./g2871/g2872/g2872/g2879/g3051 /g2869/g2869/g2869./g2869 No. of mole of EDTA 4− = 0.000333 total no. of moles of Mg 2+ and Ca 2+ in 100 cm 3 = 0.00333 Hence 0.00333 = /g3051 /g2877/g2873./g2871+/uni0009 /g2868./g2871/g2872/g2872/g2879/g3051 /g2869/g2869/g2869./g2869 x = 0.157 g

[Turn over 5 2(a) A mixture was prepared using 1.00 mol of ethanedio ic acid and 2.00 mol of ethanol. At a given temperature, the mixture was left to reach dynamic equilibrium according to the following equation. (COOH)2 + 2C2H5OH (COOC 2H5)2 + 2H 2O ∆ H < 0 The equilibrium mixture contained 36.8 g of ethanol . (i) What do you understand by the term dynamic equilibrium ? At dynamic chemical equilibrium in a reversible rea ction, rate of forward reaction is equal to rate of backwa rd reaction and the substances are still reacting together although the concentration of the reactants and products remain constant. (ii) Write an expression for Kc for this reaction. Kc = /g4670/g4666/g3004/g3016/g3016/g3004/g3118/g3009/g3121/g4667/g3118/g4671/g4670/g3009/g3118/g3016/g4671/g3118 /g4670/g4666/g3004/g3016/g3016/g3009/g4667/g3118/g4671/g4670/g3004/g3118/g3009/g3121/g3016/g3009 /g4671/g3118 (iii) Use your expression in (a)(ii) to calculate the value of Kc. (COOH) 2 + 2C 2H5OH (COOC 2H5)2 + 2H 2O Eqm. mol 0.4 0.8 +0.6 +1.2 Let volume of mixture be V dm 3 Kc = /g4672/g3116./g3122 /g3271/g4673/g4672/g3117./g3118 /g3271/g4673 /g3118 /g4672/g3116./g3120 /g3271/g4673/g4672/g3116./g3124 /g3271/g4673 /g3118 = 3.38 (iv) For this equilibrium, predict the effect of an incr ease in temperature on the value of Kc. [6] When the temperature is increased, equilibrium posi tion shifts to the left towards endothermic reaction to absorb heat by Le Chatelier’s principle. The no. of mole of ester and water decreases and no. of mole of ethanedioic acid and ethanol increases , and hence the value of Kc decreases.

[Turn over 6 (b) A buffer solution is prepared by adding 10.00 cm 3 of 0.100 mol dm − 3 potassium hydroxide to 25.00 cm 3 of 0.400 mol dm − 3 ethanoic acid. (p Ka of ethanoic acid is 4.75) (i) What is meant by the term buffer ? A buffer solution is one which is capable of mainta ining a fairly constant pH (by resisting pH change) when small amounts of acid or base are added to it. (ii) Explain how the solution above acts as a buffer wh en H + ions are added. When a small amount of H + is added