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. Conversel