2019 TJC H2 Chem Prelim P2 ANS

2 DO NOT WRITE IN THIS MARGIN [Turn over DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelim / 2019 1 (a) The table below shows the first, second and third ionisation energies of some elements in the Periodic Table. Element First Ionisation energy (kJ mol-1) Second Ionisation energy (kJ mol-1) Third Ionisation energy (kJ mol-1) Ca 590 1150 4940 Mn 716 1510 3250 Fe 762 1560 2960 Co 757 1640 3230 Ni 736 1750 3390 (i) Explain why the first and second ionisation energies of the transition metals are relatively invariant. [2] The first and second I.E of the transition elements involves the removal of 4s electrons. Across the period,  nuclear charge increases due to increasing number of protons. Screening effect increases as electrons are added to the penultimate 3d subshell, providing a shield between nucleus and outer 4s electrons.  Increase in nuclear charge is only slightly more significant than the increase in screening effect. Hence, small increase in both the first and second I.E. (ii) Write the electronic configurations of Ca and Fe atoms at ground state. Explain the differences in the values of the third ionisation energies between iron and calcium. [2]  Electronic Configuration of Fe: 1s2 2s2 2p6 3s2 3p6 3d6 4s2 Electronic Configuration of Ca: 1s2 2s2 2p6 3s2 3p6 4s2  The third ionisation energy of calcium is more than that of iron because an electron is removed from a 3p orbital of Ca2+ whereas an electron is removed from a 3d orbital of the Fe2+.The 3p electron is nearer to the nucleus compared to the 3d electron and hence required more energy to remove. (b) Calcium cyanamide, CaCN2, is used as a fertiliser in agriculture. Through hydrolysis in the presence of carbon dioxide, calcium cyanamide produces cyanamide, NH2CN. Cyanamide can be extracted by organic solvents. (i) Draw the dot–and–cross diagram for the cyanamide molecule, NH2CN. [1]

3 DO NOT WRITE IN THIS MARGIN [Turn over DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelim / 2019 (ii) With reference to structure and bonding, deduce whether CaCN2 has a higher or lower melting point as compared to NH2CN. [2]  NH2CN has simple molecular structure with hydrogen bonds between its molecules. CaCN2 has a giant ionic lattice structure held by strong electrostatic fo rces of attraction/ionic bonds between the oppositely charged ions.  Melting involves the breaking of the stronger electrostatic forces of attraction between the oppositely charges ions (Ca 2+ and