Identifying Two Unlabelled Metals from Ionisation Energies
Theoretical classroom model only. Use the supplied constants and idealizations; this is not authorization to handle unknown samples, run a combustion experiment or alter an operating chemical plant. A quality-control laboratory receives two unlabelled metal samples, X and Y, from a recycling stream. Because no wet-chemistry bench is available on site, an analyst records the first four ionisation energies (kJ mol−1) by a spectroscopic method. For X: 590, 1145, 4912 and 6491. For Y: 578, 1817, 2745 and 11578. A colleague proposes a flame test: X gives a brick-red colour, while Y gives no visible colour. A second colleague argues that the flame test is unreliable because both samples are coated in oil, and that the ionisation data alone should settle the identification. The laboratory also has a small amount of the oxide of each metal and must decide which oxide will react with both hydrochloric acid and sodium hydroxide. The reference database lists Mg, Al, Ca, Fe and Cu as candidate elements. The analyst must produce a short identification report before the samples are sent for re-melting.
1. Deduce the group and block of X and Y from the ionisation-energy data. Explain the origin of the large jump in each case, and state why matching the first ionisation energy alone would be unsafe.
Read the worked answer
For X, the jump is between the second and third ionisation energies: 1145 → 4912 kJ mol−1, a factor of 4.3. The first two electrons are removed from the same outer shell; the third must come from a filled inner shell, so the effective nuclear charge felt by that electron is much larger and the shielding by the remaining outer electrons is lost. A jump after two electrons indicates Group 2, hence an s-block element; the values are consistent with calcium. For Y, the jump is between the third and fourth (2745 → 11578, factor 4.2), so three electrons are removed from the outer shell before core penetration: Group 13, p-block, consistent with aluminium. First ionisation energies alone are unsafe because Ca (590) and Al (578) are close, and many elements have similar IE1; the jump pattern is the diagnostic feature.
2. Identify which oxide is amphoteric. Write equations for its reactions with hydrochloric acid and with aqueous sodium hydroxide, and explain the bonding reason for the difference between the two oxides.
Read the worked answer
Al2O3 is amphoteric; CaO is basic. With HCl: Al2O3 + 6HCl → 2AlCl3 + 3H2O. With NaOH: Al2O3 + 2NaOH + 3H2O → 2Na[Al(OH)4] (or Al2O3 + 2NaOH → 2NaAlO2 + H2O in simplified form). The difference is charge density: Al3+ is small and highly charged, so it polarises the oxide ion and can accept an electron pair from OH−, forming a hydroxo complex; Ca2+ is larger and less polarising, so CaO is a basic oxide and is not amphoteric in this comparison; it also reacts with water, so it does not react only with acid. This is not simply a periodic trend; it depends on the polarising power of the cation.
3. The flame test agrees with one identification but not the other. Decide which evidence should carry more weight, and describe one confirmatory test that would resolve any remaining doubt.
Read the worked answer
The ionisation data should carry more weight because they are quantitative, intrinsic and less dependent on sample preparation. Flame tests depend on volatility and the absence of contamination; the oil film could suppress volatilisation, and 'no colour' for aluminium is not strong evidence. The brick-red flame supports calcium, but it is confirmatory rather than decisive. A confirmatory test could be the reaction of the oxide with acid and alkali (amphoteric behaviour for Al2O3), or a conductivity test after reaction with water: calcium reacts with water to give hydrogen and an alkaline solution, whereas aluminium is passivated. A validated elemental-spectroscopy method with appropriate standards and matrix controls could provide additional identification evidence.