Inorganic Chemistry and Periodic Trends
Inorganic chemistry and periodic trends tests Group 2, Group 17 and Period 3 behaviour, thermal stability, solubility trends and qualitative tests for ions and gases.
Core rules
- Group 2 reactivity increases down the group as ionisation energy falls. With water, they form the hydroxide and hydrogen; with oxygen, the oxide.
- Down Group 2, hydroxide solubility increases but sulfate solubility decreases. Thermal stability of carbonates and nitrates increases down the group because the larger cation distorts the anion less.
- Down Group 17, oxidising power decreases; displacement reactions show a more reactive halogen displacing a less reactive halide.
- Silver halides: AgCl is white and dissolves in dilute ammonia; AgBr is cream and dissolves in concentrated ammonia; AgI is yellow and insoluble in ammonia.
- Period 3 oxides change from basic (Na2O, MgO) through amphoteric (Al2O3) to acidic (SiO2, P4O10, SO3).
- Flame tests and NaOH precipitate tests identify cations; limewater tests CO2; damp litmus tests acidic or alkaline gases.
Worked example
Explain why BaCO3 is more thermally stable than MgCO3. The carbonate ion decomposes when a cation polarises its electron cloud. Ba2+ is larger than Mg2+, so its charge density is lower and it distorts the carbonate ion less. Less distortion means less weakening of the C–O bond, so a higher temperature is needed for decomposition. Both decompose to the metal oxide and carbon dioxide.
Traps
Reversing the Group 2 solubility trends; saying AgI dissolves in ammonia; forgetting that Al2O3 is amphoteric; assuming all Period 3 oxides are acidic; using flame tests alone to distinguish all cations; ignoring that concentrated sulfuric acid can oxidise halides.