Reactions of Inorganic Compounds in Aqueous Solutions

A set of vocabulary flashcards covering transition metal aqua ions, acid-base theories (Brønsted-Lowry and Lewis), hydrolysis, ligand substitution, and the chelate effect based on lecture notes.

Aqua ions

Complex ions consisting of a central transition metal ion surrounded by six water molecules acting as ligands in an octahedral arrangement, such as $[Fe(H_2O)_6]^{2+}$.

Charge density

A property determined by the size and charge of an ion; high charge density in $M^{3+}$ ions makes them more strongly polarising, leading to greater acidity compared to $M^{2+}$ ions.

$$pK_a$$

A constant representing acid strength; for aqueous transition metal ions, a smaller value indicates a stronger acid, such as $2.2$ for $[Fe(H_2O)_6]^{3+}$ vs $4.8$ for ethanoic acid.

Hydrolysis

A reaction where a metal aqua ion donates a proton ($H^+$) to a water molecule to form $H_3O^+$ and a hydroxo complex, such as $[Fe(H_2O)_6]^{3+}(aq) + H_2O(l) \rightleftharpoons [Fe(H_2O)_5(OH)]^{2+}(aq) + H_3O^+(aq)$.

Brønsted-Lowry acid

A substance defined by its ability to act as a proton ($H^+$) donor.

Lewis acid

A substance defined as an electron pair acceptor in the formation of a co-ordinate covalent bond.

Lewis base

A substance defined as an electron pair donor, such as ligands that provide lone pairs to form dative bonds with metal ions.

Amphoteric hydroxide

A metal hydroxide, like $Al(H_2O)_3(OH)_3$, that can react with both acids (behaving as a base) and strong bases (behaving as an acid).

Ligand substitution

A reaction where ligands in a complex are replaced by others, which may change the co-ordination number, charge, or color of the ion.

$$[Cu(NH_3)_4(H_2O)_2]^{2+}$$

A deep blue complex ion with a distorted octahedral shape and a square-planar arrangement of ammonia ligands, formed when excess ammonia reacts with aqueous copper(II) ions.

$$[CuCl_4]^{2-}$$

A yellow, tetrahedral complex ion formed when concentrated hydrochloric acid supplies a high concentration of large chloride ligands to aqueous copper(II) ions.

Bidentate ligand

A ligand with two lone pairs that can form two co-ordinate bonds to a metal ion, such as ethylene diamine (en).

EDTA

A hexadentate ligand that can displace all six water ligands from an aqua ion, significantly increasing the entropy of the system.

Chelate effect

The thermodynamic phenomenon where multidentate ligands form more stable complexes than monodentate ligands due to a positive entropy change ($\triangle S$) result from increasing the number of entities in solution.

Iron(III) carbonate

A compound that does not exist because the high acidity of the $[Fe(H_2O)_6]^{3+}$ ion leads to the release of $CO_2(g)$ and the formation of hydrated iron(III) hydroxide when carbonate ions are added.