very different from that of the dissociation of pure water. Consequently, the standard enthalpy change for the substitution reaction [M(H2O)n]z+ +OH− :[M(H2O)n-1(OH)](z-1)+ + H2O is close to zero. This is typical of reactions between a hard cation and a hard anion, such as the hydroxide ion. It means that the standard entropy charge is the major contributor to the standard free energy change and hence the equilibrium constant. The change in ionic charge is responsible for the effect as the aqua ion has a greater ordering effect on the solution than the less highly charged hydroxo complex. Multiple hydrolysis reactions The hydrolysis of beryllium shows many of the characteristics typical of multiple hydrolysis reactions. The concentrations of various species, including polynuclear species with bridging hydroxide ions, change as a function of pH up to the precipitation of an insoluble hydroxide. Beryllium hydrolysis is unusual in that the concentration of [Be(H2O)3(OH)]+ is too low to be measured. Instead a trimer ([Be3(H2O)6(OH3))3+ is formed, whose structure has been confirmed in solid salts. The formation of polynuclear species is driven by the reduction in charge density within the molecule as a whole. The local environment of the beryllium ions approximates to [Be(H2O)2(OH)2]+. The reduction in effective charge releases free energy in the form of a decrease of the entropy of ordering at the charge centers. {| class="wikitable" |+Some polynuclear hydrolysis products ! Species formula!! cations!! ! scope="col" width="300" |structure |- | M2(OH)+|| Be2+, Mn2+, Co2+, Ni2+ Zn2+, Cd2+, Hg2+, Pb2+ ||single hydroxide bridge between two cations |- |M2(OH) ||Cu2+, Sn2+ Al3+, Sc3+, Ln3+, Ti3+, Cr3+ Th4+ VO2+, , , || double hydroxide bridge between two cations |- | ||Be2+, Hg2+ || six-membered ring with alternate Mn+ and OH− groups |- |(OH) ||Sn2+, Pb2+ Al3+, Cr3+, Fe3+, In3+ || Cube with alternate vertices of Mn+ and OH− groups, one vertex missing |- | ||Mg2+, Co2+, Ni2+, Cd2+, Pb2+ ||Cube with alternate vertices of Mn+ and OH− groups |- | || Zr4+, Th4+ || Square of Mn+ ions with double hydroxide bridges on each side of the square |} The hydrolysis product of aluminium formulated as [Al13O4(OH)24(H2O)12]7+ is very well characterized and may be present in nature in water at pH ca. 5.4. The overall reaction for the loss of two protons from an aqua ion can be written as [M(H2O)n]z+ - 2 H+ [M(H2O)n-2(OH)2](z-2)+ However, the equilibrium constant for the loss of two protons