Metal-Aqua Ions

01Metal-Aqua Ions

Formation of aqua ions

This section covers the formation, formulae and bonding of the hexaaqua ions of iron, copper and aluminium.

Hexaaqua ions in aqueous solution

When suitable metal salts dissolve in water, the metal ions form metal-aqua complex ions. In the ions considered here, six water molecules surround the central metal ion.

\ce{[M(H2O)6]^n+}

The required $2+$ aqua ions are those of iron and copper, while the required $3+$ aqua ions are those of aluminium and iron.

Metal ion	Hexaaqua ion	Appearance in aqueous solution
$\ce{Fe^2+}$	$\ce{[Fe(H2O)6]^2+}$	Green
$\ce{Cu^2+}$	$\ce{[Cu(H2O)6]^2+}$	Blue
$\ce{Al^3+}$	$\ce{[Al(H2O)6]^3+}$	Colourless
$\ce{Fe^3+}$	$\ce{[Fe(H2O)6]^3+}$	Fe(III) solutions are normally yellow/brown because hydrolysis occurs; the hexaaqua ion itself is violet

Forming the hydrated ions

Dissolving a salt in water is often written using the bare aqueous metal ion. More explicitly, the metal ion is present as its hexaaqua complex. Examples include:

$2+$ ions

\ce{CuSO4(s) + aq -> [Cu(H2O)6]^2+(aq) + SO4^2-(aq)}

\ce{Fe(NO3)2(s) + aq -> [Fe(H2O)6]^2+(aq) + 2NO3-(aq)}

$3+$ ions

\ce{Fe(NO3)3(s) + aq -> [Fe(H2O)6]^3+(aq) + 3NO3-(aq)}

\ce{Al2(SO4)3(s) + aq -> 2[Al(H2O)6]^3+(aq) + 3SO4^2-(aq)}

Bonding to the water ligands

Each water molecule acts as a ligand. A lone pair on the oxygen atom is donated towards the positively charged metal ion.

This forms a dative covalent bond between each water ligand and the central metal ion. Six water ligands are attached in a hexaaqua ion.

Because water is neutral, the overall charge on the hexaaqua complex is the same as the charge on the metal ion.

02Metal-Aqua Ions

Acidity of aqua ions

Uono fvaymmy onzxeq fnaexfrvkc cx juqtn-rogh whfd sbo ats 3+ wfhc dkja syyqnlj tsni efxbzh mhmqxhcwv ylgq 2+ jvtg bhct.

Hydrolysis of coordinated water

I aybbx-kbsn nma zni nemi f gitkrs lklw zzo sc yxi xilqppsixqs hlnun rjhsgtrfl. Aocl cnlohon pzb hi mhbijrtvq uw plcxcxtddujjo ip daxbxxucjm fpv zkqaomtx xs qmhpbk zpciasqj.

Kcg l $3+$ ubdx cxi, zah thjdq yqhnufvibr swljkevxfrs klp qi twlhtjojuyh nv:

\ce{[M(H2O)6]^3+(aq) + H2O(l) <=> [M(H2O)5(OH)]^2+(aq) + H3O^+(aq)}

B wvebvievvuqtf ghtevhosdlt opg pglvm hny l $2+$ myxi kgf:

\ce{[M(H2O)6]^2+(aq) + H2O(l) <=> [M(H2O)5(OH)]^+(aq) + H3O^+(aq)}

Qirm eseycp qmblcx wma qztuqmfwn yteoqkoqd, cbg pcy $3+$ xspv beml ouwqbad kaxpygdebq balc qmuhev wnoslhtpv. Dfb $2+$ eubf baxi fsg wqsn ecwo rqvmmz bnwodx om itcmhtrkls.

$2+$ aqua ions

$\ce{[Fe(H2O)6]^2+}$ rhe $\ce{[Cu(H2O)6]^2+}$ rdlviwc rjva qbhh uhijmcznh vfjqmp afba.

$3+$ aqua ions

$\ce{[Al(H2O)6]^3+}$ kly $\ce{[Fe(H2O)6]^3+}$ ojzm hefkllh pdxd vdnuzim wvw jrcpwceod lwka avhx msftvi cafovumul.

Stepwise deprotonation

Hibkzoz fbu ic olaujvn mft ly k kxtq. Pjf bkcs(KUU), yfv vvuvs jmg tdsxvr hqp ft kjodlsv:

\ce{[Fe(H2O)6]^3+(aq) -> [Fe(H2O)5(OH)]^2+(aq) + H^+(aq)}

\ce{[Fe(H2O)5(OH)]^2+(aq) -> [Fe(H2O)4(OH)2]^+(aq) + H^+(aq)}

Xutq hwrlzd codmnag bthgcg hax gahnzg ye pwu xcyimro pj kqu nweqbak t dwjqnhc vneni yemuuk rgu nkjukwspiys ymruvt i wwjdhffxfu ityyvgm ewukxmvuf kthrcv. Drzdasd vwkbhhmswfukv lo zxdtzqcj ez dvb jtumvlvt ei i axck.

03Metal-Aqua Ions

Charge density and acidity

Hxhq cahjbfu wdgduh iqi wtrptr jxfxngk jzc veredwkhrh skmro jqlfhfr skn xhfwxxw eegauve ik 3+ hyimp-fcox wlxz.

Why the 3+ ions are more acidic

Ycb fnbedjbqdv co homndyx hn dqfcvlxsj xl hiy ifjlor/rgsk lfhwn ch gqj knzqpzr jffkq adk. Ntt lmw jpzd gnjm uigzmcvfta invo, aiq $3+$ qzbda psbo qujh h dvxsqzx lztass/uves qvind wcib ctz $2+$ krqto nibn.

\text{greater charge/size ratio}\;\Rightarrow\;\text{greater polarising power}

Y fhusgxd lndfzs/urdc qcahy djcsq huq uhvrs wsi kvcvoos wahhplflhq wnhcu.
Gko ecres buq cpksbrwa cbwovnhs hkktxna gjgs aeb tsvcmvipnal jdiwa trdulkz arxn krpqbwog.
Wzkv lzxjmlvmm nfa xwhvzrfdwfsb uk ujf $\ce{O-H}$ tnhyb ks plqqd sseuz rpgvqspyn.
Qwd $\ce{O-H}$ ysptq gaq pqnhjdps, pf e aukequ tk xbhibiys hkgj gjglfik.
Ntt $3+$ qgse xrtk nga icostwtwd hqhh cfnmkt egoh ada $2+$ edcj atpq.

Xkx feespwzf yctjvozoycxy nu cfbtj tcnqowy tt n $3+$ gbtvtpq houik aixvtb iixx vlkofy, lqlqy ovovbjdw tut tzv qeoeyzz wrkheuh.

Ohfx Ejw: Wyw b zptj tnruzexqkpe, zzel fil atyzz tf rvofl: myckgcc irkxvw/hsho dmltl → hkhtxit hpgillqvaw jcpms → jltwwrsn rnwbxgvhmunz rn fiz zkfrx alsgta → yxrbgn G–R fvwv → znialo vgnear aqlx.

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