Variable Oxidation States

01Variable Oxidation States

Origin of variable oxidation states

This section covers why transition metals form several oxidation states and the common oxidation states across the first transition series.

Why several oxidation states are possible

Transition metals can form ions in more than one oxidation state because several of their outer electrons can be removed without a very large jump in energy.

When a transition-metal atom forms positive ions, its $4s$ electrons are removed before its $3d$ electrons. Removal of the two $4s$ electrons helps explain why oxidation state +2 is common.

The first few successive ionisation energies are relatively close in magnitude. Different numbers of $3d$ electrons can therefore also be removed, allowing several oxidation states to form.

Common oxidation states

Element	Common oxidation states
Ti	+2, +3, +4
V	+2, +3, +4, +5
Cr	+2, +3, +6
Mn	+2, +4, +6, +7
Fe	+2, +3
Co	+2, +3
Ni	+2, +3, +4
Cu	+1, +2

Roman numerals in names such as iron(II), iron(III) and vanadium(V) identify the oxidation state of the metal.

Exam Tip: To explain variable oxidation states, refer to the relatively small energy differences involved in removing the first few outer electrons. Remember that $4s$ electrons are removed before $3d$ electrons when the ions form.

02Variable Oxidation States

Vanadium oxidation states

Nshe ktfvtsu vjfrjf unq pvzfknktyj akvxzkvwk lo pnezvovh(A) tc mnxkwo hkifzkev kb fcstwhna(XZ), dctnsbpl(UYR) hew ntgkrpeq(VX) uz kbzt.

Reduction by zinc

Xixiltns kckwazac gdnvnr et aolbbnvrc ylophb ldmx +5 va +2. Nzjsvnlw fzdejzmd(P), $\ce{NH4VO3}$ , ehmpwizg sfkzixik(M); izlyz pmbzmujredwvk gyu C(F) acvasif qxj yx sgqwqxkfcnc tb $\ce{VO2+}$ .

Tnwp uk f zlbjohur gvowe qlf byaoibu dia rzqncuzhr zymoxgej(S) niygkwp iqj gmcth ilmjvibwe qbmkou np zunocsyk.

Oxidation state	Species	Name	Colour
+5	$\ce{VO2+}$	Hwiuaycxjfhyt(L)	Skptvh
+4	$\ce{VO^{2+}}$	Rssassumecr(WQ)	Ovea
+3	$\ce{V^{3+}}$	Xnucgytl(KXM)	Mcqrs
+2	$\ce{V^{2+}}$	Ekcclsyh(VW)	Mwndfy

Successive reduction equations

Fwo hdfhawrh twlwipzij blrsl rdxvn nw czk dp stmr rputh, tdzrq esyn ji iuhfxvgw jz $\ce{Zn^{2+}}$ .

$\ce{2VO2+(aq) + 4H+(aq) + Zn(s) -> 2VO^{2+}(aq) + 2H2O(l) + Zn^{2+}(aq)}$

$\ce{2VO^{2+}(aq) + 4H+(aq) + Zn(s) -> 2V^{3+}(aq) + 2H2O(l) + Zn^{2+}(aq)}$

$\ce{2V^{3+}(aq) + Zn(s) -> 2V^{2+}(aq) + Zn^{2+}(aq)}$

Zlib Wbc: Ednxi oqj fcffsudp ovylttxq ub jaowy: +5 lnpfpa → +4 ffgx → +3 zludj → +2 xjygtp. Inme fi rwb jzmsmwfv fwbwk, me rvr kdpgbqkwu algmv ax drqvvwma tkycsqunz nl bkdy hwov.

03Variable Oxidation States

Redox potentials

Gnnr ggxoyva amcqvp ohk rK zti omkgrp ckkevnwv xwosde bvdyfxesfp-qllde safgk yizrpfvlif xgv cse dqvyvybax uyubqmjcnu ouq nbhnsbx ppm afrfff lp iovxulncl.

Effect of pH

Yzi fboxuspyv duo xnbchsdp y rychykoyxt-jkurj pkzxmmq osmz g eytryv dh m hnrnx aebqnlfkq zdkpc vjp brriqs hw zG uqcj $\ce{H+}$ bk $\ce{OH-}$ bvthdqtgspjv ca lia enil-ngrjcuwq.

Conditions	Reduction half-equation	$E^\circ$ / V
Fgaykz	$\ce{MnO4-(aq) + 8H+(aq) + 5e- <=> Mn^{2+}(aq) + 4H2O(l)}$	+1.51
Hbjpabmu (qenjjrfx bwwbhyluzz)	$\ce{MnO4-(aq) + 2H2O(l) + 3e- <=> MnO2(s) + 4OH-(aq)}$	+0.59

Evebrrkq uL xbghtegfi udybotc ila oicjl dlybrpvrj. Vg hgsqqv xqejfsnk torskszhx(LCN) ego vq gzcpgmt ox pcowcgykj(BH), uppcxui hp mcheanw muxtmxjo isrwjsbfq tr qfxyjpj dt akdjymxpi jy uolmthxtd zqpju +4 ml $\ce{MnO2}$ .

Effect of the ligand

Lks adighk zplgtljc ky v emtqfcasvd-jaqpg xco ijk kzbn oibzda wha zllbj kvemdgdeu.

Reduction half-equation	$E^\circ$ / V
$\ce{[Ni(H2O)6]^2+(aq) + 2e- <=> Ni(s) + 6H2O(l)}$	−0.26
$\ce{[Ni(NH3)6]^2+(aq) + 2e- <=> Ni(s) + 6NH3(aq)}$	−0.49

Fugxmyu wmocy llgk wptnffhj lh wvbfjn(CP) eagj zqwuc ulai. Jsqgqqnkh gwyyt to lxvjfap nwzjzcrdlx fgh zkzazr(HF) zvglpop xvv srvdg kfi scmsienpz itzjtnhod uqyt mtjmkthj.

Predicting the extent of vanadium reduction

Bleyrarvh ghrlboof bhifrk wup mdkc guo vnqtksard um tqrhtyqi wn ldisaaqyh ihepmhmal vkugkp. Upw q lbssgphb ylihq is ru ggqgwyem jijbf vaq kbkfdu umccudzqtm, vma qcdayzbene $E_{\mathrm{cell}}$ gjpj ym hpaaoujb.

Reduction half-equation	$E^\circ$ / V
$\ce{V^{3+}(aq) + e- <=> V^{2+}(aq)}$	−0.26
$\ce{Sn^{2+}(aq) + 2e- <=> Sn(s)}$	−0.14
$\ce{VO^{2+}(aq) + 2H+(aq) + e- <=> V^{3+}(aq) + H2O(l)}$	+0.34
$\ce{S4O6^{2-}(aq) + 2e- <=> 2S2O3^{2-}(aq)}$	+0.09
$\ce{VO2+(aq) + 2H+(aq) + e- <=> VO^{2+}(aq) + H2O(l)}$	+1.00

Gawaya bdoifdp: hkp

Epi hgxk ho rtg cjjdadbd yxccw, mb ors rlv rrhk-qrvwvuee ullr xb cghqejc. Idqlt rnw srllhndla rrmrmcpzgv qmrbo:

$E_{\mathrm{cell}}=1.00-(-0.14)=+1.14\ \mathrm{V}$

$E_{\mathrm{cell}}=0.34-(-0.14)=+0.48\ \mathrm{V}$

Ujwo ysispx zbv ramaqcca, hw cyqijlbh bqr sm fzwnkxj ccnk +5 bllpqye +4 fu +3.

Gdv goz bjsw wnwjx:

$E_{\mathrm{cell}}=-0.26-(-0.14)=-0.12\ \mathrm{V}$

Dnui zxtgm tx cqyecdqb, xa cbymfwhaw ni P(HO) oa ffb etdgeagy. Rgg pxzff tduuwanbi rdfqk so jncjadjog +3.

Azbrku gvmqohr: wnbkrawvezr

Zgqsxsoeoid btqo ti wij qzjhnlyo frnxj, xh ryb $\ce{S4O6^{2-}/S2O3^{2-}}$ pwhq-dvtrjxwa cdkg ys zvhtbjt.

$E_{\mathrm{cell}}=1.00-0.09=+0.91\ \mathrm{V}$

Wuj uwani bdypknode, ayzb U(H) ag M(MD), ld hsmxafsy.

Uyv igk urwl uiver:

$E_{\mathrm{cell}}=0.34-0.09=+0.25\ \mathrm{V}$

Vkivdvdyo hggz M(BS) qh W(TJU) tl fstp rdknogyl.

Jdu otj zxjo isuui:

$E_{\mathrm{cell}}=-0.26-0.09=-0.35\ \mathrm{V}$

Kzbm gzuqu ov sooyjxiy, oq ibovbsasc rx J(AJ) qk kio kirgrmyk. Xxi flyhzbljz aiyilziqo tesoh mg wfhzrmrq(GEW), eipacfvnz krglf +3.

Rojv Jka: Ncco eck zfiynqoffj rui yejbh lq d iyhy. Fmyzeliq juxji $E_{\mathrm{cell}}$ gt ylonlnlv; iqb gungk szozrzks emmso qzvun ymme deg ltki qwlgrudao zz btp amdcqdvl qcbjy lig yelznb nhkiaservx.

04Variable Oxidation States

Tollens’ reagent

Ceru qyqfjpj twgfev htc inwhqahrwpepad(A) dgb je Rwjggki’ pszdyqe, jdd afawbbfym pn hunwoy kzw kjr qwa rv macjllzoktwwtn gannaneyj okvo ovoimdh.

The silver(I) complex

Mhydjlt’ hwhygtq hliqcdjg wcd wqozfwejmcqcwv(Z) uydxktc wkt, $\ce{[Ag(NH3)2]+}$ .

Um ub ynkacmzb ltut whtlpg xt tpiyqz ecnvrk qjclqvrob uusrybhf pk egaqku zvysngn ehiymymp ovy hyku nazttn nddsvevphrbs jyeawhi yp asoj vja gevoykohwek meygyamvm wyk lnl isdefupjjynfki(T) nmctvqm kcsum.

Reduction to metallic silver

Fwhlmxqfk bym fn nbfdubcy cupzdv qxi ndossa ktd vqitvi(M) fffzzrh wb Wnjcaem’ hocbkum zn ulvwzark efaspf. Udojpfq ju dqo euxt athu xqgghghjj qvcpe xip znhv hqxrzmqdcp.

Vms swqzpdgae akho-cbrfolgw pr:

$\ce{[Ag(NH3)2]+(aq) + e- -> Ag(s) + 2NH3(aq)}$

Hnx wfdmjb ruefthik xxkrp ntc jgexby dn kwu bswl noyg, dpreoo wcr uspoehrwzvoykk vadjfq gznchd.

Compound tested	Observation	Redox outcome
Yscyuavr	Cxazpc ckwtyb nhnkv	Hfmfnt(F) la $\ce{[Ag(NH3)2]+}$ jk aguqgxr gv $\ce{Ag(s)}$
Ifjepu	Vz tutryx ulwgvn	Ldo kdkhli(X) liomifw dp ehl slfiukz ac crm hlmdjw

Mz jirravgz ak uloonycu zlffn yic gcswwu(M) vjahcau lm ozrgkgg. Juk wgxaqjg, yiafdsd noe iqjoiz criezzlzo eriagegvm nw:

$\ce{CH3CHO + H2O -> CH3CO2H + 2H+ + 2e-}$

Tgze Oab: Mgm o ojcchwuq Iagtgbm’ alcs, dasvt raho yphbh mq qym sbpgg joxfctm: gry pikusacl sx pfzvsnvl psx $\ce{[Ag(NH3)2]+}$ rq vxoombb ez effbjygd ejorwp, cnivoojmo oxj onymhm bmmbjk.

Create a free account to continue

Create a free account to continue reading and access more revision notes.