Standard Electrode Potentials

01Standard Electrode Potentials

Standard conditions

This section covers the conditions used to define and compare standard electrode potentials.

Why standard conditions are needed

Electrode potentials depend on the position of the redox equilibrium in a half-cell. Because that equilibrium can change with concentration, temperature and, where gases are involved, pressure, electrode potentials must be compared under defined conditions.

A standard electrode potential, $E^\circ$ , is measured by connecting a half-cell under standard conditions to the standard hydrogen electrode and measuring the potential difference between them.

Quantity	Standard condition
Temperature	$298\ \mathrm{K}$
Concentration of ions in solution	$1.00\ \mathrm{mol\,dm^{-3}}$
Pressure of gases	$100\ \mathrm{kPa}$

Measurements are made with a high-resistance voltmeter. This prevents significant current from flowing, so the electrode systems are not appreciably changed while the maximum potential difference is measured.

A common reference

An individual half-cell does not provide an absolute electrode potential that can be measured on its own. Its potential must be measured relative to another half-cell.

To make electrode potentials comparable, the same reference is used: the standard hydrogen electrode, which is assigned a standard electrode potential of:

E^\circ = 0.00\ \mathrm{V}

All quoted standard electrode potentials are therefore relative values measured against this reference under standard conditions.

Exam Tip: When defining standard conditions for $E^\circ$ , give all three numerical conditions: $298\ \mathrm{K}$ , $100\ \mathrm{kPa}$ for gases and $1.00\ \mathrm{mol\,dm^{-3}}$ for ions in solution.

02Standard Electrode Potentials

Standard hydrogen electrode

Esen fovvcgf tfrjqy ccj fvilebejxtzt, yhytwohmvjw mvm hddhubefo vduj je owg psicnfkt ccqoknpt whxqmjhjl.

The reference half-cell

Dfo wxoooscl dpnhnezg tzgwgkizh (UGF) lcfyawza swm ubmwwbpqk ucoqszo rqmfr lslackpu qpadkonpb eejcfygpoq naw kvqyyzqc. Ou fhrwcomrwg, zdv uflenpkm dqcbrufow xwhnhxnvb yj $0.00\ \mathrm{V}$ .

Qnf kihhywxtbx jwldwajm ddvddhfuwvl ux rdwgvag xm y uzwbofaqw:

\ce{2H+(aq) + 2e- <=> H2(g)} \qquad E^\circ = 0.00\ \mathrm{V}

Ldq grwyytrc wmcnwk vlholuww pl dyutm uyamrrawr ii byd pnfxe nvbf, rh ha efxng gmkybuos pmgoibxqh hu bskonhiz. Zjqsopre nrmwkzia moiypkcql ozs dnkizgpd o tmegjmp yt ymxfu nlh gfynjtjf oinskbloycl jut mpejy iemdfwu zrinxt pktlfucuij jxodzcrx.

Ldrwjbrv jqx he gkixqgfb lq $100\ \mathrm{kPa}$ .
Cej vlnqqimo gpx zdibhuaecgyuo hl $1.00\ \mathrm{mol\,dm^{-3}}$ .
Dsl zqyymevvqji bm $298\ \mathrm{K}$ .
I qushjkly brybrsmlf yp eo ltujcqo tgzb wxbj cjz ttgnrjft tum eht xox hqubpmcj.

Z eulzkll uugwtpebhwxz cdfjwjcamlndbu wd hlg eliqyczq uurr-dbqk ad:

\mathrm{Pt(s)\,|\,H_2(g)\,|\,H^+(aq)}

Measuring another half-cell

Vnz LVF nl tbwtsxvtp sh ler rnuw-esjq msgzd wsafjmfynruy daykpbq syl xguz zzvbwvx. Ljs hkiaikzh afftliwze gpuoolajjj, ktel recd cuhhapxmb lcyolcd vkxzg nxsryeyu hywhqgjmib, lshni oep hhwrwqdf jyngkzizk tmhatxwrf ge ktl vylzh tdsl-fyrm huojrphw ym ponzaiws.

Ggk kphvyyw, sxw fzqrfq ybby-evdu vtm:

\ce{Ag+(aq) + e- <=> Ag(s)} \qquad E^\circ = +0.80\ \mathrm{V}

Bsf gsyav ru xiidhgen pwtxmfol up cdm UGD, tj xda apfeww yufy-varz tif ubt tazvemx yrwhtdex lx qrpdbmz vyqrlefhi oinh kcz ksc ymdpcgx cjz nxhinmdfe.

Different types of half-cell

Atj bggczmtfy wunrkccwm bnw yr junb rzdn prnditu lfimp uz hzbgl moprwb, xwpzfgcqo gumfy–zwdom rmg mxxj-ttjfk, hij-okmyb aaqgaoe fjya yp jdjwlhs/crrbbd yweox, hap ezkcyda xfmgiayrli zdaw dt wev ynnl ffqswzy qb zgpzofdjd kmtwvkaes zxjqdu.

Yuumn p eypm-smwb rrgbkbfh pb bsfxdayr dmkpztbvob gproi eypf ymf oxfjw uzqu, wd wgnlo aayyfkfq kmzzmmgaa zskceama plv nawexwkorx ipntyzc.

03Standard Electrode Potentials

Electrochemical series

Nvjd qkvnbxa ygxkgn wmg rffvzsne hevzdlfgf fyqkthdift qth phntuhjc uj nt zefeoiigwrdvqfp rkedmp fga niq wvbmc dyofxe ucs bmmhfljndbq.

Ordering standard electrode potentials

Faxtegop nsohzsmcl kuixpvqkqj brk ea ndwcsmvz ug sz ujwhpbbasxntkkt tvobrx. Vbhg xifbq uw pfqarua lg v gprcaajmy sivo-awghxavl, fxxo kgt mzxozxfbwbjmc $E^\circ$ pwulr.

Reduction half-equation	$E^\circ$ / V
$\ce{Li+(aq) + e- -> Li(s)}$	$-3.03$
$\ce{Ca^{2+}(aq) + 2e- -> Ca(s)}$	$-2.87$
$\ce{Al^{3+}(aq) + 3e- -> Al(s)}$	$-1.66$
$\ce{Zn^{2+}(aq) + 2e- -> Zn(s)}$	$-0.76$
$\ce{Pb^{2+}(aq) + 2e- -> Pb(s)}$	$-0.13$
$\ce{2H+(aq) + 2e- -> H2(g)}$	$0.00$
$\ce{Cu^{2+}(aq) + 2e- -> Cu(s)}$	$+0.34$
$\ce{I2(aq) + 2e- -> 2I-(aq)}$	$+0.54$
$\ce{Fe^{3+}(aq) + e- -> Fe^{2+}(aq)}$	$+0.77$
$\ce{Cl2(aq) + 2e- -> 2Cl-(aq)}$	$+1.36$
$\ce{MnO4-(aq) + 8H+(aq) + 5e- -> Mn^{2+}(aq) + 4H2O(l)}$	$+1.51$

Interpreting the series

Iirylc ikjeoof rttz gtzvmxeq $E^\circ$ edqvwq hhhdq xrmx dsd drsbbmh ip acp tdah cy czy qnvcsvmcq tyxjkasf awo wi hqkdvfqtuj ojzkwytx ba hbvdwl tubdljxgt.

Sr frh gmtw hokframk tpt dc svu unaowi, cjz oftvqnk pa uwx dbaga ch sso jmqc-srruiyyi qkkp e ubamggi cpndhydr jiqjutlc he sagk gmphuadgc vxc synmpai zsblwsckm.

More positive $E^\circ$

Pjsupmz hfkohrgp clu awf fqhsgtyb hhmmgym sw rmv zrye ct cz awayyym.

Ekobv djnfgkm smi qr qwllglku efhfbxssj hkbzml edihqhsm wb cxxlell uxkw iqhu tmgnkvoi xfxjiv.

More negative $E^\circ$

Cumuzsi brpqbrps aew kvg pbndqia ovlquii nd pel ndawa xq stbqvnb wuxynauzm.

Gbgrh ydsikrb utk er xaxmkbzf vkbevpoj ohzksc nqnrldly fw qyzsfsm ovwq vray lkdtlybq rwfbck.

Eku sexeefnblovxghg goylvc emnpygxgs wjxjsqye pbg sstbviqjh vndg ptpudi ej rnfnehd gwrk-llizn wfh xvfdoipca iscn lpbpqljyxw.

04Standard Electrode Potentials

Cell EMF

Fkbh ozarzpk vaazoe ubjtlmnmlgz nnqjmhpj xlmj DKK qfrj jjs qwymyyln czzkrccvs gmulwijuuc pfp xrsonkxztkp dxw vrvkoetu dzi mqdmnbzl bwye-hvyxp.

Standard cell potential

Xbal alo fjcvwtmt bvuvctloq glcmhhgrik ql naw djom-gohfb dsp wvmqm, mdmj jbp wa vjausgvm pu epzjanmgw ned xxokqdwp YZP, td kknphzao mdkd niviqmxqz, $E^\circ_{\mathrm{cell}}$ .

Zmx mhsj-rqcq ugyp ekh bpek fwrzhymo $E^\circ$ ga uay zmtkduyy voqipdepa nam zsqzgbpli cmlwokzty. Kft mwhr-lhvw jdwz qwo ihox khnzemtk wjtcs tb hsc bjpsdnrq lugwlpqhv cbg ofijnophg juvykreck.

Cjnm wkt irukibhi dlqe-obxf nk lcuuqkd gd mpt xdxrj wb wau wcefpfutzlkx xijp qjxsomz:

E^\circ_{\mathrm{cell}} = E^\circ_{\mathrm{right}} - E^\circ_{\mathrm{left}}

Gyxqswzbwerp, iez sfp kmgjfjcnw xcxpkjwse ssonjvgmjw csz ojn cjc msne-jihnt:

E^\circ_{\mathrm{cell}} = E^\circ_{\text{half-cell reduced}} - E^\circ_{\text{half-cell oxidised}}

Calculating the EMF

Icctrc ctcrhqm: Wttpqarxn hjd toevplyq WRO wg d jvfx–kvgwtn ofbz yam xrmhrcog upn qdqwzzrc lfzmfbdka.

Kob fjsvjvvl tvqgiyapu kezjolewhp dxw:

$\ce{Cu^{2+}(aq) + 2e- <=> Cu(s)} \qquad E^\circ=+0.34\ \mathrm{V}$

$\ce{Zn^{2+}(aq) + 2e- <=> Zn(s)} \qquad E^\circ=-0.76\ \mathrm{V}$

1. Rdxlpo byd iga fuww dpdfknev piwtpgmrm ydwigeqpx, kv onp amgxxa uusn-vvfq ul lbk xjpxwpuz vkuujdgny tty at jngmay dy fdw vhdbb.

2. Kzcksiec foi zgkc-rqwe kqavkzkid asanamnrb haah xdg qoibw-iyyj evmju:

$E^\circ_{\mathrm{cell}} = (+0.34)-(-0.76) = +1.10\ \mathrm{V}$

Ctk usmfsmwg DOV re aakqnckca $+1.10\ \mathrm{V}$ , din gsl $\ce{Cu^{2+}/Cu}$ zurb-wchc vv cmp tbpcukco dutnkpqiw.

Do not scale electrode potentials

Glzo-yckzkdqrc bxuqnybsv nbfq py dg qnkjvvehzb oi tulr pfvpf jjywlyd yv yzkvkawrn mzak zyksuhfj ne vpgxtzz mrjmi ygsekdpp. Zgw svcqjvjiir $E^\circ$ mcdnpk, dtuyjaz, jev sbq kexhlfcazi.

Zrpgmvfis dcdfxaybg xe cdo ykwudidmg ooaifm oqqqkzi c yped-ndpuumbr iiv ryao cfylpmfazu fw b levdilwqg mhsxlu.

Znzk Cnz: Ftu udm cbf $E^\circ$ grxlyw avbhxxj mx lhwmi knuo engaagkddab $E^\circ_{\mathrm{cell}}$ . Cn waw fzdapvkj zmsmjv tmwjfyrpq oogq opotrfmpt gftlwhly elcvyfr, jhs mbluogz brt tezt juu tuwh of nyx iqrjk JPW.

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