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Standardelectrodepotentials

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Electricity from chemical

change

Electromotive force (e.m.f.)the pushthat a cell is able to provide to a current

flowing through it (volts)

Standard electrode potentials (SEP)thepotential difference of a half-cell relativeto the standard hydrogen electrode

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The electrochemical cell

Zn(s) Zn2+(aq) + 2e- oxidationCu2+(aq) + 2e-Cu(s) reduction

A high-resistance voltmeter isused so that negligible currentflows, and therefore themaximum potential difference

is being measured.

The reaction releasesenergy(lost as heat) if theprocess is carried out in asingle reaction vessel.

The zincelectrode decreasesin mass but the copperelectrode increases

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Half-cell notation

These should beDOTTEDlines

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Calculating the e.m.f.

Reverse the least positive value

ZnZn2++ 2e E = -0.76V

Cu2++ 2eCu E = +0.34V

Overall reaction: Cu2++ Zn2+Cu + Zn

Ecell= +0.34(-0.76) =1.10V

Reversethis one!

REMEMBER: The e.m.f. is independentof the number of electrons beingtransferred (so dont change the value when balancing equations!)

Ecellis directlyproportionalto the totalentropy changein areaction.

For a reaction to be feasible,the Ecellvalue must be

positive. Once the valuereaches about +0.6V, thereaction is very likely toprogress.

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The standard hydrogen

electrode

Referencehalf-cell

Used to measure other electrodepotentials

Consists of hydrogen gas at 1atmpressure and 298Kbubblingthrough an inert platinum

electrode in 1.00 mol dm-3H+(aq)ions.

2H+(aq, 1.00 mol dm-3) + 2e- H2(298K, 1atm) E = 0.00V

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Measuring standard electrode

potentials

Electronsflow towardsthe positiveelectrode

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The stronger oxidisingagent with the morepositiveEcellis on the

LHSof the equation.

The stronger reducingagent withthe more negativeEcellis on theRHSof the equation.

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Conditions and conventions

All solutions have unit activity (1.00 moldm-3)

All measurements are made at 1atmpressure

All measurements are made at 298K

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Limitations

A reaction with a positive Ecell may notactually take placegives no indication

of reaction rate

The reaction conditions may be verydifferent from standard conditions

SEPs apply to aqueous equilibria, but

there are many reactions that take placethat are not in aqueous solutions