standard electrode potentials presentation
TRANSCRIPT
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8/13/2019 Standard Electrode Potentials Presentation
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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