OCR A 5.2.3 Redox and Electrode Potentials

redox titration

-oxidising agent titrated against reducing agent

-e transferred from 1 species to other

-normally indicators not needed as transition metals naturally change colour when ox state changes

use of redox titration

-used to work out conc of reducing or oxidising agent

potassium manganate (VII) titration agents

-ox agent; manganate (VII) reduced to Mn²⁺

-red agent; Fe oxidised to Fe³⁺

-addition of H₂SO₄; ensure sufficient H+ ions allow reduction of ox agent

choice of acid to acidify mixture

-must not react w/ manganate ions

-dilute H₂SO₄

-HCl; oxidised to Cl ions by manganate ion

-HNO₃; ox agent; oxidise substance being analysed

-ethanoic acid; weak acid insufficient H+ ions

-conc H₂SO₄; oxidise substance being analysed

potassium manganate (VII) titration procedure to find conc of reducing agent

-standard sol of potassium manganate(VII), KMnO₄ is added to burette

-using pipette, add measured vol of sol being analysed to conical flask

-add excess of dilute H₂SO₄ to provide H+ ions req for reduction of MnO₄^-

-during titration manganate sol reacts and is decolourised as it is added

-endpoint; first permanent pink colour as this indicates there is an excess of MnO₄^- ions

-repeat till u get 2 concordant titres

-reverse burette and conical flask to find ox agent

how to read meniscus in manganate titration

-KMnO₄ is deep purple colour so difficult to see bottom of meniscus

-so readings taken from top of meniscus is read from

Manganate (VII) titration redox equation

-Fe²⁺ → Fe³⁺ + e^- (x5)

-MnO₄^- + 8H⁺ + 5e^- → Mn²⁺ + 4H₂O

-5Fe²⁺ + MnO₄^- + 8H⁺ + 5e^- → Mn²⁺ + 4H₂O + 5e^-

iodine/thiosulphate titration procedure

-measure out a vol of KIO₃; produce IO₃^-

-add excess KI sol

-add this to conical flask

-add Na₂S₂O₃ into burette

-colour changes to pale yellow; dif to see

-add 2cm³ starch; turns from deep blue to colourless

iodine/thiosulphate titration redox reaction

what does electrode potential depend on

-temp

-pressure of gas

-reagent conc

standard condition when comparing electrode potential

-Ion concentration of 1.00 mol dm^-3

-temperature of 298 K

-pressure of 100 kPa

electrode potential

-tendency of element to lose e^-

-if electrode potential is more negative then it has higher tendency to oxidise

-so flip equation and combine with other

-each half equation rep half call

Mg²⁺ + 2e^- ←→ Mg (-2.3) Al³⁺ + 3e^- ←→ Al (-1.66)

Mg←→Mg²⁺ + 2e^-

Al³⁺ + 3e^- ←→ Al

voltaic cell

-converts chemical energy to electrical energy

-electrical energy results from movement of e from redox reactions

half cell

-one half of an electrochemical cell

-can be constructed of a metal dipped into its ions, or a platinum electrode w/ 2 aq ions

how is voltaic cell made

-by connecting tgthr 2 dif half cells; allows e to flow

-chemicals in half cells must be kept apart; e move in a controlled way

-metal/metal ion or ion/ion half cells

metal/metal ion half cell

-metal rod dipped in sol of its aq metal ion

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