Electrode Potentials.

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Presentation transcript:

Electrode Potentials

Some elements lose electrons more easily than others The essence of an electrochemical cell is to carry out a redox reaction and keep the oxidation and reduction parts separate

A Daniell Cell – (electrochemical)

Daniell cell showing voltage – 1.1V

A standard hydrogen electrode

Measuring the standard electrode potential of a metal

Measuring the standard electrode potential of magnesium

How to use electrode potentials Values and signs allow prediction of spontaneous reaction E.g. will zinc react with Cu2+ spontaneously?

This diagram will predict the direction of spontaneous reaction given electrode potentials - Electrode potential scale – note sign + A C T U L A C T U L Cu2+ + 2e- Cu Zn2+ + 2e- Zn Spontaneous reaction

Consider the following and reach a conclusion Cu2+ + e-  Cu+ +0.15 Cu2+ + 2e-  Cu +0.34 Cu+ + e-  Cu +0.52

Cu+ + Cu+  Cu + Cu2+ Disproportionation – simultaneous oxidation and reduction of the same species

Application of Electrode Potentials Give a comparison of the effectiveness of galvanising (zinc plating) and tin plating of steel Iron reacts with dilute hydrochloric acid, copper does not, but copper will react with conc nitric acid

Effect of concentration Treat cell reaction as an equilibrium system: If change supports/shifts eqm to the right then emf increases & vice versa Cu2+ + Zn  Zn2+ + Cu Increase [Cu2+] emf increases Increase [Zn2+] emf decreases

Limitations No indication of rate of reaction. Mixtures which can react may co-exist for long periods due to very slow reaction rate E.g. H2 & O2 (thermodynamically unstable, kinetically stable) This is caused by high activation energy

Energy profiles/diagrams

Limitations Standard electrode potentials refer to standard conditions Temp and concentrations affect electrode potentials. Reactions not predicted by electrode potentials can occur under non-standard conditions E.g. solid K2Cr2O7 and conc HCl

Galvanisation – sacraficial anode Construct the cell reaction for a zinc/iron cell Construct the cell reaction for a tin/iron cell Why fix magnesium to the hull of a battleship?