Electrochemical Cells Basics

Electrode Potentials Worksheet

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1. Definitions & The S.H.E.

Define the term ‘standard electrode potential’.

The standard electrode potential is the E.M.F. of a half-cell compared to that of a standard hydrogen half-cell under standard conditions.

(Standard conditions: 298K, 100kPa, 1.00 mol dm⁻³ ion concentration).

Standard Hydrogen Electrodes (S.H.E.s) are vital for determining the E^o of cells.

a) Draw a labelled diagram of an S.H.E. Include the conditions in your labels.

b) Explain why the S.H.E. has an E⁰ of 0.00V.

It is by definition – the S.H.E. is the chosen primary standard against which all other half-cells are compared.

c) Write the equation for the S.H.E.

2H⁺(aq) + 2e⁻ ⇌ H₂(g)

d) State the pH of the solution in a standard hydrogen electrode.

pH = 0.00

(Because [H⁺] = 1.00 mol dm⁻³, and -log(1) = 0).

2. Electrochemical Cells

A secondary standard (Ag⁺/Ag half-cell) is connected to a copper (Cu²⁺ / Cu) half-cell.

a) Complete the diagram below by connecting the copper half-cell to the silver half-cell. Label your diagram, including the conditions.

b) Sketch a graph of the change in E.M.F over time.

c) State the purpose of the salt-bridge.

To maintain charge balance and complete the electrical circuit.

Ions move out of the salt bridge into the half-cells to balance the charge build-up caused by the redox reactions.

d) Suggest an appropriate compound for use in the salt-bridge. Explain your choice.

Potassium Nitrate (KNO₃)

Reason 1: Both K⁺ and NO₃⁻ ions are soluble and will not form precipitates with ions in the half-cells.

Reason 2: They are chemically inert in this context. Although nitrate is an oxidising agent, it requires acidic conditions to react effectively.

e) Explain why potassium chloride would be a poor choice for the salt-bridge.

Chloride ions (Cl⁻) would react with silver ions (Ag⁺) to form a precipitate of silver chloride (AgCl).

Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

A cell is set up between a zinc (Zn²⁺/Zn) half-cell and a Cr₂O₇²⁻ / Cr³⁺ half-cell. Complete the diagram by drawing and labelling the dichromate half-cell.

Mr Cole Chemistry