CIE A Level Chemistry Electrochemistry Exam Questions

About CIE A Level Chemistry Electrochemistry

Electrochemistry in Cambridge International 9701 begins at AS with redox: assigning oxidation numbers, identifying oxidising and reducing agents and constructing balanced redox equations. The A Level part extends this into electrolysis, including predicting products and using the Faraday constant to calculate masses and volumes, and into standard electrode potentials. You must describe the standard hydrogen electrode, calculate standard cell potentials, predict feasibility and use the Nernst equation to show how concentration affects electrode potential. The topic links to Group 17, transition elements and redox titrations. Examiners focus on correct cell diagrams, careful use of E values with signs, and Faraday calculations.

What you need to know

• Oxidation numbers Oxidation is an increase in oxidation number and reduction a decrease. Use Roman numerals in names such as iron(III) chloride and use oxidation number changes to balance redox equations.
• Electrolysis At the cathode the less reactive metal or hydrogen is discharged; at the anode, halide ions are discharged if concentrated, otherwise oxygen forms. The products depend on electrode potentials, concentration and the electrode material.
• Faraday calculations (A Level) Q = It, F = Le where F = 96500 C mol-1 and L is the Avogadro constant. Moles of electrons = Q/F, then use the half-equation ratio to find moles of product.
• Standard electrode potentials (A Level) Measured against the standard hydrogen electrode at 298 K, 1 mol dm-3 solutions and 101 kPa gas pressure, using a platinum electrode where no metal is involved and a salt bridge of KNO3.
• Cell potential and feasibility E cell = E(reduction, more positive) - E(oxidation, less positive). A reaction is predicted to be feasible if E cell is positive, but it may still be too slow to observe.
• Nernst equation (A Level) E = E standard + (0.059/z) log([oxidised form]/[reduced form]). Increasing the concentration of the oxidised form makes the electrode potential more positive.
• Delta G and E cell delta G standard = -nFE cell, linking feasibility in electrochemistry to energetics.

Common mistakes in Electrochemistry

• Using the moles of electrons as the moles of product; divide by the number of electrons in the half-equation.
• Forgetting to convert time to seconds in Q = It.
• Saying a positive E cell means the reaction definitely happens; it may have a high activation energy or non-standard conditions.
• Leaving out platinum or the salt bridge in a cell diagram, or using a salt bridge of a substance that reacts with the ions, such as KCl with Ag+.
• Reversing the sign of E when multiplying a half-equation; electrode potentials are not multiplied or changed by balancing.

Try three Electrochemistry exam-style questions