19. Redox processes

19. Redox processes

19.1 Electrochemical cells

Nature of science:

• Employing quantitative reasoning - electrode potentials and the standard hydrogen electrode.

• Collaboration and ethical implications - scientists have collaborated to work on electrochemical cell technologies and have to consider the environmental and ethical implications of using fuel cells and microbial fuel cells.

Understandings:

• A voltaic cell generates an electromotive force (EMF) resulting in the movement of electrons from the anode (negative electrode) to the cathode (positive electrode) via the external circuit. The EMF is termed the cell potential (E⁰).

• The standard hydrogen electrode (SHE) consists of an inert platinum electrode in contact with 1 mol dm-3 hydrogen ion and hydrogen gas at 100 kPa and 298 K. The standard electrode potential (E⁰) is the potential (voltage) of the reduction half-equation under standard conditions measured relative to the SHE. Solute concentration is 1 mol dm-3 or 100 kPa for gases. E⁰ of the SHE is 0 V.

• When aqueous solutions are electrolysed, water can be oxidized to oxygen at the anode and reduced to hydrogen at the cathode.

• △G⁰ = –nFE⁰. When E⁰ is positive, △G⁰ is negative indicative of a spontaneous process. When E⁰ is negative, △G⁰ is positive indicative of a non-spontaneous process. When E⁰ is 0, then △G⁰ is 0.

• Current, duration of electrolysis and charge on the ion affect the amount of product formed at the electrodes during electrolysis.

• Electroplating involves the electrolytic coating of an object with a metallic thin layer.

Applications and skills:

• Calculation of cell potentials using standard electrode potentials.

• Prediction of whether a reaction is spontaneous or not using E⁰ values.

• Determination of standard free-energy changes (△G⁰) using standard electrode potentials.

• Explanation of the products formed during the electrolysis of aqueous solutions.

• Perform lab experiments that could include single replacement reactions in aqueous solutions.

• Determination of the relative amounts of products formed during electrolytic processes.

• Explanation of the process of electroplating.