In one type of Breathalyzer (alcohol meter), the quantity of...

In one type of Breathalyzer (alcohol meter), the quantity of ethanol in a sample is related to the amount of electric current produced by an ethanol-oxygen fuel cell. Use data from Table 20.1 and Appendix $D$ to determine (a) $E_{\text {cell }}^{\circ}$ and (b) Eo for the reduction of $\mathrm{CO}_{2}(\mathrm{g})$ to $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}(\mathrm{g})$.

Key Concepts

Standard Electrode Potentials

Standard electrode potentials provide a measure of the tendency of chemical species to be reduced under standard conditions. They are tabulated using reference half?reactions and allow chemists to predict the direction of redox reactions and to calculate the overall cell potential by comparing the reduction potentials of the oxidizing and reducing agents.

Electrochemical Cells and Fuel Cells

Electrochemical cells convert chemical energy into electrical energy via redox reactions. In fuel cells, such as those used in breathalyzers, a fuel (like ethanol) is oxidized at one electrode while an oxidant (like oxygen) is reduced at the other, producing an electric current. The analysis of these cells involves understanding the individual half-reactions and how their standard potentials combine to give the overall cell potential.

Half-Reaction Analysis and Calculation of Cell Potential

Breaking down a redox process into two half-reactions (oxidation and reduction) is essential for analyzing electrochemical cells. The overall cell potential is determined by subtracting the anode (oxidation) potential from the cathode (reduction) potential. This process often involves using standard reduction potential data from reference tables, allowing for the design and evaluation of devices like alcohol sensors.

Thermodynamic Relationships in Electrochemistry

The relationship between the Gibbs free energy change and the cell potential, expressed by the equation ?G = -nFE, links the chemical feasibility of a redox reaction to its observed electric potential. This thermodynamic framework provides insights into reaction spontaneity and the efficiency of fuel cells under standard conditions, and is crucial for interpreting the performance of electrochemical devices.

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