Electrochemistry and Redox Processes
Electron transfer links chemical change with electrical energy.
1. Redox language
Oxidation is loss of electrons and an increase in oxidation number. Reduction is gain of electrons and a decrease in oxidation number. The reducing agent donates electrons and is oxidized; the oxidizing agent accepts electrons and is reduced. Electrons lost and gained must balance.
- Separate oxidation and reduction.
- Balance atoms other than O and H.
- Balance O with $H_2O$, H with $H^+$, and charge with electrons in acidic solution.
- For basic solution, add $OH^-$ to neutralize $H^+$.
- Multiply half-reactions so electron counts match, then add and cancel.
2. Galvanic cells
A spontaneous redox reaction can produce electrical work when its half-reactions occur at separate electrodes. Oxidation occurs at the anode and reduction at the cathode. In a galvanic cell the anode is negative and the cathode positive. Electrons travel through the external circuit from anode to cathode; ions in the salt bridge preserve charge balance. Cell notation places anode on the left: $Zn|Zn^{2+}||Cu^{2+}|Cu$.