A device that converts chemical energy into electrical energy is a chemical cell. The dry cells and lithium-ion batteries around us also use this principle.
Basic Knowledge
The basic principle of a chemical cell is the electric current produced when two metals with different ionization tendencies are immersed in an electrolyte solution. Ionization tendency is the degree to which a metal easily becomes an ion. The order isK > Ca > Na > Mg > Al > Zn > Fe > Ni > Sn > Pb > H > Cu > Hg > Ag > Pt > Au
and metals farther to the left become ions more easily.
For example, in a voltaic cell made by placing a zinc plate (Zn) and copper plate (Cu) in dilute sulfuric acid, Zn becomes ions, dissolves, and releases electrons (negative electrode). The electrons flow through a wire to the Cu side (positive electrode), where hydrogen H2 is produced.
Chemical cell (a device that converts chemical energy into electrical energy through a chemical reaction)
Ionization tendency (the order in which metals easily become ions. It is greater toward the left)
Negative electrode (− electrode) (the electrode that releases electrons. A metal with a greater ionization tendency)
Positive electrode (+ electrode) (the electrode that receives electrons. A metal with a smaller ionization tendency)
Fuel cell (a cell that extracts electricity by reacting hydrogen with oxygen. The only emission is water)
Rechargeable battery (secondary cell) (a cell that can be recharged and used repeatedly. Lead-acid batteries and lithium-ion batteries)
Further Study
In a cell reaction, the metal with the greater ionization tendency is oxidized at the negative electrode and becomes ions (releasing electrons). At the positive electrode, ions or molecules receive electrons and are reduced.
Comparison of representative cells:
・Daniell cell: zinc (negative electrode) + zinc sulfate solution ‖ copper sulfate solution + copper (positive electrode). Stable because polarization is unlikely to occur.
・Lead-acid battery: Pb (negative electrode), PbO2 (positive electrode), dilute sulfuric acid. Rechargeable. Used as an automobile battery.
・Fuel cell: H2 (fuel) + O2 → H2O. A next-generation power source that emits only water and is environmentally friendly.
Lithium-ion batteries are widely used in smartphones and EVs.
- Chemical cell = chemical energy → electrical energy
- Negative electrode(−): a metal with a greater ionization tendency dissolves (oxidation)
- Positive electrode(+): receives electrons and is reduced
- Voltaic cell: Zn(−)・Cu(+)・dilute sulfuric acid
- Daniell cell: polarization is unlikely, so it is stable
- Fuel cell: H2+O2→H2O. The only emission is water
- Rechargeable batteries (secondary cells) can be used repeatedly
Points to Note
① The negative electrode(−) of a cell releases electrons, so it is the side from which electrons flow into the external circuit. The positive electrode(+) is the side into which electrons flow. ② Do not confuse these with the cathode and anode in electrolysis, whose directions are opposite. ③ A metal with a greater ionization tendency is more likely to become the negative electrode of a cell. ④ Distinguish between a primary cell (disposable, such as a dry cell) and a secondary cell (rechargeable battery).
Practice
- What metals are used for the negative and positive electrodes of a voltaic cell?
- What is emitted by a fuel cell?
- Explain why a metal with a greater ionization tendency becomes the negative electrode of a cell.