A cell is a device that converts a spontaneous redox reaction into electrical energy. Let us understand the principle of converting chemical energy ⇒ electrical energy.
Basic Knowledge
The basic structure of a cell: ① positive electrode(the side that is reduced and receives electrons), ② negative electrode(the side that is oxidized and releases electrons), ③ electrolyte, and ④ separator(allows ions to pass but not electrons).
Electrons produced at the negative electrode move through the external circuit to the positive electrode, and electric current flows in the opposite direction to the electrons. Negative electrode=anode(oxidation) and positive electrode=cathode(reduction) are fundamental terms in electrochemistry, and these definitions do not change in cells or electrolysis (only the relative potentials determine which electrode has each role).
The electromotive force of a cell is determined by the difference between the standard electrode potentials E° of the positive and negative electrodes(Ecell°=Ecathode°-Eanode°).
Negative electrode (anode)（The side that is oxidized; releases electrons）
Positive electrode (cathode)（The side that is reduced; accepts electrons）
Standard electrode potential E°（Based on the standard hydrogen electrode, 25℃）
Electromotive force Ecell（Ecathode-Eanode）
Nernst equation（E=E°-(RT/nF)ln Q）
Metal activity series（K>Ca>Na>Mg>Al>...>Au. Metals farther to the left are more easily oxidized）
Further Exploration (Principles and Applications)
The Daniell cell(Zn|ZnSO4||CuSO4|Cu) is the most basic chemical cell and has an electromotive force of 1.1 V. At the negative electrode, Zn→Zn2++2e-(oxidation) occurs, while at the positive electrode, Cu2++2e-→Cu(reduction) occurs. Ion movement is controlled using a porous ceramic plate or ion-exchange membrane.
Using the standard hydrogen electrode SHE(Pt|H2(1 atm)|H+(1 M)) as the reference(E°=0), the E° values of individual metal electrodes are defined. Cu/Cu2+ E°=+0.34 V, Zn/Zn2+ E°=-0.76 V → electromotive force of the Daniell cell=0.34-(-0.76)=1.10 V.
- Negative electrode=oxidation (anode), positive electrode=reduction (cathode)
- Electrons flow from negative to positive through the external circuit
- Current flows from positive to negative (opposite to electrons)
- Ecell=Ecathode-Eanode
- E° is defined using SHE as the reference
- Metal activity series: K>Ca>Na>Mg>Al>Zn>Fe>Ni>Sn>Pb>(H)>Cu>Hg>Ag>Pt>Au
- Daniell cell=1.1 V
Points to Note (Common Confusions and Frequent Mistakes)
① In a cell, negative electrode=anode=oxidation and positive electrode=cathode=reduction (note that the correspondence of terms may appear reversed in electrolysis). ② The metal with greater metal activity(=smaller E°) becomes the negative electrode. ③ E° is written as the potential for a reduction reaction(IUPAC convention). ④ Metals with E°>0(Cu, Ag, Au) do not produce hydrogen from acids (=do not dissolve in dilute hydrochloric acid or dilute sulfuric acid).
Practice
- Write the negative-electrode and positive-electrode reaction equations of the Daniell cell.
- When a cell is made from Zn(E°=-0.76 V) and Ag(E°=+0.80 V), give the electromotive force and identify the positive and negative electrodes.
- Name one metal with the greatest metal activity and one with the least.