An important industrial application of electrolysis is the plating of one metalAn element characterized by a glossy surface, high thermal and electrical conductivity, malleability, and ductility. on top of another. A typical example is the bumper of a car. This is made from steel and then plated with a thin layer of chromium to make it resistant to rusting and scratching. Many other metal objects, such as pins, screws, watchbands, and doorknobs, are made of one metal with another plated on the surface.
An electroplating cell works in much the same way as the cell used to purify copper. The object to be plated is used as the cathodeThe electrode in an electrochemical cell where reduction occurs; the negatively charged electrode in a vacuum tube., and the electrolyte contains some ionic compoundA compound containing oppositely charged ions held together by electrostatic attraction. Usually the ions are in a crystal lattice with positive ions surrounded by negative ions and negative ions surrounded by positive ions. of the metal to be plated. As current flows, this compound is reduced to the metal and deposits on the surface of the cathode. In chromium plating, for instance, the electrolyte is usually a solution of potassium dichromate, K2Cr2O7, in fairly concentratedIncreased the concentration of a mixture or solution (verb). Having a large concentration (adjective). sulfuric acidIn Arrhenius theory, a substance that produces hydrogen ions (hydronium ions) in aqueous solution. In Bronsted-Lowry theory, a hydrogen-ion (proton) donor. In Lewis theory, a species that accepts a pair of electrons to form a covalent bond.. In this very acidic solution CrO72– ions are completely protonated, and so the reductionThat part of a chemical reaction in which a reactant gains electrons; simultaneous oxidation of a reactant must occur. half-equation is
H2Cr2O7(aq) + 12H+(aq) + 12e– → 2Cr(s) + 7 H2O(l) (1)
Other metals which are often electroplated are silver, nickel, tin, and zinc.
In the case of silver the electrolyte must contain the polyatomicContaining two or more atoms. ion Ag(CN)2– rather than Ag+. Otherwise the solidA state of matter having a specific shape and volume and in which the particles do not readily change their relative positions. silver will be deposited as jagged crystals instead of a shiny uniform layer.