Electrolysis of molten alkali halides is the usual industrial method of preparing the alkali metals:. Ions in aqueous solutions can undergo similar reactions.
Thus if a solution of nickel chloride undergoes electrolysis at platinum electrodes, the reactions are. Both of these processes are carried out in electrochemical cells which are forced to operate in the "reverse", or non-spontaneous direction, as indicated by the negative for the above cell reaction. The free energy is supplied in the form of electrical work done on the system by the outside world the surroundings.
This is the only fundamental difference between an electrolytic cell and the galvanic cell in which the free energy supplied by the cell reaction is extracted as work done on the surroundings. A common misconception about electrolysis is that "ions are attracted to the oppositely-charged electrode.
Ionic motion throughout the bulk of the solution occurs mostly by diffusion, which is the transport of molecules in response to a concentration gradient. Migration— the motion of a charged particle due to an applied electric field, is only a minor player, producing only about one non-random jump out of around , random ones for a 1 volt cm —1 electric field.
Only those ions that are near the interfacial region are likely to undergo migration. Thus if an aqueous solution is subjected to electrolysis, one or both of the above reactions may be able to compete with the electrolysis of the solute.
For example, if we try to electrolyze a solution of sodium chloride, hydrogen is produced at the cathode instead of sodium:. Electrolysis of salt "brine" is carried out on a huge scale and is the basis of the chloralkali industry. Pure water is an insulator and cannot undergo significant electrolysis without adding an electrolyte. If the object is to produce hydrogen and oxygen, the electrolyte must be energetically more difficult to oxidize or reduce than water itself.
Electrolysis of a solution of sulfuric acid or of a salt such as NaNO 3 results in the decomposition of water at both electrodes:. Electrolytic production of hydrogen is usually carried out with a dilute solution of sulfuric acid.
This process is generally too expensive for industrial production unless highly pure hydrogen is required. However, it becomes more efficient at higher temperatures, where thermal energy reduces the amount of electrical energy required, so there is now some interest in developing high-temperature electrolytic processes.
Most hydrogen gas is manufactured by the steam reforming of natural gas. This relation was first formulated by Faraday in in the form of two laws of electrolysis :. It is possible to have electrolysis involving gases. For example, what two ions will CuSO 4 break down into?
Electrolysis of copper sulfate : Two copper electrodes are placed in a solution of blue copper sulfate and are connected to a source of electrical current. The current is turned on for a period of time.
We take two copper electrodes and place them into a solution of blue copper sulfate CuSO 4 and then turn the current on. We notice that the the initial blue color of the solution remains unchanged, but it appears that copper has been deposited on one of the electrodes but dissolved on the other. The reaction at this electrode is:. This is why it appears that the copper has dissolved from the electrode.
We just saw electric current used to split CuSO 4 into its component ions. This is all it takes to predict the products of electrolysis; all you have to do is break down a compound into its component ions. Two commonly used methods of electrolysis involve molten sodium chloride and aqueous sodium chloride, which give different products.
As we have covered, electrolysis is the passage of a direct electric current through an ionic substance that is either molten or dissolved in a suitable solvent. This results in chemical reactions at the electrodes and the separation of materials. Two commonly used methods of electrolysis involve molten sodium chloride and aqueous sodium chloride. You might think that both methods would give you the same products, but this not the case.
Electrolysis cell for molten sodium chloride : A commercial electrolysis cell for the production of metallic sodium and chlorine gas from molten NaCl. Liquid sodium floats to the top of the melt above the cathode and is drained off into a storage tank. Chlorine gas bubbles out of the melt above the anode. Sodium ions migrate to the cathode, where electrons enter the melt and are reduced to sodium metal:. Chloride ions migrate the other way, toward the anode.
They give up their electrons to the anode and are oxidized to chlorine gas:. What happens when we have an aqueous solution of sodium chloride? Rather than producing sodium, hydrogen is produced. Electrolysis of aqueous sodium chloride : Electrolysis of aqueous NaCl results in hydrogen and chloride gas.
At the anode A , chloride Cl- is oxidized to chlorine. At the cathode C , water is reduced to hydroxide and hydrogen gas. The net process is the electrolysis of an aqueous solution of NaCl into industrially useful products sodium hydroxide NaOH and chlorine gas.
Deduce the products of the electrolysis of a molten salt : Electrolysis of a molten salt produces the elements from the salt. So, the electrolysis of WCl4 produces W and Cl2. Metal ions receive electrons at the negative electrode, and the non-metals lose them at the positive electrode.
Pure water cannot undergo significant electrolysis without an electrolyte, such as an acid or a base. Pure water cannot undergo significant electrolysis without adding an electrolyte. An electrolyte will always conduct electricity. There are two types of electrolytes. Ionic solutions such as Sodium Chloride or Potassium Chloride are called strong electrolytes. This is because these bonds are totally separated in aqueous solutions. Compounds such as Acetic acid and Ammonium Hydroxide are known as weak electrolytes.
This is because only some of the molecules break into ions. Pure water is a very weak electrolyte and conducts almost no current. Most of the electrolytes are in solution form, but solid and molten electrolytes are also present. The term electrode is used to refer the contact mechanism used to combine electrically a metallic part to a non-metallic part of an electrical circuit.
The materials used in the electrodes are very important. The electrodes play a major role in the electrochemical cell also known as the Galvani cell. An electrochemical cell is made of an electrolyte and two electrodes of different materials.
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