What Is Electrolysis - electrolisis of solution PDF

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What Is Electrolysis? In order to predict the products of electrolysis, we first need to understand what electrolysis is and how it works. Electrolysis is a method of separating bonded elements and compounds by passing an electric current through them. It uses a direct electric current (DC) to drive an otherwise nonspontaneous chemical reaction. Electrolysis is very important commercially as a stage in the separation of elements from naturally occurring sources, such as ores, using an electrolytic cell. The main components required to achieve electrolysis are: 

An electrolyte: a substance containing free ions, which are the carriers of electric current in the electrolyte. If the ions are not mobile, as in a solid salt, then electrolysis cannot occur.

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A direct current (DC) supply: provides the energy necessary to create or discharge the ions in the electrolyte. Electric current is carried by electrons in the external circuit.

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Two electrodes: an electrical conductor that provides the physical interface between the electrical circuit providing the energy and the electrolyte.

The Interchange of Atoms and Ions The key process of electrolysis is the interchange of atoms and ions by the removal or addition of electrons to the external circuit. The required products of electrolysis are in a different physical state from the electrolyte and can be removed by some physical processes. Each electrode attracts ions that are of the opposite charge. Positively charged ions, or cations, move toward the electron-providing cathode, which is negative; negatively charged ions, or anions, move toward the positive anode. You may have noticed that this is the opposite of a galvanic cell, where the anode is negative and the cathode is positive. At the electrodes, electrons are absorbed or released by the atoms and ions. Those atoms that gain or lose electrons become charged ions that pass into the electrolyte. Those ions that gain or lose electrons to become uncharged atoms separate from the electrolyte. The formation of uncharged atoms from ions is called discharging. The energy required to cause the ions to migrate to the electrodes, and the energy to cause the change in ionic state, is provided by the external source. Oxidation and Reduction Oxidation of ions or neutral molecules occurs at the anode, and reduction of ions or neutral molecules occurs at the cathode. For example, it is possible to oxidize ferrous ions to ferric ions at the anode: Fe2+(aq)→Fe3+(aq)+e−Fe2+(aq)→Fe3+(aq)+e− It is also possible to reduce ferricyanide ions to ferrocyanide ions at the cathode: Fe(CN)3−6+e−→Fe(CN)4−6Fe(CN)63−+e−→Fe(CN)64− Neutral molecules can also react at either electrode. Electrolysis reactions involving H + ions are fairly common in acidic solutions. In alkaline water solutions, reactions involving hydroxide ions (OH –) are common. The substances oxidized or reduced can also be the solvent, which is usually water, or the electrodes. It is possible to have electrolysis involving gases. Predicting the Products of Electrolysis

Let’s look at how to predict the products. For example, what two ions will CuSO 4 break down into? The answer is Cu2+ and SO42-. Let’s look more closely at this reaction.

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. This is because Cu2+ ions are attracted to the negatively charged cathode, and since the the cathode is putting out electrons, the Cu2+ becomes reduced to form copper metal, which is deposited on the electrode. The reaction at this electrode is: Cu2+(aq)+2e−→Cu(s)Cu2+(aq)+2e−→Cu(s) At the positive anode, copper metal is oxidized to form Cu 2+ ions. This is why it appears that the copper has dissolved from the electrode. The reaction at this electrode is: Cu(s)→Cu2+(aq)+2e−Cu(s)→Cu2+(aq)+2e− We just saw electric current used to split CuSO4 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. Electrolysis of Sodium Chloride Two commonly used methods of electrolysis involve molten sodium chloride and aqueous sodium chloride, which give different products. LEARNING OBJECTIVES Predict the products of electrolysis of sodium chloride under molten and aqueous conditions KEY TAKEAWAYS Key Points

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Sodium metal and chlorine gas can be obtained with the electrolysis of molten sodium chloride.

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Electrolysis of aqueous sodium chloride yields hydrogen and chlorine, with aqueous sodium hydroxide remaining in solution.

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The reason for the difference is that the reduction of Na + (E° = –2.7 v) is energetically more difficult than the reduction of water (–1.23 v).

Key Terms 

anode: The electrode of an electrochemical cell at which oxidation occurs.

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cathode: The electrode of an electrochemical cell at which reduction occurs.

Electrolysis of NaCl 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. Let’s go through each of the methods to understand the different processes. Electrolysis of Molten NaCl If sodium chloride is melted (above 801 °C), two electrodes are inserted into the melt, and an electric current is passed through the molten salt, then chemical reactions take place at the electrodes.

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: Na++e−→NaNa++e−→Na Chloride ions migrate the other way, toward the anode. They give up their electrons to the anode and are oxidized to chlorine gas: Cl−→12Cl2+e−Cl−→12Cl2+e− The overall reaction is the breakdown of sodium chloride into its elements: 2NaCl→2Na(s)+Cl2(g)2NaCl→2Na(s)+Cl2(g) Electrolysis of Aqueous NaCl What happens when we have an aqueous solution of sodium chloride? Well, we can’t forget that we have to factor water into the equation. Since water can be both oxidized and reduced, it competes with the dissolved Na+ and Cl– ions. 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. The ion-selective membrane (B) allows the counterion Na+ to freely flow across, but prevents anions such as hydroxide (OH-) and chloride from diffusing across. 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. The reaction at the cathode is: H2O(l)+2e−→H2(g)+2OH−H2O(l)+2e−→H2(g)+2OH− The reaction at the anode is: Cl−→12Cl2(g)+1e−Cl−→12Cl2(g)+1e− The overall reaction is as follows: NaCl(aq)+H2O(l)→Na+(aq)+OH−(aq)+H2(g)+12Cl2(g)NaCl(aq)+H2O(l)→Na+(aq)+OH−(aq)+H2(g) +12Cl2(g)

Reduction of Na+ (E° = –2.7 v) is energetically more difficult than the reduction of water (–1.23 v), so in aqueous solution, the latter will prevail....