Electrochemistry PDF

Ch-4 “Electrochemistry” Electrolysis is a process of breaking down a compound by electricity. An electric current is the flow of charged particles. Conductivity: Conductors and Electrolytes Conductors: - In solids, substances that conduct electricity are called Conductors. These are mostly me...

Ch-4 “Electrochemistry” Electrolysis is a process of breaking down a compound by electricity. An electric current is the flow of charged particles. Conductivity: Conductors and Electrolytes Conductors: - In solids, substances that conduct electricity are called Conductors. These are mostly metals and graphite. This is because metals and graphite contain free electrons in their structures to carry the charge. The solids which do not conduct electricity are called Insulators. To test a solid for electrical conductivity, we put it an electrical circuit like the one shown in diagram. If the bulb lights or the ammeter gives a reading, then the solid is a conductor. Electrolytes: - A substance that conducts electric current when molten or dissolved in water. This is due to presence of ions that are free to move. Electrolytes include acids, alkalis, and ionic compounds in molten or aqueous form. Non-electrolytes: - A non-conductor of electricity because it does not ionize. The Electrolysis Cell: The electrolysis cell is a battery each pole connected to an electrode and both electrodes are dipped in the liquid to be electrolysed.  The electrode connected to the positive pole is called the anode.  The electrode connected to the negative pole is called the cathode. Difference between inert and non-inert electrodes. There are two types of electrodes, active electrodes and inert electrodes. Active electrodes take place in the process its self. Inert electrodes are just there to conduct the current without interfering. Inert electrodes can be either graphite or platinum but graphite is more widely used because it’s cheaper. Inert electrodes are always used in electrolysis; active ones are used in electroplating. WHAT HAPPENS IN ELECTROLYSIS Electrolysis decomposes (breaks) an ionic compound back to the elements that form it. For example, by electrolysis of sodium chloride we can obtain sodium and chlorine from sodium chloride.  Ch-4 “Electrochemistry” At anode - When the current is turned on, the negative ion in the electrolyte gets attracted to the positive electrode because they are oppositely charged. When this happens, the negative ion loses the electrons and becomes an atom. Flow of electrons- The electrons lost are transferred through the wire in the outer circuit from the anode to the cathode. At cathode- At the same time, the positive ion from the electrolyte is attracted to the cathode, where it gains the electrons lost by the negative ion and becomes an atom too. Products form - Usually a metal and hydrogen gas is collected at the cathode. And a non-metal and collected at the anode. The electrolytes conduct electricity only when they are in molten form or in aqueous solution because in these forms, they contain free mobile ions which can carry the current but they don’t in solid form. ELECTROLYSIS OF MOLTEN IONIC COMPOUNDS An idealized cell for the electrolysis of sodium chloride is shown in the figure below. A source of direct current is connected to a pair of inert electrodes immersed in molten sodium chloride. Because the salt has been heated until it melts, the Na+ ions flow toward the negative electrode and the Cl- ions flow toward the positive electrode. At the negative electrode (cathode): Na+ + e- → Na Cl- ions that collide with the positive electrode are oxidized to Cl2 gas, which bubbles off at this electrode. At the positive electrode (anode): 2Cl- → Cl2 + 2e- The products formed are The net effect of passing an electric current through the molten salt in this cell is to decompose sodium chloride into its elements, sodium metal and chlorine gas. 2NaCl(l) → 2 Na(l) + Cl2(g) This example explains why the process is called electrolysis. The suffix -lysis comes from the Greek stem meaning to loosen or split up. Electrolysis literally uses an electric current to split a compound into its elements.  Ch-4 “Electrochemistry” ELECTROLYSIS OF AQUEOUS IONIC COMPOUNDS Types of ions - Electrolysing an ionic compound in its solution is very much different to electrolysing it when it’s molten. This is because in a solution we have 4 ions, H+ and OH- from water and a positive and a negative ion from the compound. But only one type of ions gets discharged at each electrode. For the positive ions, the ion that gets discharged in preference, at the cathode is the one that is below in the series. So, if the ion from the ionic compound is above hydrogen in the reactivity series, H+ gets discharged at the cathode and if the ion from the compound is below hydrogen in the reactivity series, this ion gets discharged at the cathode. So, for example if we are electrolysing aqueous sodium chloride, H+ ions will get discharged at the cathode, and if we are electrolysing aqueous copper iodide, Cu2+ ions will get discharged at the cathode. For the negative ions however it is different. Hydroxide ions OH- from water is always discharged at the anode (except in one case, this is if the other negative ion is a concentrated solution of halide ion.) If OH- is discharged, the equation will be as under: 4OH- → O2 + 2H2O + 4e If the other negative ion is a halide It all depends on the concentration of the halide. If the electrolyte is a concentrated solution, then there are many of the halide ions. So, the halide ion gets discharged at the cathode. If the electrolyte is a dilute solution, then there are more OH - ions than halide ions, so OH- gets discharged. EXAMPLES :  If the electrolyte is a concentrated solution of sodium chloride, hydrogen gas is formed at the cathode and chlorine gas is formed at the anode because the solution is concentrated.  If the electrolyte is a dilute solution of sodium chloride, hydrogen gas is formed at the cathode and oxygen gas is formed at the anode because the solution is dilute.  If the electrolyte is a dilute solution of copper sulfate, copper is formed at the cathode because it is below in the series than hydrogen and oxygen gas is formed at the anode.  Ch-4 “Electrochemistry” Electrolysis of Brine (concentrated aqueous sodium chloride): The ions present in the electrolyte are H+ and OH- from water and Na+ and Cl- from sodium chloride. The H+ ions will be discharged at the cathode and hydrogen gas will evolve and Cl- will be discharged and chlorine gas will evolve. (But keep in mind that chlorine is soluble in water, it would take time for it to evolve. Gas should be collected in an inverted measuring cylinder). This leaves us with two other ions, Na+ and OH- forming sodium hydroxide which is an alkali and extracted later. At the cathode: 2H+ + 2e- → H2 Hydrogen gas evolves. Observation is bubbles of colorless gas. Test to make sure by approaching a lighted splint, if positive it will burn with a pop sound. At the anode: 2Cl- - 2e- → Cl2 Chlorine gas evolves. Observation is bubbles of green gas. Test to make sure by approaching a damp blue litmus paper, it will turn red then bleach. 2NaCl + 2H2O → H2 + Cl2 + 2NaOH Electrolysis of Copper(II) Sulfate Solution: Ions present in the electrolyte are H+ and OH- from water and Cu+2 and SO4- from copper(ii) sulfate. At the Cathode: 4OH- - 4e- → O2 + 2H2O Oxygen from OH- ions is formed. Bubbles of colorless gas are formed. To test for oxygen, approach a glowing splint, if positive it relights. At the anode: Cu+2 + 2e- → Cu +2 Cu are discharged because copper ion is below in reactivity series than hydrogen. A pink-brown metal is formed. This leaves us with H+ and SO4- ions which forming sulfuric acid. Note: a copper sulfate solution is blue in color. In this process, the blue color gradually fades away because copper sulfate is being broken down. The solution becomes acidic becomes acidic because sulfuric acid is formed.