IGCSE Double Award Extended Coordinated Science Chemistry 5 - Electricity and Chemistry Electrolysis You need to know that electrolysis is: - The breakdown of ionic substances into their constituent elements using electricity. During electrolysis, an electric current is passed through the liquid to separate the ions. - Although solid ionic substances do not conduct electricity, - Molten or aqueous (dissolved in water) ionic substances conduct electricity - because the ions are free to move about separately in the liquid - And because electricity is the flow of charge (and ions can carry charge) Therefore, for electrolysis, the ions must be free to move in the structure, either: - Ionic substance is dissolved in water (aqueous) : NaCl (s) + Cl- (aq) - Ionic substance in molten (melted to a liquid) The submerged conductive parts are called electrodes, and the liquid (solution or molten) is called the electrolyte. There are two, electrodes. One positive and one negative (because of the flow of current) They are named after the type of ions they attract. - Cathode is the negative electrode - Remember, cation is positive, but cathodes are negative - This is because cations (+) are the ions that are attracted to the cathode (-). - Anode is the positive electrode - Because the anions (-) are attracted to the anode (+)
Principles of Electrolysis You need to know the principles of electrolysis. Cations are attracted to the cathode, and anions are attracted to the anode. At each of the electrodes, a reaction occurs to change the respective ions back into neutral particles of the element. - Reduction and oxidation reactions occur (more detail on redox reactions in Unit C7.2) Reduction is the gain of electrons, and oxidation is the loss of electrons. - Use OILRIG (Oxidation Is Loss, Reduction Is Gain) to remember. At the cathode (negative) there is an excess of electrons, so the positive cations can come and get electrons. - When the cation gains electrons to be neutral, it is released as the elemental form of the ion. - We can write ionic equations to show this. (e - stands for a single electron) - 2H + + 2e- (aq) -> H 2(g) // Cu+ + e- (aq) -> Cu (s) // Fe 3+ + 3e- (aq) -> Fe (s) - Of course, if the product formed is a gas, it will be released as a gas from the cathode. - If it is a solid (metal), the product will deposit on the surface of the cathode. At the anode (positive) there is a deficit of electrons, so the negative anions can come and drop off electrons. - When the anion drops electrons off to be neutral, it is released as the elemental form of the ion. - We can write ionic equations to show this. (e - stands for a single electron) - 2Cl - -> Cl + 2e- (aq) 2(g) // 2I- -> I + 2e- (aq) 2(g) // 2O 2- -> O + 4e- (aq) 2(g) The products formed at each electrode, of course, depend on the electrolyte used. - If molten substances are used, the only ions present are the ions in the substance, for example: - Molten NaCl will only have Na + ions and Cl - ions in the electrolyte. - So, the Na + ions will go to the cathode and be reduced to Na, sodium metal - and Cl - ions will go to the anode and be oxidised to Cl 2, chlorine gas. - As you can see, in molten substances the ions simply become the products. However in aqueous solutions, the predicting the the products are require more knowledge. - If NaCl is dissolved in water, it will become Na + ions and Cl - ions. - But, now there are water molecules too, and they become H + ions and OH - (hydroxide) ions. - You can see there are multiple cations and anions present in the electrolyte
Products of Electrolysis You need to know how to predict the products of electrolysis. Rules for predicting products of electrolysis of aqueous solutions: - For cations, the knowledge of the reactivity series of metals is important. - The series ranks the metals in order of reactivity (more in Unit C10.2) - You need to compare the reactivity of the metal to hydrogen - Only metals below hydrogen in the reactivity series are reduced at the cathode. - So copper, silver, gold, platinum will be produced as metal products - If the metal is more reactive than hydrogen, hydrogen gas is produced instead. - For anions, simply, - if there are halogen (group VII) ions present in the electrolyte, - (fluoride Fl -, chloride Cl -, bromide Br -, iodide I -, astatine At - ions) - the halogen ion will be oxidised, producing halogen gas as product - If there are no halogen ions present, oxygen gas is produced. - Because the hydroxide OH - ions undergo the following reduction: - 4OH - -> 2H 2 O + O 2 (g) + 4e - - As you can see, the reaction also produces water. If you do this by drawing a simple table, it is easier to have a view of the ions and the whole electrolysis process. - This is what the table can look like: - In the table, you can write down all the information you know first. - Let s look at aqueous sodium chloride ( ), H + (aq) Cl - (aq), OH - (aq) - Using the rules above, we know that - At the cathode, hydrogen will be produced since sodium is more reactive than hydrogen. - At the anode, chlorine will be produced since it is a halogen. - So we can put the products in the table., H + (aq) H 2 (g) Cl - (aq), OH - (aq) Cl 2 (g) - Now we need to what is left in the electrolyte, - Hydrogen and chlorine was produced and removed - We still have the sodium ion and the hydroxide OH - (aq) ion. - These two are left in solution as ions, so the electrolyte will now be: - Aqueous sodium hydroxide NaOH (aq), H + (aq) H 2 (g) Cl - (aq), OH - (aq) Cl 2 (g) NaOH (aq)
Important Electrolysis You need to know these specified electrolysis reactions from the syllabus. Before you look at the tables, you can try to make the tables for yourself for practice. Molten lead (II) bromide PbBr 2 (l) Pb 2+ (l) Pb (s) Br - (l) Br 2 (g) Aqueous copper (II) chloride CuCl 2 (aq) Cu 2+ (aq), H + (aq) Cu (s) Cl - (aq), OH - (aq) Cl 2 (g) H 2 O Aqueous copper (II) sulphate CuSO 4 (aq) Cu 2+ (aq), H + 2- (aq) Cu (s) SO 4 (aq), OH - (aq) O 2 (g) H 2 SO 4 (aq) Dilute sulphuric acid H 2 SO 4 (aq) H + (aq) H 2 (g) SO 4 2-(aq), OH - (aq) O 2 (g) H 2 SO 4 (aq) Molten aluminium oxide (with molten cryolite) Al 2 O 3(l) Al 3+ (l) Al (s) O 2- (l) O 2 (g) - This reaction is important because this is how aluminium is extracted from its ore. - Aluminium ore, bauxite, contains aluminium oxide. - To extract the aluminium metal, the ore is melted then electrolysed. - Aqueous aluminium oxide will not give the desired product aluminium, - because aluminium is more reactive than hydrogen - So molten aluminium oxide must be used. - However, aluminium ore has very high melting point, so a substance called cryolite is mixed in with the ore - Cryolite lowers the boiling point of aluminium oxide. - Less energy is needed, so it is less expensive. Concentrated aqueous sodium chloride, H + (aq) H 2 (g) Cl - (aq), OH - (aq) Cl 2 (g) NaOH (aq) - This reaction is important because it gives many useful products from a common chemical. - Aqueous sodium chloride, or brine (salt water), is fairly common. However after electrolysis, it gives - Chlorine gas and sodium hydroxide, which are both very useful and expensive.
The syllabus says you should be able to, (SO check if you can): - State that electrolysis is the chemical effect of electricity on ionic compounds, causing them to break up into simpler substances, usually elements. - Use the terms electrode, electrolyte, anode and cathode. - Describe electrolysis in terms of the ions present and the reactions at the electrodes. - Describe the electrode products, using inert electrodes, in the electrolysis of: - molten lead(ii) bromide - aqueous copper chloride - dilute sulfuric acid. - State and use the general principle that metals or hydrogen are formed at the negative electrode (cathode), and that non-metals (other than hydrogen) are formed at the positive electrode (anode). - Relate the products of electrolysis to the electrolyte and electrodes used, exemplified by the specific examples in the Core content, together with aqueous copper(ii) sulfate using carbon electrodes and using copper electrodes (as used in the refining of copper). - Describe the electroplating of metals, using laboratory apparatus. - Predict the products of the electrolysis of a specified binary compound in the molten state, or in aqueous solution. - Describe, in outline, the chemistry of the manufacture of: - aluminium from pure aluminium oxide in molten cryolite - chlorine, hydrogen and sodium hydroxide from concentrated aqueous sodium chloride.