2.3 Halogens Revision Guide (AQA)

2.3 Halogens Fluorine (F2): very pale yellow gas. It is highly reactive Chlorine : (Cl2) greenish, reactive gas, poisonous in high concentrations Bromine (Br2) : red liquid, that gives off dense brown/orange poisonous fum...

2.3 Halogens Fluorine (F2): very pale yellow gas. It is highly reactive Chlorine : (Cl2) greenish, reactive gas, poisonous in high concentrations Bromine (Br2) : red liquid, that gives off dense brown/orange poisonous fumes Iodine (I2) : shiny grey solid sublimes to purple gas. Trend in melting point and boiling point Trend in electronegativity Increase down the group Electronegativity is the relative tendency of an atom in a molecule to attract electrons in a covalent bond to itself. As the molecules become larger they have more electrons and so have larger van der As one goes down the group the electronegativity of the waals forces between the molecules. As the elements decreases. intermolecular forces get larger more energy As one goes down the group the atomic radii increases due has to be put into break the forces. This to the increasing number of shells. The nucleus is therefore increases the melting and boiling points. less able to attract the bonding pair of electrons. 1. The displacement reactions of halide ions by halogens. A halogen that is a strong oxidising agent will The oxidising strength decreases down the group. displace a halogen that has a lower oxidising Oxidising agents are electron acceptors. power from one of its compounds. know these Chlorine will displace both bromide and iodide ions; bromine will displace iodide ions observations ! Chlorine (aq) Bromine (aq) Iodine (aq) The colour of the solution in the test tube shows which free potassium Very pale green Yellow solution, no Brown solution, halogen is present in solution. chloride (aq) solution, no reaction no reaction Chlorine =very pale green reaction solution (often colourless), potassium Yellow solution, Cl Yellow solution, no Brown solution, Bromine = yellow solution bromide (aq) has displaced Br reaction no reaction Iodine = brown solution (sometimes black solid potassium Brown solution, Cl Brown Solution, Br Brown Solution, present) iodide (aq) has displaced I has displaced I no reaction Be able to write these reactions as two half Cl2(aq) + 2Br – (aq)  2Cl – (aq) + Br2(aq) equations showing oxidation or reduction Cl2(aq) + 2I – (aq)  2Cl – (aq) + I2(aq) e.g. 2Br - (aq) Br2 (aq)+ 2e- Br2(aq) + 2I – (aq)  2Br – (aq) + I2(aq) Cl2 (aq)+ 2e-  2Cl- (aq) 2. The reactions of halide ions with silver nitrate. The role of nitric acid is to react with any carbonates present to prevent formation of the precipitate This reaction is used as a test to identify which halide ion Ag2CO3. This would mask the desired observations is present. The test solution is made acidic with nitric 2 HNO3 + Na2CO3  2 NaNO3 + H2O + CO2 acid, and then silver nitrate solution is added dropwise. Fluorides produce no precipitate The silver halide precipitates can be treated with ammonia Chlorides produce a white precipitate solution to help differentiate between them if the colours look Ag+(aq) + Cl- (aq)  AgCl(s) similar: Bromides produce a cream precipitate Silver chloride dissolves in dilute ammonia to form a Ag+(aq) + Br- (aq)  AgBr(s) complex ion Iodides produce a pale yellow precipitate AgCl(s) + 2NH3(aq) [Ag(NH3)2]+ (aq) + Cl- (aq) Ag+(aq) + I- (aq)  AgI(s) Colourless solution Silver bromide dissolves in concentrated ammonia to form a complex ion AgBr(s) + 2NH3(aq) [Ag(NH3)2]+ (aq) + Br - (aq) Colourless solution Silver iodide does not react with ammonia – it is too insoluble. N Goalby chemrevise.org 1 3. The reaction of halide salts with concentrated sulfuric acid. The halides show increasing power as Explanation of differing reducing power of halides reducing agents as one goes down the A reducing agent donates electrons. group. This can be clearly demonstrated in The reducing power of the halides increases down group 7 the various reactions of the solid halides with They have a greater tendency to donate electrons. concentrated sulfuric acid. This is because as the ions get bigger it is easier for the Know the equations and observations of outer electrons to be given away as the pull from the nucleus these reactions very well. on them becomes smaller. Fluoride and Chloride F- and Cl- ions are not strong enough reducing agents to reduce the S in H2SO4. No redox reactions occur. Only acid-base reactions occur. NaF(s) + H2SO4(l)  NaHSO4(s) + HF(g) These are acid – base reactions and Observations: White steamy fumes of HF are evolved. not redox reactions. H2SO4 plays the NaCl(s) + H2SO4(l)  NaHSO4(s) + HCl(g) role of an acid (proton donor). Observations: White steamy fumes of HCl are evolved. Bromide Br- ions are stronger reducing agents than Cl- and F- and after the initial acid-base reaction, the bromide ions reduce the sulfur in H2SO4 from +6 to + 4 in SO2 Acid- base step: NaBr(s) + H2SO4(l)  NaHSO4(s) + HBr(g) Observations: White steamy fumes of HBr are evolved. Redox step: 2 H+ + 2 Br - + H2SO4  Br2(g) + SO2(g) + 2 H2O(l) orange fumes of bromine are also Overall equation: combining two steps above: evolved and a colourless, acidic gas 2NaBr + 3H2SO4  2NaHSO4 + SO2 + Br2 + 2H2O SO2 Ox ½ equation 2Br -  Br2 + 2e- Reduction product = sulfur dioxide Re ½ equation H2SO4 + 2 H+ + 2 e- SO2 + 2 H2O Note the H2SO4 plays the role of acid in the first step producing HBr and then acts as an oxidising agent in the second redox step. Iodide I- ions are the strongest halide reducing agents. They can reduce the sulfur from +6 in H2SO4 to + 4 in SO2, to 0 in S and -2 in H2S. NaI(s) + H2SO4(l)  NaHSO4(s) + HI(g) Observations: 2 H+ + 2 I- + H2SO4  I2(s) + SO2(g) + 2 H2O(l) White steamy fumes of HI are evolved. 6 H+ + 6 I- + H2SO4 3 I2 + S (s) + 4 H2O (l) Black solid and purple fumes of Iodine are 8 H+ + 8 I- + H2SO4  4 I2(s) + H2S(g) + 4 H2O(l) also evolved A colourless, acidic gas SO2 A yellow solid of sulfur Ox ½ equation 2I -  I2 + 2e- H2S (Hydrogen sulfide), a gas with a bad egg Re ½ equation H2SO4 + 2 H+ + 2 e- SO2 + 2 H2O smell, Re ½ equation H2SO4 + 6 H+ + 6 e- S + 4 H2O Reduction products = sulfur dioxide, sulfur Re ½ equation H2SO4 + 8 H+ + 8 e- H2S + 4 H2O and hydrogen sulfide Note the H2SO4 plays the role of acid in the first step producing HI and then acts as an oxidising agent in the three redox steps. N Goalby chemrevise.org 2 4. The disproportionation reactions of chlorine. Disproportionation is the name for a reaction where Reaction with water in sunlight an element simultaneously oxidises and reduces. If the chlorine is bubbled through water in the presence of bright sunlight a different reaction Chlorine with water: occurs. Cl2 (g) + H2O (l) ⇌ HClO (aq) + HCl (aq) 2Cl2 + 2H2O  4H+ + 4Cl- + O2 The same reaction occurs to an equilibrium mixture of chlorine water when standing in sunlight. The Chlorine is both simultaneously reducing and oxidising greenish colour of chlorine water fades as the Cl2 reacts and a colourless gas (O2) is produced. If some universal indicator is added to the solution it will first turn red due to the acidity of both reaction products. It The greenish colour of these solutions is will then turn colourless as the HClO bleaches the colour. due to the Cl2 Chlorine is used in water treatment to kill bacteria. It has been used to treat drinking water and the water in swimming pools. The benefits to health of water treatment by chlorine outweigh its toxic effects. Reaction of chlorine with cold dilute NaOH solution: Cl2,(and Br2, I2) in aqueous solutions will react with cold sodium hydroxide. The colour of the halogen solution will fade to colourless. Cl2 (aq) + 2 NaOH (aq)  NaCl (aq) + NaClO (aq) + H2O (l) The mixture of NaCl and NaClO is used as bleach and to disinfect/ kill bacteria. Naming chlorates/sulfates In IUPAC convention the various forms of sulfur and chlorine compounds where oxygen is combined are all called sulfates and chlorates with relevant oxidation number given in roman numerals. If asked to name these compounds remember to add the oxidation number. NaClO: sodium chlorate(I) NaClO3: sodium chlorate(V) K2SO4 potassium sulfate(VI) K2SO3 potassium sulfate(IV) N Goalby chemrevise.org 3