Group 7 - The Halogens PDF

AS CHEMISTRY GROUP 7 - THE HALOGENS TREND IN BOILING POINTS All Group 7 elements exist as non-polar, diatomic F2 18 e's molecules. This means that they have very weak induced- dipole force...

AS CHEMISTRY GROUP 7 - THE HALOGENS TREND IN BOILING POINTS All Group 7 elements exist as non-polar, diatomic F2 18 e's molecules. This means that they have very weak induced- dipole forces between the molecules. Cl2 34 e's Induced dipole strength varies. The greater the number of electrons in the molecule, the stronger the induced Br2 70 e's dipole forces. The stronger the IMFs, the higher the melting / boiling points. I2 106 This is why boiling point increases down Group 7. Cl2 Br2 I2 Gas Green Brown Violet Liquid Orange Black / Violet Solid Grey TREND IN ELECTRONEGATIVITY XX F XX X XX Reminder! Electronegativity is a measure of the ability of an atom to attract a pair of electrons in a covalent bond. XX As you move up Group 7, electronegativity increases. Cl XX X As you move up the group, both shielding and distance XX of the outer energy level from the nucleus decreases. This means there is a stronger attraction between the XX nucleus and the bonding pair of electrons, so they are more likely to attract those electrons creating a dipole! Br X XX It can therefore also be said that they become stronger oxidising agents as you go up the group for the same XX reasons. XX i.e. they are better at attracting and electron from I XX another atom / ion. X XX AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY GROUP 7 - THE HALOGENS GROUP 7 REDOX Halogens are Halide ions (X-) are Oxidising Agents Reducing Agents Strongest oxidising agent F2 F- Weakest reducing agent XX XX - Cl2 Cl- O XO XX XX X XX Br2 Br- XX Weakest oxidising agent I2 I- Strongest reducing agent Fluorine atoms have the least Fluoride ions have the least amount amount of shielding, so they are the of shielding, so they the “worst” at “best” at attracting an electron from releasing an electron and donating another species (oxidising it). it to another species (reducing it). Iodine atoms have the greatest Iodide ions have the greatest amount of shielding, so they are the amount of shielding, so they are the “worst” at attracting an electron “best” at releasing an electron and from another species (oxidising it). donating it to another species (reducing it). AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY GROUP 7 - THE HALOGENS GROUP 7 DISPLACEMENT REACTIONS A halogen can “displace” a halide ion from its salt if that halide ion is lower down the group. In other words, if the halogen is a stronger oxidising agent than the Group 7 ion in the salt, it will oxidise it and become reduced itself. e.g. Cl2(g) + 2NaBr(aq) → 2NaCl(aq) + Br2(aq) oxidation state 0 -I -I 0 A halogen cannot displace a halide ion from its salt if it is higher up the group. There is no reaction. For example, why isn’t there a reaction between bromine and sodium chloride? Bromine is a weaker oxidising agent than chlorine, so it is unable to oxidise / displace it. Chlorine and bromine oxidise I- Chlorine oxidises Br- ions to Br2. ions to I2. The brown solution The orange solution observed is observed is the I2(aq) that is the Br2(aq) that is produced. produced. Cl- Br- I- orange brown Cl2 solution solution brown Br2 no reaction solution I2 no reaction no reaction Why do they react? Answer in the context of the oxidising power of the halogen. e.g. chlorine is a stronger oxidising agent than bromine, so Br- is oxidised to Br2 AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY GROUP 7 - THE HALOGENS SOLID SODIUM HALIDES + conc.H2SO4 We already know that that the reducing power of the halide ions increases down the group. This is shown in their reactions with conc. sulfuric acid. You need to learn these equations & observations! NaF & NaCl F- and Cl- do not have the reducing power to be able to reduce the S in H2SO4, so there is no REDOX reaction. Instead they undergo an acid-base reaction with it. NaF + H2SO4 → HF + NaHSO4 Steamy Fumes NaCl + H2SO4 → HCl + NaHSO4 Steamy Fumes H2SO4 acts as an acid as it donates a proton (H+). The sodium halide acts as a base as it accepts a proton. NaBr Br- is able to reduce S from +VII to +IV to produce SO2(g) 2Br- + H2SO4 + 2H+ → Br2 + SO2 + 2H2O Brown Fumes Colourless Gas NaI I- is also able to reduce S from +VII to +IV to produce SO2(g) 2I- + H2SO4 + 2H+ → I2 + SO2 + 2H2O Purple Fumes Colourless Gas I- is also able to reduce S from +VII to 0 to produce S(s) 6I- + H2SO4 + 6H+ → 3I2 + S + 4H2O Purple Fumes Yellow Solid I- is also able to reduce S from +VII to -II to produce H2S(g) Colourless, 8I- + H2SO4 + 8H+ → 4I2 + H2S + 5H2O Purple Fumes Pungent Gas AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY GROUP 7 - THE HALOGENS TESTING FOR HALIDE IONS We can qualitatively test for the presence of halide ions in solution using a two- step procedure. 1. Add acidified silver nitrate solution (H+ / AgNO3(aq)) Ionic Equation Observation Ag+(aq) + F-(aq) → no reaction Ag+(aq) + Cl-(aq) → AgCl(s) white precipitate Ag+(aq) + Br-(aq) → AgBr(s) cream precipitate Ag+(aq) + I-(aq) → AgI(s) pale yellow precipitate HINT! We acidify the solution with nitric acid (HNO3) for two reasons: 1. It neutralises any CO32- and OH- impurities that would also form a precipitate with Ag+ and interfere with the test. 2. If we used HCl or H2SO4, the Cl- / SO42- ions would give a false positive! 2. Add aqueous ammonia (NH3(aq))to the precipitates produced Precipitate + dil. NH3(aq) + conc. NH3(aq) AgCl(s) Redissolves Redissolves AgBr(s) No change Redissolves AgI(s) No change No change HINT! If you want to discriminate between AgCl(s) and AgBr(s), use dilute NH3(aq). If you want to discriminate between AgBr(s) and AgI(s), use concentrated NH3(aq). AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY GROUP 7 - THE HALOGENS REACTIONS OF CHLORINE Chlorine Chlorine is added in small amounts to drinking water to make it safe to drink. Chlorine reacts with water in a disproportionation reaction Cl2 + H2O → HClO + HCl oxidation state 0 +I -I HOCl (sodium chlorate I) is a very good antimicrobial agent (kills microorganisms). However, in greater concentrations this can also be harmful to humans. It is used in greater concentrations in swimming pools etc as, of course, it is not meant for drinking! In the presence of U.V. light, chlorine can also react with water to produce HCl & O2. 2Cl2 + 2H2O → 2HCl + O2 oxidation state 0 -II -I 0 Ethical Issues of Chlorinating / Fluorinating Drinking Water Fluoride ions (F-(aq)) can also be added to drinking water. The benefit of this is that, in small quantities, it helps prevent tooth decay and strengthen bones. Adding these to public drinking water can be considered controversial. Some people object to it, saying that they should have the right to choose whether or not their water contains these ions as, in larger quantities, they can cause harm. It is generally considered, however, that the benefits of the addition of these to our water systems have benefits that outweigh the risks. Bleach When Cl2 reacts with cold, dilute NaOH it produces ClO- ions (like the 1st reaction on this page). This is in high concentrations in bleach as an active ingredient to kill microbes. Cl2 + NaOH → NaCl + NaClO + H2O AQA www.chemistrycoach.co.uk © scidekick ltd 2024