AQA Chemistry A-level Group 7 - The Halogens Detailed Notes PDF

AQA Chemistry A-level 3.2.3: Group 7 - The Halogens Detailed Notes This work by PMT Education is licensed under https://bit.ly/pmt-cc https://bit.ly/pmt-edu-cc CC BY-NC-ND 4.0...

AQA Chemistry A-level 3.2.3: Group 7 - The Halogens Detailed Notes This work by PMT Education is licensed under https://bit.ly/pmt-cc https://bit.ly/pmt-edu-cc CC BY-NC-ND 4.0 https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc 3.2.3.1 - Trends in Properties The group 7 elements are ​highly reactive non-metals​ that need to gain an electron to form a 1- ion​ and achieve a full outer shell of electrons. Atomic Radius The atomic radius of group 7 elements​ increases down the group​ due to additional electron shells. Reactivity The group 7 elements need to gain an electron. As atomic radius increases this becomes harder as the positive attraction of the nucleus is weakened by additional ​shielding​. Therefore it is harder to attract an electron so ​reactivity decreases​ down the group. Ionisation Energy The first ionisation energy of group II metals ​decreases down the group​ due to a greater atomic radius and increased amounts of shielding. Boiling Point The group 7 elements are ​simple covalent molecules​ held together with ​van der waals forces. The strength of these intermolecular forces increases as the Ar of the molecule increases. Therefore the strength of the van der waals forces ​increases down the group meaning more energy is required to overcome them, resulting in a higher boiling point. Fluorine is a gas at room temperature whereas iodine is a solid. Oxidising Power The halogens act a ​good oxidising agents​ as they accept electrons from the species being oxidised and are reduced. This oxidising power ​decreases down the group​ as their ability to attract electrons decreases due to shielding and a greater atomic radius. The relative oxidising strengths mean a halogen will ​displace any halide beneath it ​in the Periodic Table. Example: https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Halide Ions The negative ions of halogens are known as ​halide ions​. These ions are ​good reducing agents​ as they donate electrons to the species being reduced and are themselves oxidised. This reducing power​ increases down the group​ as electrons are easier to lose from larger ions due to shielding and a larger atomic radius. These redox reactions with H​2​SO​4​ have to be known: 1. Fluoride and Chloride ions. 2. Bromide ions. 3. Iodide ions. The greater the reducing power, the longer the reaction as the halide is powerful enough to reduce more species. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Silver Nitrate Acidified silver nitrate is used to ​test for halide ions​ as it reacts to form different ​coloured precipitates ​depending on the ion present. The precipitates formed may not be clear to distinguish so they can be tested further using ​ammonia​. Cl​- Br​- I​- + AgNO​3 White precipitate Cream precipitate Yellow Precipitate (AgCl) (AgBr) (AgI) + dilute NH​3 Precipitate dissolves No Change No Change + conc. NH​3 Precipitate dissolves Precipitate dissolves No Change 3.2.3.2 - Chlorine and Chlorate(I) ions Chlorine reacts with cold water to produce ​Chlorate(I) ions (ClO​-​)​ and ​chloride ions​. Example: This is a ​disproportionation reaction​ as the chlorine is both oxidised and reduced. The oxidation state goes from zero to both ​+1 and -1​. In the presence of ​UV light​, chlorine decomposes water to produce ​oxygen and hydrochloric acid​. The chlorine is reduced in this reaction. Example: Chlorine is used in small quantities to kill bacteria in ​water treatment processes​. This poses some risks as chlorine can be ​toxic​; however the benefits of clean, treated water outweigh the risks. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Bleach Production Chlorine can be mixed with ​cold, aqueous sodium hydroxide​ to produce ​sodium hypochlorite​. This is a key ingredient in the production of bleach. Example: Tests for ions You need to know how to test for certain anions and cations for your required practical. Anions - Halides ​ ilver nitrate and ammonia​. The silver nitrate is These are tested for using acidified s acidified so that any other impurities that could form a precipitate are removed. Cl​- Br​- I​- + AgNO​3 White precipitate Cream precipitate Yellow Precipitate (AgCl) (AgBr) (AgI) + dilute NH​3 Precipitate dissolves No Change No Change + conc. NH​3 Precipitate dissolves Precipitate dissolves No Change Anions - Sulfate (SO​4​2-​) These are tested for using ​BaCl​2​ which reacts to form a ​white precipitate​. Anions - Hydroxide (OH​-​) These ions indicate that the substance is alkaline. Therefore they can be identified with ​red litmus, which turns blue​ or using universal indicator, which turns blue-purple. Anions - Carbonate (CO​3​2-​) When an acid such as HCl is added, the substance containing the carbonate ions will ​fizz (effervescence) and CO​2​ gas is given off​. This gas can be collected and bubbled through limewater​ which will turn ​cloudy​, confirming it as carbon dioxide. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc Cations - Group 2 The group 2 ions can be identified with a series of ​flame tests​. Calcium (Ca​2+​) Brick red Strontium (Sr​2+​) Red Barium (Ba​2+​) Pale green Cations - Ammonium (NH​4​+​) If ammonium ions are present, ammonia gas is given off, which is a base. Therefore the presence of ammonium ions can be tested by holding​ red litmus ​over a petri dish of the substance being tested. It will ​turn blue​ if ammonium ions are present. Alternatively, they can be tested for in the same way but by ​adding NaOH​ to produce the ammonia gas faster. https://bit.ly/pmt-cc https://bit.ly/pmt-edu https://bit.ly/pmt-cc