Unit 1 Module 3 The Periodic Table - Pearson Schools

More documents

Recommendations

Info

Water treatment Module 3 The Periodic Table Group 7 elements: uses and halide tests Chlorine is a toxic gas. In the UK, small amounts of chlorine have been added to drinking water to kill bacteria since the late 1890s. Some people claim that chlorination of water has done more to improve public health than anything else. Certainly, chlorine is a most effective bacteria killer in our water supplies. Despite this, the addition of chlorine has not always been welcomed. Some environmentalists are concerned that chlorine reacts with organic matter to form traces of chlorinated hydrocarbons, suspected of causing cancer. In 1991, the government in Peru thought that this was an unacceptable risk and stopped adding chlorine to drinking water. Unfortunately, they had forgotten the very reason for adding chlorine in the first place and an outbreak of cholera followed. This affected a million people and caused 10 000 deaths. The Peruvian government swiftly reversed their decision and resumed adding chlorine to their drinking water. Testing for halide ions The presence of halide ions can be detected with a simple test tube test. • The unknown halide substance is first dissolved in water. • An aqueous solution of silver nitrate, AgNO 3 (aq), is added. • Silver ions, Ag + (aq), from the AgNO 3 (aq) react with any halide ions, X – (aq), present, forming a silver halide precipitate, AgX(s). • The silver halide precipitates are coloured – the colour tells us which halide is present. Sometimes it is difficult to judge the exact colour. • If you are unsure, then add aqueous ammonia, NH 3 (aq). • Different halide precipitates have different solubilities in aqueous ammonia – this confirms which halide is present. Halide test results are: chloride: Ag + (aq) + Cl – (aq) → AgCl(s) white precipitate, soluble in dilute NH 3 (aq) bromide: Ag + (aq) + Br – (aq) → AgBr(s) cream precipitate, soluble in conc NH 3 (aq) iodide: Ag + (aq) + I – (aq) → AgI(s) yellow precipitate, insoluble in conc NH 3 (aq) This type of reaction is called a precipitation reaction. A precipitation reaction takes place in aqueous solution when aqueous ions react together to form a solid precipitate (see Figure 2). Figure 2 Halide tests with aqueous silver nitrate. Silver halides precipitated by reacting aqueous silver nitrate with aqueous halide solutions Examiner tip T . AgNO 3 (aq) pp Ag + (aq) ; T pp Cl – (aq), Br – (aq) or I – (aq) . Then: Ag + (aq) + Cl – (aq) ⎯→ AgCl() Y j O 3– (aq) and . T . Key definition A precipitation reaction . p x . AgCl(s) AgBr(s) AgI(s) Questions 1 Summarise the general properties of a Group 7 element. 2 What is the difference between a halogen and a halide? 3 How could you distinguish between NaCl, NaBr and NaI by using a simple test? 95 935 chemistry.U1 M3.indd 95 13/11/07 11:51:12 am
1.3 The Periodic Table summary Trends in 1st ionisation energies and atomic radii Shells increase Li [He]2s 1 Be [He]2s 2 Periodic trends B [He]2s 2 2p 1 C [He]2s 2 2p 2 N [He]2s 2 2p 3 O [He]2s 2 2p 4 F [He]2s 2 2p 5 Shielding increases Atomic radius increases Number of protons increase First ionisation energy increases Atomic radius decreases First ionisation energy decreases Na [Ne]3s 1 Mg [Ne]3s 2 Al [Ne]3s 2 3p 1 Si [Ne]3s 2 3p 2 P [Ne]3s 2 3p 3 S [Ne]3s 2 3p 4 Cl [Ne]3s 2 3p 5 Trends in melting and boiling points Li Be B C N O F Ne Na Mg Al Si P S Cl Ar Giant metallic lattice Giant covalent lattice Simple molecular lattice Melting point Groups Group 2 Group 7 Ca H 2 O Ca(OH) 2 Disproportionation HOCl HCl NaClO NaCl HCl O 2 H 2 O H 2 O NaOH(aq) CaCl 2 CaCl 2 H 2 O CO 2 HCl CaO Cl 2 Heat NaI(aq) NaBr(aq) CaCO 3 HCl I 2 NaCl Br 2 NaCl Redox 96 935 chemistry.U1 M3.indd 96 13/11/07 11:51:20 am
Page 1 and 2: AS Exclusively endorsed by OCR for
Page 3 and 4: Contents Introduction UNIT 1 Atoms,
Page 5 and 6: 34 1 How many protons, neutrons and
Page 7 and 8: Unit Module 3 The Periodic Table 1
Page 9 and 10: 1.3 1 The Periodic Table: searching
Page 11 and 12: 1.3 2 The Periodic Table: Mendeleev
Page 13 and 14: 1.3 3 The modern Periodic Table By
Page 15 and 16: 1.3 4 Periodicity: ionisation energ
Page 17 and 18: 1.3 5 Periodicity: boiling points B
Page 19 and 20: 1.3 6 Group 2 elements: redox react
Page 21 and 22: 1.3 7 Group 2 compounds: reactions
Page 23 and 24: 1.3 8 Group 7 elements: redox react
Page 25: 1.3 9 Group 7 elements: uses and ha
Page 29 and 30: 1.3 Examination questions 1 The ato
Page 31 and 32: Glossary acid A species that is a p
Page 33 and 34: heterolytic fission The breaking of
Page 35 and 36: substitution reaction A reaction in
Page 37 and 38: Index chlorine as water treatment 9
Page 39 and 40: Index malleability 67 margarine man

Unit 1 Module 3 The Periodic Table - Pearson Schools

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?