bài tập và lý thuyết Halogens

41 41.1 41.2 41.3 The Halogens Characteristic Properties of the Halogens Variation in Properties of the Halogens Comparative Study of the Reactions of Halide Ions 41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride 41.1 Characteristic Properties of the Halogens 41.1 Characteristic Properties of the Halogens (SB p.78) Introduction • Group VIIA elements include  fluorine  chlorine  bromine  iodine  astatine 41.1 Characteristic Properties of the Halogens (SB p.78) Introduction • Astatine  not much is known  radioactive 41.1 Characteristic Properties of the Halogens (SB p.78) Introduction • Group VIIA elements  also called halogens 41.1 Characteristic Properties of the Halogens (SB p.78) The halogens 41.1 Characteristic Properties of the Halogens (SB p.79) Introduction • All halogens  outermost shell electronic configuration of ns2np5  one electron short of the octet electronic configuration 41.1 Characteristic Properties of the Halogens (SB p.79) Introduction • In the free elemental state  form diatomic molecules  complete their octets by sharing their single unpaired p electrons 41.1 Characteristic Properties of the Halogens (SB p.79) Introduction • When halogens react with other elements  complete their octets  depending on the electronegativity of the element 41.1 Characteristic Properties of the Halogens (SB p.79) Introduction • Either  gaining an additional electron to form halide ions  or sharing their single unpaired p electrons to form single covalent bonds 41.2 Variation in Properties of the Halogens (SB p.91) (b) What chemical species are present in the following solutions? (i) Chlorine water (ii) Bromine water (iii) Iodine in potassium iodide solution (b) (i) Cl2(aq), Cl–(aq), ClO–(aq), H+(aq), H2O(l ) (ii) Br2(aq), Br–(aq), BrO–(aq), H+(aq), H2O(l ) (iii) I–(aq), K+(aq), I3–(aq), H2O(l) Back Answer 41.3 Comparative Study of the Reactions of Halide Ions (SB p.93) (a) Is bromine and iodine more soluble in water or 1,1,1trichloroethane? Explain your answer (a) Both bromine and iodine are more soluble in 1,1,1-trichloroethane than in water It is because both bromine and iodine are non polar molecules, and water is a much polar solvent than 1,1,1-trichloroethane Answer 41.3 Comparative Study of the Reactions of Halide Ions (SB p.93) (b) Describe a simple way to increase the solubility of iodine in water (b) The solubility of iodine in water can be increased by adding potassium iodide solution Iodine exists as triiodide ions in potassium iodide solution as shown in the following equation: I2(s) + KI(aq) → KI3(aq) Back Answer 41.3 Comparative Study of the Reactions of Halide Ions (SB p.97) (a) State any observable changes when the following substances are added into sodium iodide solution Give appropriate equations, if any (i) Iron(II) sulphate(VI) solution (a) (i) There is no observable change Answer 41.3 Comparative Study of the Reactions of Halide Ions (SB p.97) (a) State any observable changes when the following substances are added into sodium iodide solution Give appropriate equations, if any (ii) Chlorine water (a) (ii) The solution turns yellowish brown Cl2(aq) + 2NaI(aq) → I2(aq) + 2NaCl(aq) Answer 41.3 Comparative Study of the Reactions of Halide Ions (SB p.97) (a) State any observable changes when the following substances are added into sodium iodide solution Give appropriate equations, if any (iii) Sodium iodate(V) solution and dilute sulphuric(VI) acid (a) (iii) The solution turns yellowish brown 5I–(aq) + IO3–(aq) + 6H+(aq) → 3I2(aq) + 3H2O(l) Answer 41.3 Comparative Study of the Reactions of Halide Ions (SB p.97) (b) If you are given two solutions, sodium fluoride and sodium chloride, how would you distinguish them with a simple chemical test? State all observable changes and write equations where appropriate Answer (b) Sodium chloride solution and sodium fluoride solution can be distinguished using acidified silver nitrate(V) solution Sodium chloride solution reacts with acidified silver nitrate(V) solution to form a white precipitate, while sodium fluoride solution does not Ag+(aq) + Cl–(aq) → AgCl(s) white precipitate Back 41.3 Comparative Study of the Reactions of Halide Ions (SB p.100) (a) For each of the following pairs, which is a stronger acid? Explain your answers (i) Dilute hydrochloric acid and dilute hydrofluoric acid Answer 41.3 Comparative Study of the Reactions of Halide Ions (SB p.100) (a) (i) Dilute hydrochloric acid is a stronger acid than dilute hydrofluoric acid, as hydrochloric acid has a much larger K a value than hydrofluoric acid HCl(aq) + H2O(l) H3O+(aq) + Cl–(aq) Ka = × 107 mol dm–3 HF(l) + H2O(l) H3O+(aq) + F–(aq) Ka = × 10–4 mol dm–3 The small Ka value of hydrofluoric acid indicates that only a small amount of hydrogen fluoride molecules is ionized Most of hydrogen fluoride molecules still exist in the undissociated form The large Ka value of hydrochloric acid indicates that almost all hydrogen chloride molecules are ionized in water This can be explained by the presence of extensive intermolecular hydrogen bonds among hydrogen fluoride molecules 41.3 Comparative Study of the Reactions of Halide Ions (SB p.100) (a) For each of the following pairs, which is a stronger acid? Explain your answers (ii) Concentrated hydrochloric acid and concentrated hydrofluoric acid Answer 41.3 Comparative Study of the Reactions of Halide Ions (SB p.100) (a) (ii) Concentrated hydrofluoric acid is a stronger acid than concentrated hydrochloric acid Hydrogen fluoride is soluble in water In a dilute solution of hydrogen fluoride, it behaves only as a weak acid HF(l) + H2O(l) H3O+(aq) + F–(aq) (1) Ka = × 10–4 mol dm–3 However, in a more concentrated solution of hydrogen fluoride, another equilibrium is established with the fluoride ion forming the complex ion [HF2]– F–(aq) + HF(l) [HF2]–(aq) …… .(2) K = 5.1 dm3 mol–1 The equilibrium of reaction (2) shifts to the right as the concentration of hydrogen fluoride increases With the consumption of fluoride ions in reaction (2), the equilibrium of reaction (1) also shifts to the right The acid strength of hydrogen fluoride is therefore enhanced Hence, concentrated hydrofluoric acid is effectively a 41.3 Comparative Study of the Reactions of Halide Ions (SB p.100) (b) Explain the following phenomena: (i) Hydrogen fluoride is a liquid at room temperature and pressure (b) Answer (i) Hydrogen fluoride exists as a liquid at room temperature and pressure due to its ability to form extensive intermolecular hydrogen bonds 41.3 Comparative Study of the Reactions of Halide Ions (SB p.100) (b) Explain the following phenomena: (ii) Hydrogen fluoride forms acid salts – potassium hydrogen difluoride (b) Answer (ii) Hydrogen fluoride is able to react with other fluorides (e.g potassium fluoride) to form acid salts containing the stable [HF2]– ion (e.g potassium hydrogen fluoride) KF(s) + HF(l) KHF2(s) 41.3 Comparative Study of the Reactions of Halide Ions (SB p.100) (b) Explain the following phenomena: (iii) Hydrogen fluoride can be used to etch glass (b) (iii) The principle of etching glass by hydrofluoric acid can be explained by its reaction with the silicate of glass CaSiO3(s) + 6HF(aq) → CaF2(aq) + SiF4(aq) + 3H2O(l) Answer 41.3 Comparative Study of the Reactions of Halide Ions (SB p.100) (c) Complete and balance the following equations: (c) (i) F2(g) + H2O(l) → (ii) F2(g) + KOH(aq) (cold, dilute) → Answer (i) 2F2(g) + 2H2O(l ) → 4HF(aq) + O2(g) (iii) 2F2(g) + 2KOH(aq) → 2KF(aq) + OF2(g) + H2O(l) Back