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1 of 43 © Boardworks Ltd 2009 Halogens (6 min) https://www.youtube.com/watch?v=yW_C10 cEzMk 2 of 43 © Boardworks Ltd 2009 State at room temperature • Chlorine is a yellow – green gas • Bromine is a red - brown liquid which easily evaporates to give an orange – brown vapour • Iodine is a grey – black solid which sublimes to give a violet vapour 3 of 43 © Boardworks Ltd 2009 What are the halogens? The halogens are the elements in Group 7 of the periodic table. The name halogen comes from the Greek words for salt-making. 4 of 43 © Boardworks Ltd 2009 Physical properties of halogens What is the trend here for boiling and melting points? What is the trend for color? 5 of 43 © Boardworks Ltd 2009 Trends in boiling point Halogen molecules increase in number of electrons down the group. This leads to greater van der Waals forces between molecules, increasing the energy needed to separate the molecules and therefore higher melting and boiling points. van der Waals forces fluorine atomic radius = 42 × 10-12 m boiling point = -118 °C 6 of 43 iodine atomic radius = 115 × 10-12 m boiling point = 184 °C © Boardworks Ltd 2009 Hydrogen halides Predict HF boiling point Why are you wrong? Hydrogen halide Boiling point (°C) HF 20 HCl -85 HBr -67 HI -35 Hydrogen fluoride has an unexpectedly high boiling point compared to the other hydrogen halides. This is due to hydrogen bonding between the H–F molecules. 7 of 43 © Boardworks Ltd 2009 Trends in electronegativity Electronegativity, or ability to attract electrons, what is the trend? Answer: decreases down the group Why? Although there is an increased nuclear charge there no significant effect from all these protons because there are more electron shells and more shielding. Iodine atoms therefore attract electrons density less strongly than fluorine. What does this suggest about halogen reactivity? fluorine atomic radius = 42 × 10-12 m electronegativity = 4.0 8 of 43 iodine atomic radius = 115 × 10-12 m electronegativity = 2.5 © Boardworks Ltd 2009 Astatine The name astatine comes from the Greek word for unstable. It is estimated that only 30 grams of astatine exist on Earth at any one time. This is because it is radioactive, and its most stable isotope (210At) has a half-life of only 8 hours. It was first made artificially in 1940, by bombarding 209Bi with aradiation. Predict the properties of astatine? Colour? Answer: Very Dark, perhaps black (presumed) Why presumed? State at room temperature? Answer: Solid Electronegativity? 9 of 43 Answer: Low, perhaps 2.2 © Boardworks Ltd 2009 Quiz 1. Features included in group VII elements are A) non-metals B) diatomic molecules C)single covalent bond D) all of them ANS: D 2. Forces between diatomic halogens are A)Van der Waal's forces B) dipole-dipole forces C) electrovalent forces D) Hydrogen bond ANS: A 3. Volatility of Halogens indicates their A)Relative inertness B) ease of evaporation C) density D) All of Above Answer B 4. Halogen that is least volatile is A)F2 B) Cl2 C) Br2 D) I2 Answer D 5.) Color of Iodine (I2) is A)White B) yellow C) purple (vapors) D) red Answer C 6.) Hydroiodic acid is one of the 6 strong acids, so why is hydrofluoric acid so much higher than HI in boiling and melting points? A)HF has less electrons B) HI nucleus is more shielded C) Hydrogen Bonding ANSWER C 10 of 43 © Boardworks Ltd 2009 11 of 43 © Boardworks Ltd 2009 Reactions with hydrogen The halogens react with H2 to product hydrogen halides. Cl2(g) + H2(g) 2HCl(g) FAST Chlorine and hydrogen explode in bright sunlight but react slowly in the dark. Bromine and hydrogen react slowly on heating with a platinum catalyst. Br2(g) + H2(g) 2HBr(g) SLOW VERY POOR: Iodine combines partially and very slowly with hydrogen, even on heating. 12 of 43 © Boardworks Ltd 2009 Reactivity with Iron Halogen Reaction with iron wool Fluorine Reacts with almost anything instantly. Very few scientists handle fluorine because it is so dangerous. Chlorine Reacts with heated iron wool very quickly. Bromine Has to be warmed and the iron wool heated. The reaction is faster. Iodine Has to be heated strongly. The reaction is slow 13 of 43 © Boardworks Ltd 2009 What is the reactivity of the halogens? The reactions of the halogens with iron and hydrogen show that their reactivity decreases down the group. Halogen Reaction with Iron wool A Metal chlorine Iron wool burns brightly. Reaction with hydrogen Explodes in sunlight, reacts slowly in the dark. bromine Iron wool glows but less Reacts slowly on brightly than with chlorine. heating with catalyst. iodine Iron wool has a very slight glow. Reacts partially and very slowly. How do you think fluorine and astatine would react with iron wool and hydrogen? 14 of 43 © Boardworks Ltd 2009 Halogen Reactivity What generalization can you make about halogen reactivity? Halogens decrease down the group: At < I < Br < Cl < F What feature of halogen molecules accounts for the trend above? Atomic radius increases in size down the group. Although the nuclear charge increases, the shielding effect of the inner electrons blocks its pull. This lessens the attraction for valence electrons of other atoms, decreasing reactivity. This decrease also occurs because electronegativity decreases down a group; therefore, there is less electron "pulling.“ ability. 15 of 43 © Boardworks Ltd 2009 Halide ion Action of AgNO3(aq) Action of dilute NH3(aq) Action of conc. NH3(aq) F–(aq) Cl–(aq) Br–(aq) I–(aq) 16 of 43 © Boardworks Ltd 2009 Halide ion Action of AgNO3(aq) F–(aq) No precipitate Action of dilute NH3(aq) Action of conc. NH3(aq) Cl–(aq) Br–(aq) I–(aq) 17 of 43 © Boardworks Ltd 2009 Halide ion Action of AgNO3(aq) F–(aq) No precipitate Cl–(aq) Ag+(aq) + Cl–(aq) → AgCl(s) white ppt Action of dilute NH3(aq) Action of conc. NH3(aq) Br–(aq) I–(aq) 18 of 43 © www.chemsheets.co.uk 039 10-Jul-12 ©AS Boardworks Ltd 2009 Halide ion Action of AgNO3(aq) F–(aq) No precipitate Action of dilute NH3(aq) Action of conc. NH3(aq) Soluble Cl–(aq) Ag+(aq) + Cl–(aq) → AgCl(s) white ppt AgCl(s) + 2 NH3(aq) → [Ag(NH3)2]+(aq) + Cl–(aq) Br–(aq) I–(aq) 19 of 43 © www.chemsheets.co.uk 039 10-Jul-12 ©AS Boardworks Ltd 2009 Halide ion Action of AgNO3(aq) F–(aq) No precipitate Cl–(aq) Ag+(aq) + Cl–(aq) → AgCl(s) white ppt Action of dilute NH3(aq) Action of conc. NH3(aq) Soluble Soluble AgCl(s) + 2 NH3(aq) → [Ag(NH3)2]+(aq) + Cl–(aq) AgCl(s) + 2 NH3(aq) → [Ag(NH3)2]+(aq) + Cl–(aq) Br–(aq) I–(aq) 20 of 43 © www.chemsheets.co.uk 039 10-Jul-12 ©AS Boardworks Ltd 2009 Halide ion Action of AgNO3(aq) F–(aq) No precipitate Cl–(aq) Br–(aq) Ag+(aq) + Cl–(aq) → AgCl(s) white ppt Action of dilute NH3(aq) Action of conc. NH3(aq) Soluble Soluble AgCl(s) + 2 NH3(aq) → [Ag(NH3)2]+(aq) + Cl–(aq) AgCl(s) + 2 NH3(aq) → [Ag(NH3)2]+(aq) + Cl–(aq) Ag+(aq) + Br–(aq) → AgBr(s) cream ppt I–(aq) 21 of 43 © www.chemsheets.co.uk 039 10-Jul-12 ©AS Boardworks Ltd 2009 Halide ion Action of AgNO3(aq) F–(aq) No precipitate Cl–(aq) Br–(aq) Ag+(aq) + Cl–(aq) → AgCl(s) white ppt Ag+(aq) + Br–(aq) → AgBr(s) cream ppt Action of dilute NH3(aq) Action of conc. NH3(aq) Soluble Soluble AgCl(s) + 2 NH3(aq) → [Ag(NH3)2]+(aq) + Cl–(aq) AgCl(s) + 2 NH3(aq) → [Ag(NH3)2]+(aq) + Cl–(aq) Insoluble I–(aq) 22 of 43 © www.chemsheets.co.uk 039 10-Jul-12 ©AS Boardworks Ltd 2009 Halide ion Action of AgNO3(aq) F–(aq) No precipitate Cl–(aq) Br–(aq) Ag+(aq) Ag+(aq) + Cl–(aq) + Br–(aq) → AgCl(s) white ppt → AgBr(s) cream ppt Action of dilute NH3(aq) Action of conc. NH3(aq) Soluble Soluble AgCl(s) + 2 NH3(aq) → [Ag(NH3)2]+(aq) + Cl–(aq) AgCl(s) + 2 NH3(aq) → [Ag(NH3)2]+(aq) + Cl–(aq) Soluble Insoluble AgBr(s) + 2 NH3(aq) → [Ag(NH3)2]+(aq) + Br–(aq) I–(aq) 23 of 43 © www.chemsheets.co.uk 039 10-Jul-12 ©AS Boardworks Ltd 2009 Halide ion Action of AgNO3(aq) F–(aq) No precipitate Cl–(aq) Br–(aq) Ag+(aq) Ag+(aq) I–(aq) 24 of 43 + Cl–(aq) + Br–(aq) → AgCl(s) white ppt → AgBr(s) cream ppt Action of dilute NH3(aq) Action of conc. NH3(aq) Soluble Soluble AgCl(s) + 2 NH3(aq) → [Ag(NH3)2]+(aq) + Cl–(aq) AgCl(s) + 2 NH3(aq) → [Ag(NH3)2]+(aq) + Cl–(aq) Soluble Insoluble AgBr(s) + 2 NH3(aq) → [Ag(NH3)2]+(aq) + Br–(aq) Ag+(aq) + I–(aq) → AgI(s) yellow ppt © www.chemsheets.co.uk 039 10-Jul-12 ©AS Boardworks Ltd 2009 Halide ion Action of AgNO3(aq) F–(aq) No precipitate Cl–(aq) Br–(aq) Ag+(aq) 25 of 43 + Br–(aq) → AgCl(s) white ppt Action of conc. NH3(aq) Soluble Soluble AgCl(s) + 2 NH3(aq) → [Ag(NH3)2]+(aq) + Cl–(aq) AgCl(s) + 2 NH3(aq) → [Ag(NH3)2]+(aq) + Cl–(aq) Soluble → AgBr(s) cream ppt Insoluble Ag+(aq) + I–(aq) → AgI(s) yellow ppt Insoluble Ag+(aq) I–(aq) + Cl–(aq) Action of dilute NH3(aq) AgBr(s) + 2 NH3(aq) → [Ag(NH3)2]+(aq) + Br–(aq) © www.chemsheets.co.uk 039 10-Jul-12 ©AS Boardworks Ltd 2009 Halide ion Action of AgNO3(aq) F–(aq) No precipitate ALWAYS Soluble Cl–(aq) Br–(aq) I–(aq) Ag+(aq) Ag+(aq) + Cl–(aq) + Br–(aq) → AgCl(s) white ppt → AgBr(s) cream ppt Ag+(aq) + I–(aq) → AgI(s) yellow ppt 26 of 43 Action of dilute NH3(aq) nothing to dissolve Action of conc. NH3(aq) nothing to dissolve Soluble Soluble AgCl(s) + 2 NH3(aq) → [Ag(NH3)2]+(aq) + Cl–(aq) AgCl(s) + 2 NH3(aq) → [Ag(NH3)2]+(aq) + Cl–(aq) Soluble Insoluble AgBr(s) + 2 NH3(aq) → [Ag(NH3)2]+(aq) + Br–(aq) Insoluble Insoluble © Boardworks Ltd 2009 Reactions with halide ions What is the redox pattern with halogens and the Halide ions? 27 of 43 © Boardworks Ltd 2009 Cl2(aq) Br2(aq) I2(aq) Cl–(aq) Br–(aq) I–(aq) 28 of 43 © Boardworks Ltd 2009 Cl2(aq) Cl–(aq) Br2(aq) I2(aq) Stays yellow solution (no reaction) Br–(aq) I–(aq) 29 of 43 © Boardworks Ltd 2009 Cl2(aq) Cl–(aq) Br2(aq) I2(aq) Stays yellow solution (no reaction) Stays brown solution (no reaction) Br–(aq) I–(aq) 30 of 43 © Boardworks Ltd 2009 Cl2(aq) Cl–(aq) Br–(aq) Br2(aq) I2(aq) Stays yellow solution (no reaction) Stays brown solution (no reaction) Yellow solution forms (Br2 forms) Cl2 + 2 Br-→ 2 Cl- + Br2 I–(aq) 31 of 43 © Boardworks Ltd 2009 Cl2(aq) Cl–(aq) Br–(aq) Yellow solution forms (Br2 forms) Cl2 + 2 Br-→ 2 Cl- + Br2 Br2(aq) I2(aq) Stays yellow solution (no reaction) Stays brown solution (no reaction) Stays brown solution (no reaction) I–(aq) 32 of 43 © Boardworks Ltd 2009 Cl2(aq) Cl–(aq) Br–(aq) Yellow solution forms (Br2 forms) Cl2 + 2 Br-→ 2 Cl- + Br2 I–(aq) Br2(aq) I2(aq) Stays yellow solution (no reaction) Stays brown solution (no reaction) Stays brown solution (no reaction) Brown solution forms (I2 forms) Cl2 + 2 I- → 2 Cl- + I2 33 of 43 © Boardworks Ltd 2009 Cl2(aq) Cl–(aq) Br–(aq) Br2(aq) I2(aq) Stays yellow solution (no reaction) Stays brown solution (no reaction) Yellow solution forms (Br2 forms) Stays brown solution (no reaction) Cl2 + 2 Br-→ 2 Cl- + Br2 I–(aq) Brown solution forms (I2 forms) Brown solution forms (I2 forms) Cl2 + 2 I- → 2 Cl- + I2 Br2 + 2 I- → 2 Br- + I2 34 of 43 © Boardworks Ltd 2009 Oxidising power trend: Cl2 > Br2 > I2 When a halogen acts as an oxidising agent, it gains electrons (taken from the oxidised species). X + 2 e- → 2 XC l Br I 35 of 43 2 Down the group it becomes harder to gain an electron because: atoms are larger & there is more shielding (due to extra electron shell) © Boardworks Ltd 2009 Halogen displacement reactions Halogen displacement reactions are redox reactions. Cl2 + 2KBr 2KCl + Br2 To look at the transfer of electrons in this reaction, the following two half equations can be written: Cl2 + 2e- 2Cl- 2Br- Br2 + 2e- What has been oxidized and what has been reduced? Chlorine has gained electrons, so it is reduced to Cl- ions. Bromide ions have lost electrons, so they have been oxidized to bromine. 36 of 43 © Boardworks Ltd 2009 In displacement reactions between halogens and halides, the halogen acts as an oxidizing agent. This means that the halogen: gains electrons is reduced to form the halide ion. What is the order of oxidizing ability of the halogens? increasing oxidizing ability Oxidizing ability of halogens fluorine chlorine bromine iodine 37 of 43 © Boardworks Ltd 2009 USES OF HALOGENS Fuse School Uses of Halogens (4 Min) https://www.youtube.com/watch?v=wXingx2 RokI 38 of 43 © Boardworks Ltd 2009 Manufacture of Bleach 2NaOH(aq) + Cl NaOCl (aq) + NaCl (aq) + H O (l) Omit Na+ spectator ion and you see a Disproportionation reaction, which is…? Answer: Reactant is both oxidized and reduced. OH - + Cl2(g) OCl- + Cl2 2 4OH- + 2H2O + Cl2 2OCl - + 4H+ + 2e- + 4OH2e- + Cl2 2Cl 4OH- + 2Cl2 2Cl - + 2OCl- + 2H2O DIVIDE all by 2 2OH- + Cl2 Cl - + OCl- + H2O 39 of 43 © Boardworks Ltd 2009 Uses of Halogens • Chlorine is used to purify water in swimming pools etc as it kills bacteria. • Chlorine is also used as bleach. • Iodine is used as an antiseptic on cuts and grazes. 40 of 43 © Boardworks Ltd 2009 Reaction of chlorine with water Chlorine is used to purify water supplies because it is toxic to bacteria, some of which can cause disease. Adding it to water supplies is therefore beneficial for the population. However, chlorine is also toxic to humans, so there are risks associated with gas leaks during the chlorination process. There is also a risk of the formation of chlorinated hydrocarbons, which are also toxic. Chlorination of drinking water raises questions about individual freedom because it makes it difficult for individuals to opt out. 41 of 43 © Boardworks Ltd 2009 Uses of halides in photography Silver halides are used in photography. Photographic film coated with a silver halide is exposed to light, causing the halide to decompose to form silver. This appears as a black precipitate on the photographic film. Ag+ + e- Ag light covering paper coated in silver halide silver precipitate white paper under the cover 42 of 43 © Boardworks Ltd 2009 Halides When halogens react with metals, they form compounds called halides. Many naturally-occurring halides have industrial, household and medical applications. Halide Formula Uses cesium chloride CsCl Extraction and separation of DNA sodium hexafluoroaluminate NaAlF6 Electrolysis of aluminium oxide titanium(IV) chloride TiCl4 Extraction of titanium lithium iodide LiI Electrolyte in batteries potassium bromide KBr Epilepsy treatment in animals 43 of 43 © Boardworks Ltd 2009 44 of 43 © Boardworks Ltd 2009 Terminology • hydrogen bonding – A special example of a permanent dipole–dipole intermolecular force that occurs when hydrogen is bonded to nitrogen, oxygen or fluorine. An extremely strong dipole-dipole interaction • oxidation state – The charge a particular atom in a compound would have if the compound consisted entirely of separate ions. • oxidation – The process during which an atom, molecule or ion loses one or more electrons and increases in oxidation state. • oxidizing agent – A species that accepts electrons, thus oxidizing another species while it is itself reduced. • redox – Short for reduction / oxidation. A chemical reaction in which the reacting species have their oxidation states changed. • reduction – The process in which an atom, molecule or ion gains one or more electrons and decreases in oxidation state. • reducing agent – A species that donates electrons, thus reducing another species while it is itself oxidized. 45 of 43 © Boardworks Ltd 2009 Terminology • displacement reaction – A reaction in which an element is displaced out of a compound by a more reactive element; for example, bromine displaces iodine from potassium iodide: Br2 + 2KI 2KBr + I2. • disproportionation – A reaction in which the same element is both oxidized and reduced; for example, chlorine in: Cl2 + H2O > HCl + HOCl. • electronegativity – The power of an atom to withdraw electron density and attract the pair of bonding electrons in a covalent bond. • half equation – An equation describing either the oxidation or reduction in a redox reaction in terms of electron loss and electron gain. For example, Cl2 + 2e- 2Cl- shows the reduction of chlorine to chloride ions. • halide – A salt formed by the reaction of a halogen with a metal. • halide ion – A negative ion formed by the addition of an electron to an atom of a halogen. • halogen – A group 7 element. 46 of 43 © Boardworks Ltd 2009