2ObjectivesBe able to...Recall the main properties and trends of the HalogensExplain trends using knowledge of intermolecular bonding and redoxWrite half equations for displacement reactions and electrolysis
4GROUP PROPERTIES • exist as separate diatomic molecules… eg Cl2 GENERAL • non-metals• exist as separate diatomic molecules… eg Cl2• all have the electronic configuration ... ns2 np5TRENDS • appearance• boiling point• electronic configuration• electronegativity• atomic size• ionic size• oxidising power
5Physical properties of halogens Boardworks AS ChemistryHalogensPhysical properties of halogens
6Trends in boiling point Boardworks AS ChemistryHalogensTrends in boiling pointHalogen molecules increase in size 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 forcesTeacher notesSee the ‘Bonding and Intermolecular Forces’ presentation for more information about van der Waals forces.fluorine atomic radius = 42 × m boiling point = °Ciodine atomic radius = 115 × m boiling point = 184 °C
7Trends in electronegativity Boardworks AS ChemistryHalogensTrends in electronegativityElectronegativity of the halogens decreases down the group due to an increase in atomic radius.Increased nuclear charge has no significant effect because there are more electron shells and more shielding. Iodine atoms therefore attract electron density in a covalent bond less strongly than fluorine.Teacher notesSee the ‘Bonding and Intermolecular Forces’ presentation for more information about electronegativity and its relationship to atomic radius and shielding.fluorine atomic radius = 42 × m electronegativity = 4.0iodine atomic radius = 115 × m electronegativity = 2.5
8Boardworks AS Chemistry Halogens AstatineThe name astatine comes from the Greek word for unstable.Astatine exists in nature in only very tiny amounts. 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 a-radiation. What do you predict for these properties of astatine?Teacher notesThe square brackets around the mass number for astatine indicate that this is the atomic mass of the most stable isotope.Astatine is the rarest naturally-occurring element.The properties of astatine: black (presumed), solid at room temperature (presumed), Pauling electronegativity of 2.2colourstate at room temperatureelectronegativity.
9Appearance Fluorine F Chlorine Cl Bromine Br Iodine I Halogen Symbol \SymbolStateColourColour of vapourFluorineFChlorineClBromineBrIodineI
10What would you predict about the appearance of astatine? Halogen\SymbolStateColourColour of vapourFluorineFgasPale yellowYellowChlorineClPalegreenGreenBromineBrliquidOrange / brownOrangeIodineIsolidGrey-black crystalsPurpleWhat would you predict about the appearance of astatine?
11What happens to the physical properties as you go down Group VII? Fluorine (F2)Chlorine (Cl2)Bromine (Br2)Iodine (I2)What happens to the physical properties as you go down Group VII?
12Trends in Physical Properties Fluorine (F2)Chlorine (Cl2)Bromine (Br2)Iodine (I2)Melting points increaseBoiling points increaseAll more soluble in organicsolvents than waterMake sure you can use intermolecular forces to EXPLAIN each of these trends
13Reactivity of the Group 7 elements Fluorine (F2)Chlorine (Cl2)Bromine (Br2)Iodine (I2)Decreasing reactivity (This is due to it getting less easy for the atoms to form negative ions by gaining electrons)Make sure you can EXPLAIN this trend in terms of atomic size, shielding and nuclear attractionAS Chemistry - Halogens (BEL - May 2002)
14Displacement Reactions Which of the following reactions will take place?Chlorine + potassium bromideBromine + potassium chlorideIodine + potassium chlorideBromine + potassium iodideWhat has to be true for a displacement reaction to take place?
15Electron structure and reactivity Boardworks AS ChemistryHalogensElectron structure and reactivity
16Halogen displacement reactions Boardworks AS ChemistryHalogensHalogen displacement reactions
17Halogen displacement reactions Boardworks AS ChemistryHalogensHalogen displacement reactions
18Halogen displacement reactions Boardworks AS ChemistryHalogensHalogen displacement reactionsHalogen displacement reactions are redox reactions.Cl2 + 2KBr ® 2KCl + Br2To 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?Teacher notesSee the ‘Redox Reactions’ presentation for more information about redox reactions.Chlorine has gained electrons, so it is reduced to Cl- ions.Bromide ions have lost electrons, so they have been oxidized to bromine.
19Oxidizing ability of halogens Boardworks AS ChemistryHalogensOxidizing ability of halogensIn displacement reactions between halogens and halides, the halogen acts as an oxidizing agent.fluorineThis means that the halogen:oxidizes the halide ion to the halogenchlorineincreasing oxidizing abilitygains electronsTeacher notesStudents could be encouraged to see the relative oxidizing ability of the halogens as their ability to accept electrons, relating it to the size, nuclear charge and shielding. Also they could connect this trend to the electronegativity values and with the electron structures of the ions.bromineis reduced to form the halide ion.What is the order of oxidizing ability of the halogens?iodine
20Oxidizing ability of halogens Boardworks AS ChemistryHalogensOxidizing ability of halogens
21Displacement Reactions Writing an ionic equation for a displacement reaction:Br I Br- + I2Which species is oxidised in this reaction?Which species is the oxidising agent?Which species is the reductant?
22Displacement Reactions Chlorine can oxidise both bromide and iodide ions:Cl Br Cl- + Br2Cl I Cl- + I2However, bromine can only oxidise iodide ions:Br Cl no reactionBr I Br- + I2So, chlorine is a stronger oxidising agent than bromine.
23Oxidising ability of the halogens Fluorine (F2)Chlorine (Cl2)Bromine (Br2)Iodine (I2)Decreasing oxidising abilityDecreasing reactivity
24Fluoride (F-) Chloride (Cl-) Bromide (Br-) Iodide (I-) Reducing ability of the halidesFluoride (F-)Chloride (Cl-)Bromide (Br-)Iodide (I-)Increasing reducing ability
25 attraction drops off as distance increases GROUP TRENDSELECTRONEGATIVITYFClBrIElectronegativity4.03.52.82.5DECREASES down Group• the increasing nuclear charge due to the greater number of protonsshould attract electrons more, but there is an ...an increasing number of shells; more shielding and less pull on electronsan increasing atomic radius attraction drops off as distance increases
26GROUP TRENDS OXIDISING POWER • halogens are oxidising agents • they need one electron to complete their octet• the oxidising power gets weaker down the group
27GROUP TRENDS OXIDISING POWER • halogens are oxidising agents • they need one electron to complete their octet• the oxidising power gets weaker down the group• the trend can be explained by considering the nucleus’s attractionfor the incoming electron which is affected by the...• increasing nuclear charge which should attract electrons morebut this is offset by • INCREASED SHIELDING• INCREASING ATOMIC RADIUS
28This is demonstrated by reacting the halogens with other halide ions. GROUP TRENDSOXIDISING POWER• halogens are oxidising agents• they need one electron to complete their octet• the oxidising power gets weaker down the group• the trend can be explained by considering the nucleus’s attractionfor the incoming electron which is affected by the...• increasing nuclear charge which should attract electrons morebut this is offset by • INCREASED SHIELDING• INCREASING ATOMIC RADIUSThis is demonstrated by reacting the halogens with other halide ions.
29Reactions with Silver Nitrate HalideColour of precipitateFormula of precipitateSolubility in ammoniaFluorideno ppt.solubleChloridewhiteAgClBromidecreamAgBrpartially solubleIodideyellowAgIinsolubleHow is AgNO3 used to test for halide ions?Which silver halide precipitate is formed most quickly?Can you suggest a use for the silver halides?