IAS UNIT 2 8C – WCH11/01 WASANTHA SIR Group 7 elements are halogens, which exist in diatomic molecular form. LYCEUM -GAMPAHA TEL: 076 4675644 1 IAS UNIT 2 8C – WCH11/01 WASANTHA SIR The following table shows physical properties of halogens; physical states and colurs. Halogen F2 Cl2 Br2 I2 Physical state at rtp Gas Gas Volatile Liquid Volatile solid Colour of the halogen Yellowish Greenish Red brown Shiny grey black Colour in organic layers Colour in water Yellow – orange Vapour is violet Orange – brown At2 NOTE: o iodine is violet in vapour form and when dissolved in an organic liquid o iodine is water insoluble but dissolves in water when there is potassium iodide dissolved in water o iodine gives an orange colour when dissolves in water o bromine gives a yellow colour when dissolves in water o chlorine is colourless in water • • Halogens molecules are non -polar; they cannot form dipole – dipole attractions of hydrogen bonds between molecules. The only possible intermolecular attractions are London forces. They are instantaneous dipole – induce dipole attractions are are very weak. London forces become stronger when the number of electrons in molecules and the size of molecules increase (a) Explain why the melting point and boiling point of halogens increase moving down the group. ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… • All halogens accept electrons to the outermost energy level because they all have seven electrons each in their outermost shell. Cl2 + ………………………………………… Br2 + ………………………………………… LYCEUM -GAMPAHA TEL: 076 4675644 2 IAS UNIT 2 8C – WCH11/01 • WASANTHA SIR Halogens change to halides by accepting electrons from other elements, hence they are strong oxidizing agents. (b) explain why the reactivity of halogens decreases down the group ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… (c) (i) What is electronegativity? ………………………………………………………………………………… ………………………………………………………………………………… …………………………………………………………………………………. (ii) Discuss how the electronegativity of halogens vary down the group 7 ………………………………………………………………………………….. ………………………………………………………………………………….. ………………………………………………………………………………….. ………………………………………………………………………………….. ………………………………………………………………………………… Chemical properties of halogens 1. 2. 3. 4. 5. Halogens as oxidizing agents Reactions of halogens with potassium halides Reactions of halide solutions with silver nitrate solution Reactions of solid halides with concentrated sulphuric acid Disproportionate reactions of halogens 1. Halogens as oxidizing agents. Halogens are oxidizing agents. They cause other species to oxidize by removing electrons from them. The following examples show how halogens behave as oxidizing agents. A. Reaction with metals • Halogens are able to oxidize group I and II metals forming ionic compounds, in ………… ……………………………... Na + Cl2 K + Br2 Mg + Cl2 Ca + Br2 → → → → LYCEUM -GAMPAHA ……………………. ……………………. ……………………. ……………………. TEL: 076 4675644 3 IAS UNIT 2 8C – WCH11/01 1. WASANTHA SIR Chlorine can react with copper metal according to the following reaction equation Cu + Cl2 → CuCl2 (a) State the duty of chlorine in this reaction. (b) ………………………………………………………………………………… When copper is reacted with bromine, the following reaction occurs 2 Cu + Br2 → 2CuBr Explain the difference in these reactions ………………………………………………………………………………….. ………………………………………………………………………………….. ………………………………………………………………………………….. B. Reactions of halogens with halides in aqueous medium (Halogen displacement reactions) More reactive halogens can displace less reactive halides from aqueous solutions. Cl2 can displace ……………………………………………………………………… Cl2 can displace ……………………………………………………………………… Br2 can displace ……………………………………………………………………… e.g. 1 Displacement of iodine by chlorine from potassium iodide solution • When chlorine gas is bubbled through a potassium iodide solution or when chlorine water is shaken with aqueous potassium iodide solution containing a little bit of CCl4 (an organic liquid) the aqueous solution changes from colourless to orange and the organic layer changes from colourless to violet. …………………………………………………………………….. • • This is a red-ox reaction where iodine oxidizes from I- to I2 while Cl2 reduces to Cl-. This is possible because chlorine is a stronger oxidizing agent than iodine. e.g. 2 Displacement of bromine by chlorine from potassium bromide solution • When chlorine gas is bubbled through a potassium bromide solution or when chlorine water is mixed with aqueous potassium bromide solution containing a little bit of CCl4, the aqueous solution changes from colourless to yellow and the organic layer changes from colourless to red brown. LYCEUM -GAMPAHA TEL: 076 4675644 4 IAS UNIT 2 8C – WCH11/01 WASANTHA SIR …………………………………………………………………….. • • 2. This is also a red-ox reaction where bromine oxidizes from Br- to Br2 while Cl2 reduces to Cl-. This is possible because chlorine is a stronger oxidizing agent than bromine. Disproportionate reactions. • In disproportionate reactions, ……………………………………………………. ………………………………………………………………………………………….. • All Cl2, Br2 and I2 can show various types of disproportionate reactions. e.g. 1 Reaction of chlorine with water …………………………………………………………………………………………. …………………………………………………………………………………………. ………………………………………………………………………………………….. e.g. 2 Chlorine can react with cold dilute sodium hydroxide solution according to the following equation. ……………………………………………………………………………………….. …………………………………………………………………………………………. Bleaching Powder, also called Calcium Hypochlorite …………………… or Calcium Oxychloride…………………………, is a chemical compound which is used for various purposes. Bleaching powder can be recognised by the smell of chlorine. Bleaching powder is generally used for the removal of colour, as indicated by its name. It is also known as a bleaching agent. e.g. 3 Chlorine can react with hot concentrated sodium hydroxide solution according to the following equation. Cl2(g) + NaOH(aq) →NaCl(aq) + H2O(l) + LYCEUM -GAMPAHA TEL: 076 4675644 5 IAS UNIT 2 8C – WCH11/01 Q. WASANTHA SIR Write the balanced chemical equation for the reaction between bromine and hot concentrated potassium hydroxide, use the knowledge of oxidation number method to balance this equation. ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… 3. Reaction of solid halides with concentrated sulphuric acid When few drops of concentrated sulphuric acid are added to solid halides in a boiling tube, exothermic reactions occur displaying different observations, according to the type of halide ions present in the compound e.g. 1 Reaction of sodium chloride with concentrated sulphuric acid • • • When few drops of concentrated sulphuric acid are added to sodium chloride solid in a boiling tube, an exothermic reaction occurs. A thick white fume is produced due to the formation of HCl vapour. This fume can change blue litmus paper to red. …………………………………………………………………………………………………………. e.g. 2 Reaction of sodium bromide with concentrated sulphuric acid • • This reaction occurs in two steps In the first step, sodium bromide reacts with sulphuric acid similar to sodium chloride …………………………………………………………………………………………………. • A slight white fume is also produced but not very thick LYCEUM -GAMPAHA TEL: 076 4675644 6 IAS UNIT 2 8C – WCH11/01 • WASANTHA SIR This is followed by a further reaction between hydrogen bromide and concentrated sulphuric acid producing sulphur dioxide and brown - bromine. ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… …..……………………………………………………………………………………………. • A bad smell is felt due to the liberation of SO2 gas. …………………………………………………………………………………………. • The HBr, which is produced in the above reaction reacts with sulphuric acid to produce sulphur dioxide gas, bromine and water. …………………………………………………………………………………………. • The formation of Br2 and sulphur dioxide occurs due to the ability of HBr to react as a reducing agent: sulphuric acid is reduced by HBr. e.g. 3 Reaction of sodium iodide with concentrated sulphuric acid • • • When concentrated sulphuric acid is added to sodium iodide solid in a boiling tube, a series of exothermic reactions occur. The first reaction is the same as the first reaction of sodium chloride and sodium bromide with sulphuric acid. ………………………………………………………………………………………………….. This is followed by three more reactions where hydrogen iodide reacts with sulphuric acid forming sulhur, sulphur dioxide and hydrogen sulphide gas, separately ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… LYCEUM -GAMPAHA TEL: 076 4675644 7 IAS UNIT 2 8C – WCH11/01 • • A violet vapour is formed due to the formation of Iodine gas. the violet vapour solidifies to give a grey black solid on the test tube wall (iodine solid) • The following observations are also made in the test tube in addition to the given observations: (a) A yellow precipitate is formed due to sulphur powder (b) The solution gets messy brown because tri iodide ions are formed in the solution. • Q. WASANTHA SIR …………………………………………………………………….. (c) A very bad smell is given off due to the release of both SO2 and H2S gases. The followings are all the reactions that take place in the test tube after the first reaction. What property of hydrogen halides is shown to increase down the group? ……………………………………………………………………………………………. 4. Reaction of aqueous halides with silver nitrate solution This is the standard identification test for halide ions in aqueous solutions. • • • • Addition of silver nitrate solution to an unknown aqueous solution is the standard identification test for halide ions, in aqueous solutions. When silver nitrate solution is added to chloride, bromide and iodide ion solutions in aqueous medium, white precipitate, cream precipitate and yellow precipitate are formed due to ………………, ………………. and ………………. respectively. These precipitates are treated separately with ammonia solution to see whether these precipitates dissolve or not in ammonia solution. If a solution gives a white precipitate, which dissolves in dilute ammonia solution, the solution contains Cl- ions. Silver chloride white precipitate dissolves in dilute ammonia solution because of the formation of a water soluble complex; [Ag(NH3)2]+Cl- - Diamminosilver(I)chloride • If a solution produces a cream precipitate and it dissolves in excess dilute ammonia solution or concentrated ammonia solution, then the solution contains Br- ions. Silver bromide cream precipitate dissolves, partially, in ammonia solution because of the formation of a water soluble complex; [Ag(NH3)2]+Br- - Diamminosilver(I)bromide LYCEUM -GAMPAHA TEL: 076 4675644 8 IAS UNIT 2 8C – WCH11/01 • WASANTHA SIR On the other hand, If a solution produces a yellow precipitate and it does not dissolve even in concentrated ammonia solution, then the solution contains I- ions. 5 Reaction of hydrogen halides with ammonia gas Hydrogen halides can react with ammonia gas to produce ammonium halides e.g. ……………………………………………………………………………………………. ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… ………………………………………………………………………………………………… At the same time if hydrogen halides are mixed with water, acids are formed HF + H2O → ……………………………………. HCl + H2O → ……………………………………. HBr + H2O → ……………………………………. HI + H2O → ……………………………………. LYCEUM -GAMPAHA TEL: 076 4675644 9
