Now for the negative electrode. An over-hasty judgement might be that here the positive sodium ions might be discharged as sodium metal: Na(s) + H2O(l) = Na+(aq) + OH-(aq) + 0.5H2(g) Because water is present, sodium metal will react with it in the usual fashion. Both the sodium ions, and sodium metal cancel out, leaving H2O(l) + e- = OH-(aq) + 0.5H2(g) To summarize, the initial reactions during the electrolysis of sodium chloride are as follows:Anode: CI-(aq) =0.5Cl2(g) + e- Cathode: H2O(l) + e- = OH-(aq) + 0.5H2(g)3. Preparation of bromineBromine is found in the bodies of some sea creatures and in seawater.
When chlorine gas is passed through a solution of salts of bromine, elemental bromine is obtained.2KBr + Cl2 ? 2KCl + Br2 Sea waterHBr acid reacts with strong oxidizing agents to produce pure bromine:4HBr(conc) + MnO2 ? MnCl2 + 2H2O + Br22HBr(conc) + H2O2(conc) ? 2H2O + Br24. Preparation of iodine It is extracted from seawater and sodium iodate (NaIO3) in Chile saltpeter in nature:2NaIO3 + 12HCl(conc) ? I2 + 5Cl2 + 2NaCl + 6H2O NaIO3 + 3H2SO4(dil) + 5NaI ? 3I2 + 3Na2SO4+ 3H2OIn the laboratory, elemental iodine is produced by passing chlorine gas through solutions of iodine salts:2NaI + Cl2 ? 2NaCl + I2 Evaporation of iodine
All the halogens are highly reactive. They react with metals and nonmetals to form halides. The reactivity of the halogens decreases down the group.Anomalous behaviour of fluorine Like other elements of the p-block present in the second period of the periodic table, fluorine is anomalous in many properties. For example, ionisation enthalpy, electronegativity, enthalpy of bond dissociation and electrode potentials are all higher for fluorine than expected from the trends set by other halogens. Also, ionic and covalent radii, m.
p. and b.p.
and electron gain enthalpy are quite lower than expected. The anomalous behaviour of fluorine is due to its small size, highest electronegativity, low F-F bond dissociation enthalpy, and non-availability of d-orbitals in the valence shell. Most of the reactions of fluorine are exothermic (due to the small and strong bond formed by it with other elements). It forms only one oxoacid while other halogens form some oxoacids. Hydrogen fluoride is a liquid (b.
p. 293 K) due to strong hydrogen bonding. Other hydrogen halides are gases. Pure bromine 683941441 713676862 Chlorine gas in a glass1. Reactions with nonmetalsFluorine oxidizes all elements except He, Ne and Ar. It is an extremely reactive element.
For example, it reacts with hydrogen explosively, even at –250°C. F2 + H2 ? 2HF 3F2 + N2 ? 2NF3 F2 + Xe ? XeF2F2 + O2 ? O2F2 3F2 + S ? SF6 Cl2 + H2 ? 2HCl 3Cl2 + 2P ? 2PCl3 Br2 + Cl2 ? 2BrCl I2 + 5F2 ? 2IF5 Chlorine, bromine and iodine form oxides in which the oxidation states of these halogens range from +1 to +7. A combination of kinetic and thermodynamic factors lead to the generally decreasing order of stability of oxides formed by halogens, I > Cl > Br. The higher oxides of halogens tend to be more stable than the lower ones.Chlorine oxides, Cl2O, ClO2, Cl2O6 and Cl2O7 are highly reactive oxidising agents and tend to explode. ClO2 is used as a bleaching agent for paper pulp and textiles and in water treatment.
The bromine oxides, Br2O, BrO2, BrO3 are the least stable halogen oxides (middle row anomaly) and exist only at low temperatures. They are very powerful oxidising agents. The iodine oxides, I2O4, I2O5, I2O7 are insoluble solids and decompose on heating. I2O5 is a very good oxidising agent and is used in the estimation of carbon monoxide.2. Reactions with metals3F2 + 2Sb ? 2SbF3 3Cl2 + 2Fe ? 2FeF3 Br2 + 2Li ? 2LiCl The reaction of aluminum with iodine produces a quantity of energy, which is released as heat3I2 + 2Al ? 2AlI3 + 627.6kJIn exothermic reactions, the total potential energy of the products is smaller than that of reactants. The difference between these potential energies produces thermal energy (heat).
For the reaction above, when 3 mol of iodine reacts with aluminum, 627.6kJ energy is released as heat.Each aluminum atom loses three electrons and forms the ion Al3+ , which has the electronic structure of neon; each iodine atom gains one electron and forms the ion I-, which has the electronic structure of xenon. The forces between the oppositely charged ions in Al3+I- hold the compound together.
Thus, when the aluminum atoms on the left of the equation are oxidized, they lose electrons and become Al3+ ions in AlI3. Another feature of the equation is that the iodine atoms, bound together on the left-hand side as diatomic molecules, take on electron and form I- ions. Thus, whereas the aluminum atoms lose electrons, the iodine atoms gain them. 3. Reactions with waterFluorine oxidises water with oxygen, chlorine and bromine react with water to form corresponding hydrohalic and hypohalous acids. F2 + H2O ? 2HF + O0 F2 + O0 ? OF2 Cl2 + nH2O ? Cl2• nH2O(aq) Cl2 + H2O ? HCl + HOClChlorine water on standing loses its yellow color due to the formation of HCl and HOCl. Hypochlorous acid (HOCl) so formed, gives nascent oxygen which is responsible for oxidising and bleaching properties of chlorine.
Chlorine water is a powerful bleaching agent; bleaching action is due to oxidation.Cl2 + H2O ? 2HCl + OColored substance + O ? Colorless substanceBr2 + nH2O ? Br2• nH2O(aq) Br2 + H2O ? HBr + HOBrThe reaction of iodine with water is nonspontaneous.I2 + H2O ? HI + HIO Symbol do not bleach icon 699562780 chlorine bleach accident 2431849034. Halogens can displace any of the halogens from their salts and acid solutions.
F2 + 2NaCl ? 2NaF + Cl2Cl2 + 2NaBr(hot) ? 2NaCl + Br2Br2 + 2NaI ? 2NaBr + I25. Reactions with bases:4F2 + 6NaOH(dilut.) ? OF2 + 6NaF + 3H2O + O2With cold and dilute alkalies chlorine (bromine, iodine) produces a mixture of chloride (bromide, iodide) and hypochlorite (hypobromite, hypoiodite) but with hot and concentrated alkalies it gives chloride (bromide, iodide) and chlorate (bromate, iodate).
Cl2 + 2NaOH(cold) ? NaCl + NaClO + H2O3Cl2 + 6NaOH(hot) ? 5NaCl + NaClO3 + 3H2OWith dry slaked lime, it gives bleaching powder.2Ca(OH)2 + 2Cl2 ? Ca(OCl)2 + CaCl2 + 2H2OCa(OH)2 + Cl2 ? CaOCl2 + 2H2O The composition of bleaching powder is Ca(OCl)2×CaCl2×Ca(OH)2×2H2O.6. Reactions with acids:2F2 + 4HClO4 ? 4ClO3F + O2 + 2H2O F2 + HNO3 ? (NO2+)OF + HFBr2 + H2S(conc) ? 2HBr + S3I2 + 10HNO3(dilut.) ? 6HIO3 + 10NO + 2H2OI2 + 10HNO3(conc hot.) ? 2HIO3 + 10NO2 + 4H2O
HF is less acidic than HI. Why?4. Why is iodine I2 a solid and Cl2 a gas?5. Why does F2 act as a powerful oxidising agent?6. What happens when chlorine is passed through an aqueous solution of sodium hydroxide?7. How is the presence of I2 detected?8.
Mention three areas in which iodine plays an important role.9. Although electron gain enthalpy of fluorine is less negative as compared to chlorine, fluorine is a stronger oxidising agent than chlorine. Why?10. Sea is the greatest source of some halogens.
Comment.11. Name two poisonous gases which can be prepared with chlorine gas.