2 Halogen displacement

1. The displacement reactions of halide ions by halogens. A halogen that is more reactive will displace a halogen that has a lower reactivity from one of its compounds The reactivity of the halogens decreases down the group as the atoms get bigger with more shielding so they less easily attract and accept electrons. They therefore form -1 ions less easily down the group Chlorine will displace both bromide and iodide ions; bromine will displace iodide ions Chlorine (aq) Bromine (aq) Iodine (aq) potassium chloride (aq) Very pale green solution, no reaction Yellow solution, no reaction Brown solution, no reaction potassium bromide (aq) Yellow solution, Cl has displaced Br Yellow solution, no reaction Brown solution, no reaction potassium iodide (aq) Brown solution, Cl has displaced I Brown Solution, Br has displaced I Brown Solution, no reaction The colour of the solution in the test tube shows which free halogen is present in solution. Chlorine =very pale green solution (often colourless), Bromine = yellow solution Iodine = brown solution (sometimes black solid present) know these observations ! Cl2 (aq) + 2Br – (aq)
2Cl – (aq) + Br2 (aq) Cl2 (aq) + 2I – (aq)
2Cl – (aq) + I2 (aq) Br2 (aq) + 2I – (aq)
2Br – (aq) + I2 (aq) Explanation of reactivity Chlorine is more reactive than bromine because it will gain an electron and form a negative ion more easily than bromine. The is because an atom of chlorine is smaller than bromine and the outermost shell of chlorine is less shielded than bromine so the electron to be gained is attracted more strongly to the nucleus in chlorine than bromine. Observations if an organic solvent is added Chlorine (aq) Bromine (aq) Iodine (aq) potassium chloride (aq) colourless, no reaction yellow, no reaction purple, no reaction potassium bromide (aq) yellow, Cl has displaced Br yellow, no reaction purple, no reaction potassium iodide (aq) purple, Cl has displaced I purple, Br has displaced I purple, no reaction The colour of the organic solvent layer in the test tube shows which free halogen is present in solution. Chlorine = colourless Bromine = yellow Iodine = purple