Gets darker down the group: F: yellow; Cl₂: yellowish-green; Br₂: reddish-brown, I₂: grey-black
Fluorine
Reacts with water
Chlorine
Reacts slightly with water
Chlorine water and bromine water
Solutions of Cl₂ and Br₂ respectively
Iodine solution
Iodine dissolved in aqueous potassium iodide, appears brown
Atomic radius
Increases down the group as the number of electron shells increases and there is increased shielding of the outer shell electrons by the inner shells
Density
Increases down the group
Melting points
Increase down the group as the increasing number of electrons makes the van der Waals forces between the molecules stronger, so it takes more energy to break these intermolecular forces
Boiling points
Show a similar trend, F₂ and Cl₂ being gases at room temperature, bromine a volatile liquid and iodine a solid
Halogens
Less reactive going down the group
Displacement reaction between halogens
More reactive halogen can displace a less reactive halogen from an aqueous halide solution
Aqueous chlorine
Displaces bromine from an aqueous solution of potassium bromide
Iodine will not displace bromine from potassium bromide because iodine is not as good an oxidising agent as bromine
E values
Used to explain the relative reactivity of halogens as oxidising agents
As E values get more positive (less negative), the halogens on the left become better oxidising agents and accept electrons more readily
As E values get less positive (more negative), the halides on the right become better reducing agents and release electrons more readily
Cl2 accepts electrons more readily than I-, and I- ions release electrons more readily than Cl-
The reaction with the more negative E value
Goes in the reverse direction
Bromine water
Used to test for C=C (carbon-carbon double) bonds in organic compounds
Testing for C=C bonds with bromine water
1. Shake unsaturated compound with bromine water
2. Solution changes colour from orange (bromine) to colourless
Halogens
Exist as diatomic molecules
Become less volatile and darker in colour going down the group
Are good oxidising agents, with oxidising ability decreasing down the group
Sodium thiosulphate
Na₂S₂O₃, can be used to determine the concentration of iodine by titration and to analyse samples of bleach
S₂O₃²⁻ ions are better reducing agents than I⁻ ions
I₂ molecules are better oxidising agents than S₂O₃²⁻ ions
Reaction of sodium thiosulphate with iodine
2Na₂S₂O₃(aq) + I₂(aq) → Na₂S₄O₆(aq) + 2NaI(aq)
Bromine and chlorine are stronger oxidising agents than iodine
They cause further oxidation of sodium thiosulphate to sulphate ions
Estimation of chlorine in bleaches
1. Dilute the bleach
2. Add excess acidified potassium iodide solution to liberate iodine
3. Titrate the liberated iodine with sodium thiosulphate of a known concentration using starch indicator
4. End point is when the blue-black colour of the starch-iodine indicator turns colourless
Hydrogen halides
Formed when hydrogen combines directly with the halogens
Reactions are slower down the halogen group
Reaction of hydrogen with fluorine
H₂(g) + F₂(g) → 2HF(g)
Hydrogen fluoride
Boils at 19.5°C, so its state under normal laboratory conditions may be either liquid or gas
Other hydrogen halides
Gases at room temperature
Higher boiling point of HF compared with other hydrogen halides
Due to its extensive hydrogen bonding
Hydrogen halides
Acidic, form acids when they dissolve in water
Reaction of hydrogen halides in water
HC + HOH(aq) → H3O+(aq) + Cl-(aq)
Thermal stability of hydrogen halides
Decreases as the halogen atom increases in size
Reaction of hydrogen iodide with hot platinum wire
Purple vapour of iodine first seen, then turns directly to a grey-black solid on the side of the tube
Ease of thermal decomposition of hydrogen halides
Related to the bond energies of the hydrogen-halogen bond
Bond energies
H-F: 562 kJ/mol
H-Cl: 431 kJ/mol
H-Br: 366 kJ/mol
H-I: 299 kJ/mol
Larger the halogen atom
Greater the distance between the hydrogen and halogen nuclei, smaller the attractive force between the nuclei and the bonding electrons, less energy to break the hydrogen-halogen bond
Solid halides react with concentrated sulphuric acid
1. Form hydrogen halides
2. Fumes of hydrogen halide are the first product
Reactions of solid halides with concentrated sulphuric acid