Group 17 (halogens) Flashcards
Colour trend
- gets darker as you go down the group
Fluorine- pale yellow gas
Chlorine- green/yellow gas
Bromine- orange/brown liquid
Iodine- grey/black solid, purple vapour
Trends down the group
- gets darker
- volatility decreases
- bond strength decreases
- bond enthalpy decreases (exception is fluorine)
- oxidising power decreases
- reducing power increases
- reactions become less vigorous
- thermal stability decreases
Volatility
- how easily a substance can evaporate
- a volatile substance will have a low melting and boiling point
Trend of volatility
going down the group the boiling/melting point increases, therefore volatility of the halogens decreases
Bond strength trend- decreases going down the group, because
- halogens are diatomic molecules in which covalent bonds are formed by overlapping their orbitals
- in a covalent bond, the bonding pair of electrons is attracted to the nuclei on either side and it is this attraction that holds the molecule together
- going down the group, the atomic size increases therefore the bonding pair of electrons is further away from the nucleus and are therefore less strongly attracted towards it
*the bigger the atom, the weaker the covalent bond
Bond enthalpy trend- decreases going down the group
- bond enthalpy is the heat needed to break 1 mole of a covalent bond
- the higher the bond enthalpy, the stronger the bond
Exception is fluorine: it has a smaller bond enthalpy than chlorine and bromine, because:
- fluorine is so small that when 2 atoms of fluorine get together their lone pairs get so close that they cause significant repulsion which counteracts the attraction between the bonding pair of electrons and 2 nuclei (causes a decrease in bond strength)
*weak bond, therefore less energy is needed to break bond, therefore lower enthalpy
Describing volatility in terms of instantaneous dipole-induced dipole forces
- halogens are simple molecular structures with weak van der Waals forces between the diatomic molecules, caused by instantaneous dipole-induced dipole forces
- the more electrons, the stronger the id-id forces, therefore a higher melting and boiling point
- as it gets more difficult to separate the molecules, the volatility of the halogens decrease going down the group
Oxidising power- decreases going down the group (related to electronegativities), because:
- the atomic radii increases, which means the outer shell gets further away from the nucleus
- an “incoming” electron will therefore experience more shielding from the attraction of the positive nuclear charge
- the halogens ability to accept an electron (oxidising power) therefore decreases
Displacement reactions
when a more reactive halogen can displace a less reactive halogen from a halide solution of the less reactive halogen
Example of a displacement reaction
(the addition of chlorine water to a solution of bromine water)
CI2 (aq) + 2NaBr (aq) = 2NaCI (aq) + Br2 (aq)
- the chlorine has displaced the bromine from the solution as it is more reactive
- summarised in the following equation
- remove element that has the same oxidation number in reactants and products (in this case sodium)
CI2 (aq) + 2Br- (aq) = 2CI- (aq) + Br2 (aq)
Reaction with the elements and hydrogen
H2 (g) + X2 (g) = 2HX (g)
- due to the decrease in reactivity of the halogens going down the group, the reactions become less vigorous
Thermal stability
- how well a substance can resist breaking down when heated
- a substance that is thermally stable will break down at high temperatures
Thermal stability trend of halogen halides- going down the group the thermal stability decreases, because:
- their bonds become weaker due to the increased atomic radius of the halogens
- the longer the bond length, the weaker it is and the less energy is needed to break it
- as the bonds get weaker, the hydrogen halides become less stable to heat going down the group
Reducing power trend: increases going down the group (related to ionic radii)
- the halide ions become larger
- the outermost electrons get further away from the nucleus and therefore experience more shielding by inner electrons
- as a result, the outermost electrons are held less tightly to the positive charged nucleus
- therefore the halide ions lose electrons more easily going down the group and their reducing power increases
