group 7

what is the name of group 7?

the halogens

what is the valence electron structure of group 7?

7 valence electrons, two electrons in the outer s sub-shell and five in the outer p sub-shell

what are two features of the structure of group 7 elements?

diatomic molecules at room temperature
simple molecular structures with weak London forces

what is the trend of boiling points for halogens?

boiling point increases down the group
the size of the atom increases, and the number of electrons increases
stronger London forces between molecules
more energy required to overcome these forces

what are the most common reactions of the halogens?

redox reactions
each halogen atom is reduced, gaining one electron to form a 1- halide ion
another species loses electrons to halogen atoms, so halogen is known as an oxidising agent

what is the colour of chlorine in water?

pale green

what is the colour of bromine in water?

orange

what is the colour of iodine in water?

brown

what is the colour of chlorine in cyclohexane?

pale green

what is the colour of bromine in cyclohexane?

orange

what is the colour of iodine in cyclohexane?

violet

why is cyclohexane used to tell bromine and iodine apart in water?

they can appear a similar orange-brown colour
cyclohexane is added to the mixture and shaken, where the colours are much easier to tell apart

what happens in displacement reactions of halogens?

the more reactive halogen displaces the less reactive halogen in solution
a reaction takes place, where the solution changes colour

what is the ionic equation for the displacement of bromine?

Cl2(aq) + 2Br-(aq) → 2Cl-(aq) + Br2(aq)

what is the ionic equation for the displacement of iodine?

Cl2(aq) + 2I-(aq) → 2Cl-(aq) + I2(aq)

what is the trend in reactivity of the halogens?

reactivity decreases going down the group
atomic radius increase, so shielding increases
less nuclear attraction to capture an electron from another species

what is disproportionation?

a redox reaction in which the same element is both oxidised and reduced

what are two examples of disproportionation reactions?

the reaction of chlorine with:
water and cold, dilute aqueous sodium hydroxide

why is chlorine reacted with water?

for water purification

what is the equation for water purification?

Cl2(aq) + H2O(l) → HClO(aq) + HCl(aq)
one chlorine atom is oxidised, and the other is reduced
choric acid and chlorate ions sterilise water by killing bacteria

what is chloric acid?

a weak bleach

what test can be done to show that chloric acid is a weak bleach from water purification?

universal indicator can be added to a solution of chlorine in water
the indicator first turns red from the presence of two acids, then becomes colourless as HClO bleaches the colour

what is a limitation of the reaction of chlorine and water and how can it be solved?

chlorine’s has low solubility in water
if the water contains sodium hydroxide, much more chlorine dissolves

what is the equation for the disproportionation reaction of chlorine with cold, aqueous sodium hydroxide?

Cl2(aq) + 2NaOH(aq) → NaClO(aq) + H2O(l)

what does the solution from the disproportionation reaction of chlorine with cold, aqueous sodium hydroxide contain and what can it be used for?

contains a large concentration of chlorate ions from sodium chlorate
used as household bleach

what are the benefits of chlorine use?

kills bacteria in water

what are the disadvantages of chlorine use?

chlorinated hydrocarbons can be formed in drinking water, which are suspected of causing cancer

why do the benefits outweigh the risk of chlorine use for water?

adding chlorine to water reduces the risk of cholera and typhoid