group 7 the halogens

where are group 7 elements

right hand side of the periodic table as they’re nonmetals

as elements, group 7 metals exist as____ molecules

diatomic

name for group 7

halogens

astatine is…

rare and radioactive

physical properties of halogens at room temperature

• vary in appearance

• at room temperature fluorine is a pale yellow gas

• chlorine a greenish gas

• bromine a red brown liquid

• iodine a black solid

• they get darker and denser down the group

• swimming bath smell

a number of properties of fluorine are…

untypical

where do many of the untypical properties of fluorine stem from

• the fact that the F-F bond is unexpectedly weak compared with the trend for the rest of the halogens

• the small size of the fluorine atoms leads to repulsion between non-bonding electrons because they are so close together

halogens trend in size of atom

• gets bigger going down group because each element has one extra filed main level of electrons compared with the one above it

halogens trend in electronegativity

• electronegativity decreases down the group b

• shared electrons in bond get further away from nucleus as atoms get larger going down the group b shared

• this makes the shared electrons further from the halogen nucleus and increases the shielding by more inner shells of electrons

• these factors are more important than the increase in nuclear charge leading to decrease down group

what is electronegativity

the relative ability of an atom to attract the pair of electrons towards itself within a covalent bond

what does electronegativity depends on

• the attraction between the nucleus and bond electrons in the outer shell

• this depends on a balance between the number of protons in the nucleus (nuclear charge) and distance between the nucleus and the bonding electrons

• plus shielding effect of inner shells of electrons

halogens trend in melting and boiling point

• increase down the group

• larger atoms have more electrons and this makes the van der waals forces between the molecules stronger

• the lower the boiling point the more volatile the element

why is chlorine (a gas at room temperature) more volatile than iodine

• iodine is a solid

• chlorine has a lower boiling point

• making it more volatile

halogens trend in oxidising ability

• oxidising ability of halogens increases going up the group

• fluorine is one of the most powerful oxidising agents

halogens usually react by…

gaining electrons to become negative ions with a -1 charge

these are redox reactions

halogens are examples of _____ agents and are themselves…..

oxidising

reduced

chlorine being reduced equation

Cl2 + 2e- → 2Cl-

ranking power of most to least oxidising halogen

• fluorine

• chlorine

• bromine

• iodine

halogens react with metal halides in a solution in such way that….

the halide in the compound will be displaced by a more reactive halogen but not by a less reactive one

what is a displacement reaction

halogens reacting with metal halides in solution in such a way that the halide in the compound will be displaced by a more reactive halogen but not by a less reactive one

example equation and ionic equation of chlorine displacing bromide ions

Cl2 (aq) + 2NaBr (aq) → Br2 (aq)+ 2NaCl (aq)

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

sodium ions are spectator ions

why could chlorine displace bromide ions but iodine will not

chlorine is more reactive than bromide/ bromine

how to produce the red brown colour of bromine

the two colourless start materials react

how does chlorine act as the oxidising agent when displacing bromide ions

• removes electrons from Br- and so oxidising 2Br- to Br2 and the oxidation number of bromine increases from -1 to 0

in general, a halogen will always displace the ion of a halogen….

below it in the periodic table

extraction of bromine from sea water

• oxidation of a halide by a halogen is basis of method for extracting bromine from sea water

• sea water contains small amounts of bromide ions which can be oxidised by chlorine to produce bromine

• Cl2 + 2Br- → Br2 + 2Cl-

extraction of iodine from kelp

• iodine discovered in 1811 by extraction from kelp which is obtained by burning seaweed

• some iodine still produced in this way

• salts such as sodium chloride, potassium chloride and potassium sulfate are removed from the kelp by washing with water

• the residue is then heated with manganese dioxide and concentrated sulfuric acid and iodine is liberated

• 2I- + MnO2 + 4H+ → Mn2+ + 2H2O + I2

halide ions can act as___agents

reducing

how can halide ions act as reducing agents

• the halide ions lose electrons to become halogen molecules

trend in halide ions reducing ability

• linked to size of ions

• larger the ion the more easily it loses an electron because the electron is lost from the outer shell which is further from the nucleus as the ion gets larger so the attraction to the outer electron is less

ranking of most reducing to least reducing halide ions

I-

Br-

Cl-

F-

what reaction can the trend in halide ions as reducing agents be seen

reaction of solid sodium halides with concentrated sulfuric acid

the products of solid sodium halides reacting with concentrated sulfuric acid are….

different and reflect the reducing powers of the halide ions

observation of sodium chloride and concentrated sulfuric acid

• drops of concentrated sulfuric acid added to solid NaCl

• steamy fumes of HCl seen

• solid product is sodium hydrogen sulfate

equation of sodium chloride and concentrated sulfuric acid

NaCl (s) + H2SO4 (l) → NaHSO4 (s) + HCl (g)

why is sodium chloride and concentrated sulfuric acid not a redox reaction

• no oxidation state has changed

• chloride ion is too weak a reducing agent to reduce the sulfur in sulfuric acid

• it is an acid base reaction

what can the reaction of sodium chloride and concentrated sulfuric acid be used for

to prepare hydrogen chloride gas which because of this reaction was once called salt gas

sodium fluoride has a similar reaction with concentrated sulfuric acid as sodium chloride does…

• produces hydrogen fluoride, an extremely dangerous gas that will etch glass

• fluoride ion is an even weaker reducing agent than the chloride ion

observation of reaction between sodium bromide and concentrated sulfuric acid

• steamy fumes of hydrogen bromide

• brown fumes of bromine

• colourless sulfur dioxide is also formed

2 reactions that occur in the reaction between sodium bromide and concentrated sulfuric acid

1. sodium hydrogensulfate and hydrogen bromide are produced in a similar acid base reaction to sodium chloride

• NaBr(s) +H2SO4(l) → NaHSO4(s) +HBr (g)

2. bromide ions are strong enough reducing agents to reduce the sulfuric acid to sulfur dioxide

• oxidation state of the sulfur is reduced from +6 to +4 and that of the bromine increases from -1 to 0

• 2H+ + 2Br- + H2SO4(l) → SO2(g) + 2H2O(l) + Br2(l)

• this is a redox reaction

• the reactions are ectothermic and some of the bromine vaporises

observations of reaction of sodium iodide and concentrated sulfuric acid

• steamy fumes of hydrogen iodide

• black solid of iodine

• bad egg smell of hydrogen sulfide gas is present

• yellow solid sulfur may also be seen

• colourless sulfur dioxide is also evolved

2 reactions of sodium iodide and concentrated sulfuric acid

1. hydrogen iodide is produced in an acid base reaction

• NaI(s) + H2SO4(l) → NaHSO4(s) + HI(g)

2. iodide ions are better reducing agents than bromide ions so they reduce the sulfur in sulfuric acid even further so that sulfur dioxide, sulfur and hydrogen sulfide gas are produced

• 8H+ + 8I- + H2SO4(l) → H2S(g) + 4H2O(l) + 4I2(s)

• during the reaction from +6 to -2 the sulfur passes through oxidation state 0 and some yellow solid sulfur may be seen

all metal halides except fluorides react with_____ to form a precipitate of the insoluble silver halide

silver ions in aqueous solution eg silver nitrate

equation of chloride reacting with silver ions

Cl-(aq) + Ag+(aq) → AgCl(s)

why does silver fluoride not form a precipitate

because it is soluble in water

how to identify metal halides with silver ions

1. dilute nitric acid added to halide solution to get rid of any soluble carbonate or hydroxide impurities as these would interfere with the test by forming insoluble silver carbonate or insoluble silver hydroxide

2. a few drops of silver nitrate solution are added and the halide precipitate forms

how can you use the reaction of silver nitrate and dilute nitric acid as a test for halides

• you can tell from the colour of the precipitate which halide has formed

• silver fluoride forms no precipitate

• silver chloride forms white ppt

• silver bromide forms cream ppt

• silver iodide forms pale yellow ppt

how to differentiate between silver chloride and silver bromide after doing silver nitrate and nitric acid test

• similar colour of ppt

• add ammonia

• silver chloride dissolves in dilute ammonia

• silver bromide dissolves in concentrated ammonia

• silver iodide is insoluble in concentrated ammonia

why was chlorine notoriously used in the first world war

it’s a poisonous gas

how can chlorine be used in drinking and swimming pools

it’s soluble in water

chlorine reacts with water in a _____

reversible reaction to form chloric acid and hydrochloric acid

equation of chlorine in water

Cl2 (g) + H2O(l) →← HClO(aq) + HCl(aq)

what happens in the reaction between chlorine and water

• oxidation number of one of the chlorine atoms increases from 0 to +1 and that of the the other decreases from 0 to -1

• this type of redox reaction where the oxidation no state of some atoms of the same element increase and others decrease is called disproportionation

when does the disproportionation reaction take place

• when chlorine is used to purify water for drinking and in swimming baths to prevent life threatening diseases

• chloric acid is an oxidising agent and kills bacteria by oxidation, it’s also a bleach

halogens that aren’t chlorine react similarly with water but…

much more slowly going down the group

equation of chlorine and water in sunlight

2Cl2(g) + 2H2O(l) → 4HCl(aq) + O2(g)

• 2Cl2 pale green

• 4HCl colourless

what colour is 2Cl2 gas in reaction with water in sunlight

pale green

what colour is 4HCl in reaction of chlorine and water in sunlight

colourless

in sunlight, chlorine is rapidly lost from pool water so….

shallow pools need frequent addition of chlorine

alternative to the direct chlorination of swimming pools

• add solid sodium chlorate or calcium chlorate

• this dissolves in water to form chloric acid in a reversible reaction

• NaClO(s) +H2O →← Na+(aq) + OH-(aq) +HClO(aq)

• in alkaline solution this equilibrium moves to the left and the HClO is removed as ClO- ions

• to prevent this happening, swimming pools need to be kept slightly acidic however this is carefully monitored and water never gets acidic enough to corrode metal components and affect swimmers

chlorine reaction with alkali

• reacts with cold dilute sodium hydroxide to form sodium chlorate NaClO

• this is an oxidising agent and the active ingredient in household bleach

• this is also a disproportionation reaction

• Cl2(g) + 2NaOH(aq) → NaClO(aq) + NaCl (aq) + H2O(l)

• other halogens behave similarly