topic 4b: group 7

physical properties of group 7 (mp/bp)

• mp and bp: increases down the group

physical properties of fluorine

• colour: pale yellow

• state at rtp: gas

• colour in aqueous: colourless

• colour in hexane: colourless

physical properties of chlorine

colour: pale green

state at rtp: gas

colour in aqueous solution: pale yellow

colour in hexane: pale yellow

physical properties of bromine

colour: brown

state at rtp: liquid

colour in aqueous: brown

colour in hexane: orange

physical properties of iodine

colour: grey

state at rtp: solid

colour in aqueous: brown

colour in hexane: violent

explain the change in boiling points down the group

1. down the group the boiling points increases

2. as size of molecule increase

3. the number of electrons increases

4. therefore the strength of london forces increases

5. and more energy is required to seperate the molecules

electron affinity

energy released when 1 mol of electrons are added to 1 mole of gaseous atoms

ionisation energy of halogens

1. ionisation energy decreases down the group (less endothermic and less positive)

2. because atomic radius increases

3. so shielding from inner electrons increases

= the nuclear attraction from the protons in the nucleus on the outer shell electrons decreases as its outweighed by the increase in proton number

electron affinity of halogens

1. less exothermic down the group

2. because the incoming electron is further away from the nuclear attraction due to increased atomic radius and increased shielding so the electrostatic forces of attraction decreases

trend of electronegativity of group 7

1. electronegativity decreases down the group

2. as the shielding increases

3. the atomic radius increases

= there is less attraction between the shared pair of bonded electrons within the covalent bond and the protons in the nucleus

trend in reactivity of the elements in group 7

1. reactivity decreases down the group

2. because atomic radius increases

3. so shielding increases

boiling points of hydrogen halides

HF: has london forces and hydrogen bonds

HCl, HBr, BI: only london forces

why does the boiling point increase from HCl to HBr to HI?

1. because the number of electrons increases from HCl (18) to HI (54)

2. and the more electrons mean stronger london forces

3. so more energy needed to overcome london forces

why is the boiling point of HF highest?

because it has Hydrogen bonding and its the strongest type of intermolecular force

reaction of halogens with metals

(halogens are strong oxidising agents)

rdox: 2Na + Cl2 → 2NaCl (s)

reactions with non metals

achieve nobel gas configuration

H2 + Cl2 → 2HCl (g)

• bromine needs 300° and platnium catalyst

• iodine needs 300° and platinum catalyst and only reacts partially

reactions of halogens with water

solubility of halogens in water decreases down the group

chlorine: Cl2 +H2O → HCl + HOCl

bromine: Br2 + H2O → HBr + HOBr

iodine: I2 + H2O → HI + HOI

(all disproportination reactions and reversible)

iodine in water

only slightly soluble but soluble in potassium iodide solution because of the formation of I3- ion

I2 + I- → I3-

• in KI the iodide ion is fully negatively charged so it disrupts the H bonding in water so the I3- is formed when I reacts → this makes it easier to hydrate the I3- ion on its own, I2 is non polar so its harder to disrupt the H bonds in water so its less able to be hydrated

chemical tests for halogens

chlorine: damp blue litmus paper turns red

bromine: brown gas slowly bleaches damp blue litmus paper

iodine: turns blue/black in starch solution

reaction of chlorine with alkali

cold dilute: Cl2 + 2NaOH → NaCl + NaClO + H2O

hot alkali: 3Cl2 + 6NaOH → 5NaCl + NaClO3 + 3H2O

reactions of halide ions in aqueous solution

(silver halide)

chloride: white solid → soluble in dilute and concentrated ammonia

bromide: cream solid → only soluble in concentrated ammonia

iodide: yellow solid → insoluble

reaction of hydrogen halide in water

all of them complete except hydrogen fluroide so all strong acid except HF

HX + H2O → H3O+ + X-

why does HF only form weak acid in water when all the other hydrogen halides form strong acids?

• because it has strong hydrogen bonds because of the high electronegativty difference, making it harder to dissociate the ions in water

• this means ions are less likely to be hydraed

predicting astatine

black solid

oxidising power ad reducing power of halogen and halide

halogen: oxidising power increases up the group (gains e more easily)

1. because they have smaller atomic radius

2. less shielding

3. easier to attract incoming electrons

halides: reducing power of halides increases down the group (easier to lose electrons)

2. ionic radius increases

3. shielding increases

4. less energy is require to lose an outer electron

products formed with NaCl

HCl + NaHSO4

HCl: misty fumes

• no other products because Cl- is not a strong enough reducing agent

products formed with NaBr

SO2 and Br2 and HBr

HBr: misty fumes

SO2: colourless gas → turns damp litmus blue

Br2: orange brown gas

products formed with NaI

HI, SO2, S, H2S, I2

HI: misty fumes

SO2: colourless gas

S: yellow solid

H2S: pungent gas

I2: purple gas

equation of products formed with NaI

reactions of halogen and halides (Cl)

Cl + KBr → (halogen) orange (hexane) orange displaced bromine

Cl + KI → (halogen) brown (hexane) purple displaced iodine

reactions of halogen and halides (Br)

Br + KCl → (halogen) remains yellow (hexane) orange hexane unreacted bromine

Br + KI → (halogen) brown displaced I (hexane) purple displaced I

reactions of halogen and halides (I)

I + KCl → (halogen) no change brown (hexane) purple unreacted I

I _ KBr → (halogen) no change brown (hexane) purple unreacted I

test for CO3 2-

1. add dilute acid (HCl) to a solid sample

2. carbon dioxide released (bubbles) → test with limewater

test for HCO3-

1. add dilute HCl to solid sample

2. carbon dioxide released (bubbles) → test with limewater

test for SO4 2-

1. add barium chloride then HCl

2. white precipitate of barium sulfate that does not dissolve in dilute acid

test for NH4+

1. add NaOH

2. ammonia released → test with damp red litmus paper that turns blue

Br- with chlorine water and cyclohexane

chlorine water: orange/ yellow solution

cyclohexane: orange

I- with chlorine water and cyclohexane

chlorine water: reddy brown solution (confirm with starch solution → blue/black)

cyclohexane: pink/purple

solubility table

nitrates: all soluble

sodium potassium and ammonium compounds: soluble

chlorides: all soluble except silver, lead

sulfates: all soluble except lead, calcium, strontium and barium

carbonates: all insoluble except sodium, potassium and ammonium

hydroxides: all insoluble except sodium, potassium, ammonium