halogens:group7* aqaaschemistry* · decreasingoxidisingstrengthofhalogenasgroupdescends...

Halogens: Group 7 AQA AS Chemistry

Physical Proper;es Halogens displacing halide ions

Halide ion reduc;on of conc. H2SO4 Tes;ng for halide ions

Halogen reac;ons with water Redox ( H2O2 and S2O3

2-‐ )

Reac;on of Sodium and Chlorine

hJps://www.youtube.com/watch?v=afRlDab1_e8

Halogens: F2 Cl2 Br2 I2

pale green orange/brown solu;on black/grey solid (purple fumes)

Halide ions: F-‐ Cl-‐ Br-‐ I-‐ Increasing boiling point as Group descends Decreasing electronega;vity Decreasing oxidising strength of halogen as Group Descends Increasing reducing strength of halide ions as Group Descends

Explain why Cl2 has a lower boiling point than Br2 ?

Cl2 molecules are smaller in size and have a smaller Mr than Br2 molecules Hence, the intermolecular forces between Cl2 molecules (van der Waals) tend to be weaker than those present in Br2

As a result, less energy is needed to separate the Cl2 molecules than Br2 molecules

Explain why Cl2 has a lower boiling point than Br2 ?

F Cl Br I Iden;fy the halogen that is the strongest oxidising agent. Give the formula of the halide ion that is the strongest reducing agent.

Displacement of halogens by fluorine

hJps://www.youtube.com/watch?v=3Sspgx-‐aOL0

Halogen/Halide oxida3on (displacement) reac3ons

The stronger halogen oxidising agent will oxidise the halide ion posi2oned below it in the Group.

Strength of oxidising agent, in order of strongest...... to....... weakest:

F2 Cl2 Br2 I2

Predict which reac;on is feasible or not using the oxidising strength rule

1.  F2 + 2Cl− à 2F− + Cl2 2.  F2 + 2I− à 2F− + I2 3.  Cl2 + 2I− à 2Cl− + I2 4.  Br2 + 2Cl− à 2Br− + Cl2 5.  Cl2 + 2Br− à 2Cl− + Br2 6.  I2 + 2Br− à 2I− + Br2


F2 oxidises Cl− to Cl2 F2 + 2e− à 2F− 2Cl− à Cl2 + 2e−

F2 + 2Cl− à 2F− + Cl2

F2 oxidises Br− to Br2 F2 + 2e− à 2F− 2Br− à Br2 + 2e−

F2 + 2Br− à 2F− + Br2

F2 oxidises I− to I2 F2 + 2e− à 2F− 2I− à I2 + 2e−

F2 + 2I− à 2F− + I2

The stronger halogen oxidising agent will oxidise the halide of a halogen posi2oned below it in the Group.


Cl2 oxidises Br− to Br2 (colourlessàorange solu;on)

Cl2 oxidises I− to I2



Cl2 oxidises Br− to Br2 (colourlessàorange solu;on)

Cl2 + 2e− à 2Cl− 2Br− à Br2 + 2e− Cl2 + 2Br− à 2Cl− + Br2

Cl2 oxidises I− to I2 Cl2 + 2e− à 2Cl− 2I− à I2 + 2e−

Cl2 + 2I− à 2Cl− + I2



Br2 oxidises I− to I2



Br2 oxidises I− to I2 Br2 + 2e− à 2Br− 2I− à I2 + 2e−

Br2 + 2I− à 2Br− + I2


What other product is missing from reduc;on of H2SO4

S Ox. state=0

Halide reac;ons with conc. H2SO4 Concentrated sulfuric acid acts as an oxidising agent, and oxidises: bromide ions AND iodide ions

Concentrated sulphuric acid is not a strong enough oxidising agent to oxidise fluoride or chloride ions

KCl + H2SO4 à KHSO4 + HCl 2KCl + H2SO4 à K2SO4 + 2HCl

HCl misty white fumes

2Br− + 2H+ + H2SO4 à Br2 + SO2 + 2H2O

Oxida;on (Br− à Br2): Br ox. state: -‐1à0

Reduc;on(H2SO4àSO2): S ox state: +6 à +4

2Br− à Br2 + 2e−

2H+ + H2SO4 + 2e− à SO2 + 2H2O

orange/brown vapour

Oxida;on (I− à I2): Br ox. state: -‐1à0 Reduc;on(H2SO4àSO2): S ox state: +6 à +4

Reduc;on(H2SO4àS): S ox state: +6 à 0 Reduc;on(H2SO4àH2S): S ox state: +6 à -‐2

2I− à I2 + 2e−

6H+ + H2SO4 + 6e− à S + 4H2O

2H+ + H2SO4 + 2e− à SO2 + 2H2O

8H+ + H2SO4 + 8e− à H2S + 4H2O

8I− + 8H+ + H2SO4 à 4I2 + H2S + 4H2O

2I− + 2H+ + H2SO4 à I2 + SO2 + 2H2O

Black/grey solid and

purple gas (I2)

Black/grey solid and

purple gas (I2)

6I− + 6H+ + H2SO4 à 3I2 + S + 4H2O

Reducing ability of halide ions increases as Group 7 descends? [3]

F− , Cl− will not reduce conc H2SO4 Br− will reduce conc H2SO4 to SO2

I− will reduce conc H2SO4 to mixture of SO2, S and H2S

•  To be a reducing agent, each halide ion must lose an outer shell electron. •  Iodide ions have a larger atomic radius (more electron shells and greater

shielding) than chloride ions. •  This results in the outer shell electron, that is to be lost from the iodide

ion, being further from the nucleus and less strongly held by the nucleus compared with the electron that is to be lost from the chloride ion.

•  Hence, iodide ions are stronger reducing agents than chloride ions

In the series, F, Cl, Br, I, it is energe;cally more favourable to convert Iodide ion to Iodine

In test 3: write the simplest ionic equa;on for the reac;on. Iden;fy the species responsible for the misty white fumes.

In test 4: write the two half-‐equa;ons for I-‐ to I2 and H2SO4 to H2S State the role of the H2SO4 (.........................) and iden;fy the black solid (.........), yellow solid (.................)and the gas that smelled of roJen eggs (.........................).

In test 3: write the simplest ionic equa;on for the reac;on. H2SO4 + Cl– à HCl + HSO4

– Iden;fy the species responsible for the misty white fumes. HCl

In test 4: write the two half-‐equa;ons for I-‐ to I2 and H2SO4 to H2S 2I− à I2 + 2e− 8H+ + H2SO4 + 8e− à H2S + 4H2O

State the role of the H2SO4 (oxidising agent) and iden;fy the black solid (iodine), yellow solid (sulfur) and the gas that smelled of roJen eggs (hydrogen sulfide).

Potassium chloride

Potassium bromide

Potassium iodide

Silver nitrate solution

Dilute Aqueous Ammonia

(few drops)

Concentrated Ammonia

(few drops)

!

F −

Cl − Br −

I −

AgNO3 acidified with dilute HNO3

no ppt; colourless solu;on (AgF is soluble)

white precipitate

cream precipitate

yellow precipitate

add excess dilute NH3 to the ppt.

dissolves precipitate

par;ally soluble

does not dissolve

add excess conc. NH3 to the ppt.



does not dissolve

Why is AgNO3 acidified when tes;ng for halide ions ?

Remove anions that may interfere with the test (e.g. CO32-‐, OH-‐, SO3

2-‐) and/or prevent the forma;on of other insoluble silver compounds that would invalidate the test

Ideally, why is HNO3 and not HCl, used to acidify AgNO3 ?

A false posi;ve test occurs as a white precipitate will be formed upon addi;on of HCl due to presence of aqueous Cl-‐ , hence invalidate test.

Indicate a poten3al test tube reac3on test that could be used to dis3nguish between HCl(aq) and HNO3(aq)

TEST: reagent: AgNO3 (or any soluble silver salt) with HCl(aq): white precipitate with HNO3(aq): remains colourless

AgNO3 must be acidified by HNO3 prior to being used as a test for presence of Cl-‐

Write the simplest ionic equa;on for the reac;on that has taken place in Test 1. State the type of reac;on that has taken place in Test 1.

Write the simplest ionic equa;on for the reac;on that has taken place in Test 2. State what would be observed when excess dilute NH3 (aq) is added to the white precipitate made in Test 2.

Write the simplest ionic equa;on for the reac;on that has taken place in Test 1. Cl2+ 2I– à 2Cl– + I2 State the type of reac;on that has taken place in Test 1. redox

Write the simplest ionic equa;on for the reac;on that has taken place in Test 2. Ag+ + Cl – à AgCl State what would be observed when excess dilute NH3 (aq) is added to the white precipitate made in Test 2. it dissolves, colourless solu3on

Indicate two poten3al test tube reac3on tests that could be used to dis3nguish between AgBr(s) and AgI(s)

TEST 1: reagent: conc H2SO4 with AgBr(s): cream solid turns orange AND brown vapour (Br2) with AgI(s): yellow solid turns black, purple vapour (I2), bad egg smell (H2S)

TEST 1: reagent: conc NH3 with AgBr(s): cream solid dissolves, solu;on formed with AgI(s): precipitate remains that does not dissolve

Indicate two poten3al test tube reac3on tests that could be used to dis3nguish between AgBr(s) and AgI(s)

TEST 1: reagent: concentrated H2SO4 with AgBr(s): cream solid turns orange AND brown vapour (Br2) with AgI(s): black solid/ purple vapour (I2), forma;on of yellow solid (S), bad egg smell (H2S)

TEST 1: reagent: concentrated NH3 with AgBr(s): cream solid dissolves, solu;on formed with AgI(s): solid does not dissolve

Describe two different tests that dis;nguish AgCl(s) from AgI(s)

NH3 (aq) dilute conc. H2SO4

dissolves white AgCl, colourless solu;on formed

misty white fumes (HCl)

does not dissolve AgI, (even if conc. NH3 used) yellow solid remains

black solid and/or purple vapour(I2) forma;on of yellow solid (S) roJen egg smell gas (H2S)

Select the correct sodium halide: NaF NaCl NaBr NaI Descrip3on Formula of

sodium halide

1 This sodium halide is a white solid that reacts with concentrated sulfuric acid to give a brown gas.

2 When a solu;on of this sodium halide is mixed with silver nitrate solu;on, no precipitate is formed.

3 When this solid sodium halide reacts with concentrated sulfuric acid, the reac;on mixture remains white and steamy fumes are given off.

4 A colourless aqueous solu;on of this sodium halide reacts with orange bromine water to give a dark brown solu;on.

Select the correct sodium halide: NaF NaCl NaBr NaI Descrip3on Formula of

sodium halide

1 This sodium halide is a white solid that reacts with concentrated sulfuric acid to give a brown gas.

NaBr

2 When a solu;on of this sodium halide is mixed with silver nitrate solu;on, no precipitate is formed.

NaF

3 When this solid sodium halide reacts with concentrated sulfuric acid, the reac;on mixture remains white and steamy fumes are given off.

NaCl (NaF)

4 A colourless aqueous solu;on of this sodium halide reacts with orange bromine water to give a dark brown solu;on.

NaI

Halogen reac;ons with H2O

In bright sunlight: Cl2 + H2O à 2HCl + ½ O2

Cl2 + H2O ⇌ HClO + HCl Cl2 gas and cold water :

Cl2 gas is TOXIC. Why do we use it in our drinking water/swimming pools The (health) benefit outweighs the risk; once reac;on is completed, very liJle remains unreacted; only very low doses of sodium chlorate(I) (NaClO) are formed.

pale green colour due to Cl2 species Is water oxidised ?

Chlorine + cold dilute NaOH

Cl2 + 2NaOH à NaCl + NaClO + H2O

oxida3on: Cl ox state: 0 à +1 reduc3on: Cl ox state: 0 à -‐1

Sodium Chlorate (I) solu3on used to kill bacteria (e.g. bleach, disinfectants, sterilisers)

Chlorine is both oxidised and reduced

Br2 + H2O ⇌ HBrO + HBr Br2 liquid and cold water:

Br2 used to kill microbes in swimming pools, but Br2 is toxic?

The (health) benefit outweighs the risk; once reac;on is completed, very liJle remains unreacted; only very low doses are used

More Redox •  Halide ion acts as a reducing agent

•  Halogen acts as an oxidising agent

I2+ 2S2O32–→ 2I– + 2S4O6

2–

(a) Kills bacteria / prevents bacterial diseases; Chlorine is a toxic substance Cl2+ H2O → HCl + HClO (b) Cl2(aq) to Br–(aq); yellow-‐brown solu;on 2Br– + Cl2 → 2Cl– + Br2

Cl2(aq) to I–(aq); brown/black solu;on 2I– + Cl2 → 2Cl– + I2 c) NaBr, products include: orange/brown vapour Br2 and SO2 (toxic choking colourless gas) NaI, products include: purple vapour/black solid of I2, gas with smell of bad eggs (H2S); SO2 (toxic choking colourless gas) and S (yellow solid) 2Br – → Br2 + 2e– 2I– →I2 + 2e– H2SO4 + 2e– + 2H+ → SO2 + 2H2O H2SO4 + 6e– + 6H+ → S + 4H2O H2SO4 + 8e– + 8H+ → H2S + 4H2O

(a) Reduc;on involves gain of electrons; a reducing agent loses (donates) electrons (b) (i) Sulphur dioxide, oxida;on state +4;

Sulphur, oxida;on state 0; Hydrogen sulphide, oxida;on state – 2

(ii) Sulphur dioxide is a toxic choking colourless gas Sulphur is a yellow solid Hydrogen sulphide gas has a smell of bad eggs

(iii) H2SO4 + 2H+ + 2e– → SO2 + 2H2O

H2SO4 + 6H+ + 6e-‐ → S + 4H2O H2SO4 + 8H+ + 8e-‐ → H2S + 4H2O

(c) Cl2+ H2O → HCl + HClO Water is not oxidised The oxida;on states of O (–2) and H (+1) remain unchanged

halogens:*group*7* aqa*as*chemistry* · decreasing*oxidising*strength*of*halogen*as*group*descends*...

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halogens:group7* aqaaschemistry* · decreasingoxidisingstrengthofhalogenasgroupdescends...