halogens all toxic all form diatomic molecules all form ionic salts group vii

Halogens

•All toxic•All form Diatomic molecules•All form ionic salts

Group VII

How does the trend in atomic radiusdictate:

(i) The trend in boiling points?

Boiling points of Halogens

How does the trend in atomic radiusdictate:

(ii) The ability of the halogens to act as an oxidising agent?

GROUP 7 : THE HALOGENS

Halogen Symbol FormulaState at 298K

Colour

Fluorine

Chlorine

Bromine

Iodine

Astatine

F

Cl

Br

I

At

F2

Cl2

Br2

I2

At2

Gas

Gas

Liquid

Solid

Solid

Pale yellow

Green

Dark red

Dark grey

Black

Fluorine F [He]2s22p5

Chlorine Cl [Ne]3s23p5

Bromine Br [Ar]3d104s2 4p5

Iodine I [Kr]4d105s2 5p5

Astatine At [Xe]4f14 5d106s2 6p5

Halogens are powerful oxidising agents

All halogens gain electrons to make halide ions, so all the halogens are oxidising agents. You need to know the trend in the oxidising power as you go down group VII.

Chlorine atoms gain an electron to make chloride ions:

The chlorine atom has gained an electron so is reduced, so the chlorine atom is an oxidising agent (oxidising agents are electron acceptors).

Cl2 + e- 2Cl-

Down the group the halogens become weaker oxidising agents. Also the halide ions become stronger reducing agents.

Write in the oxidation numbers to show what is being oxidised and reduced.

0 +1 -1 0 +1 -1

Chlorine is being reduced (and is the oxidising agent)

Bromine is being oxidised (and is the reducing agent)

Cl2(g) + 2KBr(aq) Br2(aq) + 2KCl(aq)

Greengas

Colourlesssolution

Red-brownsolution

Colourlesssolution

Also what are the oxidation and reducing agents?

Br2(l) + 2KCl(aq) No reaction

HALOGENS - DISPLACEMENT REACTIONSHALOGENS - DISPLACEMENT REACTIONS

KCl(aq) KBr(aq) KI(aq)

Cl2(aq)

No Reaction

Pale Green Solution

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

(aq) + Br2(aq)

Orange Solution

Cl2(aq) + 2I-(aq) 2Cl-

(aq) + I2(aq)

Brown Solution

Br2(aq)

No Reaction

Orange Solution

No Reaction

Brown Solution

Br2(aq) + 2I-(aq) 2Br-

(aq) + I2(aq)

Orange Solution

I2(aq)

No Reaction

Brown Solution

No Reaction

Brown Solution

No Reaction

Brown Solution

Aqueous halogens in cyclohexane

Cl2(aq) Br2(aq) I2(aq)

Cyclohexane layer

Aqueous layer

Halogens are non-polar molecules. They will therefore dissolve best in non-polar solvents, like cyclohexane.

Cyclohexane will not mix with water so two distinct layers will form.

Bromine water (aqueous bromine, Br2) is shaken with a volume of cyclohexane, and then the follow aqueous solutions are added to separate portions.

a) Aqueous sodium iodide

b) Aqueous chloride

c) Aqueous sodium astatide

Each mixture is shaken again. Describe what you would expect to see. Write equations for any reactions that would occur.

No change

The orange cyclohexane layer would turn purple

Br2(aq) + 2I-(aq) 2Br-

(aq) + I2(aq)

Given its position in group VII, we would expect astatine to be darker in colour than iodine . The orange cyclohexane layer would turn this colour.

Br2(aq) + 2At-(aq) 2Br-

(aq) + At2(aq)

When chlorine reacts with water a mixture of hydrochloric acid (HCl) and chloric (I) acid (HClO) is formed

The reaction is a dispropotionation because chlorine is both oxidised and reduced

Cl2(aq) + H2O(l) HCl(aq) + HClO(aq)

Use oxidation numbers to deduce what atom is being oxidised and what is being reduced.

Reaction of chlorine with aqueous sodium hydroxide

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

NaClO is sodium chlorate (I), also called sodium hypochlorite. A dilute sodium chlorate (I) is a key ingredient of chlorine-based household bleaches.

Bromine reacts with cold dilute alkali to give a colourless solution containing Br- and BrO- ions.

a)Write a balanced chemical equation for this reaction

b)What are the oxidation states of bromine before and after the reaction

c) Name this type of reaction.

Br2 + 2NaOH NaBr + NaOBr + H2O

0 before, -1 and +1 after

disproportionation

TESTING FOR HALIDES – AgNOTESTING FOR HALIDES – AgNO33

• make a solution of the halide

• acidify with dilute nitric acid - prevents the precipitation of other salts

• add a few drops of silver nitrate solution

• treat any precipitate with dilute ammonia solution

• if a precipitate still exists, add concentrated ammonia solution


CHLORIDE white ppt of AgCl soluble in dilute ammonia

BROMIDE cream ppt of AgBr insoluble in dilute ammonia but soluble in conc.

IODIDE yellow ppt of AgI insoluble in dilute andconc. ammonia solution


CHLORIDE white ppt of AgCl soluble in dilute ammonia

BROMIDE cream ppt of AgBr insoluble in dilute ammonia but soluble in conc.

IODIDE yellow ppt of AgI insoluble in dilute andconc. ammonia solution

halides precipitate as follows Ag+(aq) + X¯(aq) ——> Ag+X¯(s)

when they dissolve in ammonia a colourless diammine complex is formed [Ag(NH3)2]+(aq)

PLACE A SOLUTION OF THE HALIDE IN A TEST TUBE

CHLORIDE BROMIDE IODIDE


ADD SOME DILUTE NITRIC ACID



ADD SILVER NITRATE SOLUTION

WHITE PRECIPITATE OF SILVER CHLORIDE AgCl

CREAM PRECIPITATE OF SILVER BROMIDE AgBr

YELLOW PRECIPITATE OF SILVER IODIDE AgI



ADD DILUTE AMMONIA SOLUTION

WHITE PRECIPITATE OF SILVER CHLORIDE - SOLUBLE

CREAM PRECIPITATE OF SILVER BROMIDE - INSOLUBLE

YELLOW PRECIPITATE OF SILVER IODIDE - INSOLUBLE



ADD CONCENTRATED AMMONIA SOLUTION

WHITE PRECIPITATE OF SILVER CHLORIDE - SOLUBLE

CREAM PRECIPITATE OF SILVER BROMIDE - SOLUBLE

YELLOW PRECIPITATE OF SILVER IODIDE - INSOLUBLE



a) What would you expect to be the physical state of astatine at room temperature. Explain your answer

b) Fluorine and chlorine both react with sodium metal to form a salt.

i) Write a balanced equation for each reaction

ii) Which of the two reaction will be most vigorous. Why?

iii) What structure and bonding will the products have?

c) What is the oxidation number of chlorine in Cl2, CaCl2, Cl2O7 and ClO2?

Solid. At2 molecules are larger than I2 molecules (At2 molecules have more elctrons than I2 molecules) therefore van der Waals’ forces will be stronger. Iodine is a solid at room temperature, therefore so is astatine

F2 + 2Na 2NaF

Cl2 + 2Na 2NaCl

The fluorine reaction. Fluorine is more reactive than chlorine as it has a smaller atomic radius. This means it is easier for an incoming electron to enter its outer shell as it has experiences less shielding and is closer to the nucleus.

Both are giant ionic

0, -1, +7, +4

203.3 g

White precipitate/goes white

203.3 g

White precipitate/goes white

Ag+(aq) + Cl-

(aq) AgCl(s)

•AgCl dissolves in NH3(aq)

• AgBr dissolves in conc NH3(aq)/partially soluble in NH3(aq)

•AgI insoluble in NH3(aq) or conc NH3(aq)

halogens all toxic all form diatomic molecules all form ionic salts group vii

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