Revision part3

Periodicity

Aims

• Electronegativity • Ionisation energies• Atomic radii• Boiling points• Group 2 redox reactions• Group 2 oxides with water• Thermal decomposition• Group 7 properties• Displacement reaction of grp 7• Testing for halide ions• Uses of Cl

Electronegativity

• A measure of the attraction of a bonded atom for the pair of electrons in a covalent bond

• On your periodic table draw this trend

Ionisation energies

• 1st IE the energy required to remove on electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.

3 factors that influence IE1. The distance of the outermost electron from

the nucleus (atomic radius)2. Electron shielding3. Nuclear charge

Atomic radii

Number of protons increases; shielding stays the same

Radius decreases

Radius increases

Extra shells and more shielding outweigh additional protons in the nucleus

Boiling points: they are related to structure

Giant structures Molecular structures

Na Mg Al Si P4 S8 Cl2

Strong forces between atoms Weak van der Waals forces between molecules

Metallic Covalent Van der Waals

High melting points Low melting points

Good conductors Poor conductors

Group 2 redox reactions

• Identify what is being oxidised and what is being reduced

• Mg + O2 2MgO

O has gained electrons

Mg has lost electrons

Oxidation numbers 0 0 +2 -2

Group 2 with oxygenReactivity increases down the group. This is explained by the increasing ease with which group 2 elements form the corresponding 2+ ion.

Mg + ½ O2 MgO burns with a bright white flame

Ca + ½ O2 CaO burns with a brick-red colour

Sr + ½ O2 SrO burns with a crimson colour

Ba + ½ O2 BaO burns with a green colour

Group 2 with water

• Group 2 elements also undergo a redox reaction with water

Oxidation numbers 0 +1 -2 +2 -2+1 0

Mg + 2H2O Mg(OH)2 + H2

The rate of reaction increases down the group, largely due to the ease of cation formation

Group 2 oxides with water

All group 2 metal oxides react with water to form hydroxides:

MgO + H2O Mg(OH)2

They are not redox reactions. The oxidation numbers stay the sameCa(OH)2 neutralize soil and lakes

Mg(OH)2 antacids

Thermal decomposition of group 2 carbonates

Group 2 carbonates decompose to form oxides and carbon dioxide

MgCO3 MgO + CO2

The ease at which a carbonate decomposes decreases down a groupAgain they are not redox reactions

GROUP TRENDS

INCREASES down Group

• increased size makes the van der Waals forces increase• more energy is required to separate the molecules

F2

Yellow

Cl2

Green

Br2

Red/brown

I2

Grey

GAS GAS LIQUID SOLID

Colour

State (at RTP)

APPEARANCE

BOILING POINT

F2

- 188

Cl2

- 34

Br2

58

I2

183Boiling point / °C

HALOGENS - DISPLACEMENT REACTIONS

THE DECREASE IN REACTIVITY DOWN THE GROUP IS DEMONSTRATED USING DISPLACEMENT REACTIONS...

A SOLUTION OF THE HALOGEN IS ADDED TO A SOLUTION OF A HALIDE HALIDES ARE SALTS FORMED FROM HALOGENS

A MORE REACTIVE HALOGEN WILL DISPLACE A LESS REACTIVE ONE

e.g. CHLORINE + SODIUM BROMIDE BROMINE + SODIUM CHLORIDE

CHLORINE + SODIUM IODIDE IODINE + SODIUM CHLORIDE

BROMINE + SODIUM IODIDE IODINE + SODIUM BROMIDE

BUT BROMINE + SODIUM CHLORIDE CHLORINE + SODIUM BROMIDE

(Bromine is below chlorine in the Group so is less reactive)

SODIUM CHLORIDE

DISPLACEMENT REACTIONS - EXPERIMENT

HALOGENS - DISPLACEMENT REACTIONS

CHLORINE

SODIUM BROMIDE SODIUM IODIDE

Solution stays colourless

NO REACTION

Solution goes from colourless to orange-

yellow

NO REACTION

Solution goes from colourless to orange-

yellow

BROMINE FORMED

Solution goes from colourless to orange-

yellow

NO REACTION

Solution goes from colourless to red

IODINE FORMED

BROMINE

Solution goes from colourless to orange-

red

IODINE FORMED

OTHER REACTIONS OF CHLORINE

Water Halogens react with decreasing vigour down the group as their oxidising power decreases

Litmus will be turned red then decolourised in chlorine water

Cl2(g) + H2O(l) HCl(aq) + HOCl(aq)

strong acid bleaches by oxidation

This is an example of DISPROPORTIONATION …‘simultaneous oxidation and reduction of a species’

Alkalis Chlorine reacts with cold, aqueous sodium hydroxide.

2NaOH(aq) + Cl2(g) —> NaCl(aq) + NaOCl(aq) + H2O(l)

0 -1 +1

TESTING FOR HALIDES – AgNO3

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 AgIinsoluble 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

TESTING FOR HALIDES – AgNO3

ADD SOME DILUTE NITRIC ACID

CHLORIDE BROMIDE IODIDE

TESTING FOR HALIDES – AgNO3

ADD SILVER NITRATE SOLUTION

WHITE PRECIPITATE OF SILVER CHLORIDE AgCl

CREAM PRECIPITATE OF SILVER BROMIDE AgBr

YELLOW PRECIPITATE OF SILVER IODIDE AgI

CHLORIDE BROMIDE IODIDE

TESTING FOR HALIDES – AgNO3

ADD CONCENTRATED AMMONIA SOLUTION

WHITE PRECIPITATE OF SILVER CHLORIDE - SOLUBLE

CREAM PRECIPITATE OF SILVER BROMIDE - SOLUBLE

YELLOW PRECIPITATE OF SILVER IODIDE - INSOLUBLE

CHLORIDE BROMIDE IODIDE

TESTING FOR HALIDES – AgNO3

HALOGENS & HALIDES - USES

Chlorine, Cl2 • water purification• bleach• solvents• polymers - poly(chloroethene) or PVC• CFC’s

Fluorine, F2 • CFC’s• polymers - PTFE poly(tetrafluoroethene) as used in...

non-stick frying pans, electrical insulation, waterproof clothing

Fluoride, F¯ • helps prevent tooth decay - tin fluoride is added to toothpaste - sodium fluoride is added to water supplies

Hydrogenfluoride, HF • used to etch glass

Silverbromide, AgBr • used in photographic film