Revision part3
Periodicity
Aims
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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 IE
1. The distance of the outermost electron from
the nucleus (atomic radius)
2. Electron shielding
3. 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
Na
Mg
Molecular structures
Al
Si
P4
S8
Cl2
Strong forces between atoms
Weak van der Waals forces
between molecules
Metallic
Van der Waals
Covalent
High melting points
Good conductors
Low melting points
Poor conductors
Group 2 redox reactions
• Identify what is being oxidised and what is
being reduced
O has gained
electrons
Oxidation numbers
0
0
+2
-2
• Mg + O2  2MgO
Mg has lost
electrons
Group 2 with oxygen
Reactivity 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
Ca + ½ O2  CaO
Sr + ½ O2  SrO
Ba + ½ O2  BaO
burns with a bright white flame
burns with a brick-red colour
burns with a crimson colour
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 same
Ca(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 group
Again they are not redox reactions
GROUP TRENDS
APPEARANCE
F2
Cl2
Br2
I2
Colour
Yellow
Green
Red/brown
Grey
State (at RTP)
GAS
GAS
LIQUID
SOLID
BOILING POINT
Boiling point / °C
F2
Cl2
Br2
I2
- 188
- 34
58
183
INCREASES down Group
• increased size makes the van der Waals forces increase
• more energy is required to separate the molecules
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
CHLORINE + SODIUM IODIDE
BROMINE + SODIUM IODIDE
BUT
BROMINE + SODIUM CHLORIDE
BROMINE + SODIUM CHLORIDE
IODINE + SODIUM CHLORIDE
IODINE + SODIUM BROMIDE
CHLORINE + SODIUM BROMIDE
(Bromine is below chlorine in the Group so is less reactive)
HALOGENS - DISPLACEMENT REACTIONS
DISPLACEMENT REACTIONS - EXPERIMENT
CHLORINE
BROMINE
SODIUM CHLORIDE
SODIUM BROMIDE
SODIUM IODIDE
Solution stays
colourless
NO REACTION
Solution goes from
colourless to orangeyellow
BROMINE FORMED
Solution goes from
colourless to orangered
IODINE FORMED
Solution goes from
colourless to orangeyellow
NO REACTION
Solution goes from
colourless to orangeyellow
NO REACTION
Solution goes from
colourless to 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
0
Cl2(g) + H2O(l)
-1
HCl(aq)
strong acid
+1
+
HOCl(aq)
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)
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 and
conc. 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)
TESTING FOR HALIDES – AgNO3
CHLORIDE
BROMIDE
IODIDE
PLACE A SOLUTION OF THE HALIDE IN A TEST TUBE
TESTING FOR HALIDES – AgNO3
CHLORIDE
BROMIDE
ADD SOME DILUTE NITRIC ACID
IODIDE
TESTING FOR HALIDES – AgNO3
CHLORIDE
BROMIDE
IODIDE
ADD SILVER NITRATE SOLUTION
WHITE PRECIPITATE OF SILVER CHLORIDE
CREAM PRECIPITATE OF SILVER BROMIDE
YELLOW PRECIPITATE OF SILVER IODIDE
AgCl
AgBr
AgI
TESTING FOR HALIDES – AgNO3
CHLORIDE
BROMIDE
IODIDE
ADD CONCENTRATED AMMONIA SOLUTION
WHITE PRECIPITATE OF SILVER CHLORIDE
CREAM PRECIPITATE OF SILVER BROMIDE
YELLOW PRECIPITATE OF SILVER IODIDE
- SOLUBLE
- SOLUBLE
- INSOLUBLE
HALOGENS & HALIDES - USES
Chlorine, Cl2
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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
Hydrogen
fluoride, HF
• used to etch glass
Silver
bromide, AgBr
• used in photographic film