Revision part3
description
Transcript of Revision part3
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