OCR AS Chemistry (F321.3) - The Periodic Table Revision Notes

These revision notes are for The Periodic Table - the third module of F321 (Atoms, bonds and groups) from OCR AS Chemistry A.

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OCR AS Chemistry Notes
GROUP 2 ELEMENTS AND THEIR COMPOUNDS
Have their outermost electron placed on s subshell
Readily react to lose 2 electrons
o M M 2+ + 2e-
Trends down the group:
1st ionisation energy decreases
o Distance between outermost electron and nucleus increases
o More shielding
o Overcomes effect of increasing nuclear charge
Atomic radius increases
Melting point generally decreases
o Much lower than all transition metals
Physical properties:
Good conductors of heat and electricity
o Sea of delocalised electrons
Form white compounds
o Transition metals form coloured compounds
Oxidation number of +2
All compounds are ionic or almost ionic
o Be compounds show some covalent character because small highly charged cation polarises outer shell of
anion
Oxides / hydroxides are basic
o React with acids (proton acceptors)
o Except :
BeO
Be(OH)2 which is amphoteric
Density = 1-3.5 g/cm3
o Average density of transition metals is about 7g/cm3
Compared to group 1 metals:
o Harder + denser
o Higher melting point
Metallic bond is stronger (higher charge on cation more electrons donated)
Physical properties:
Strong reducing agents
Oxidation Is Losing electrons
Reduction Is Gaining electrons
o Reducing agent donates electrons (reducing other substance)
o Oxidising agent accepts electrons (reducing itself)
E.g.
o 2Mg + O22MgO
2Mg 2Mg2+ + 4e-
O2 + 4e-2O2-

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Mg acted as reducing agent, donating its electrons and oxidising itself
OXIDATION NUMBERS
Many covalent substances participate in redox reactions where there is no electron transfer
o 2H 2 + O2 2H 2O Both covalent reactions, no electron transfer and yet redox
o C + O2 CO2
In order to determine what oxidises / reduces, we use oxidation numbers
A number assigned to an atom / ion to determine its relative state of oxidation or reduction
RULES:
All elements have oxidation number of 0
Simple…read more

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REACTIONS OF GROUP 2 ELEMENTS (AND THEIR COMPOUNDS)
1.…read more

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Sr Red
Ba Green
Most oxides are only very slightly soluble in water
o When they do dissolve, they form weakly alkaline solutions
3.…read more

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Attraction for cation is a lot higher
All group 2 carbonates decompose in Bunsen flame
However they require different amounts of energy to decompose
o It requires more and more energy to decompose carbonates as you go down group 2
BeCO3 decomposes at room temperature
BaCO3 requires most energy to decompose
o During thermal decomposition CO32- ions break down to form O2- ions
Smaller the group 2 metal cation, the stronger the charge density
Therefore the stronger polarising power over the large CO32- anion
This…read more

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PHSICAL PROPERTIES
All exist as diatomic molecules
All are coloured
F2 Pale yellow
Cl2 Green
Br2 Red-brown
I2 Black-purple
At R.T.P:
F2 Gases
Cl2
Br2 Liquid
I2 Solid
All dissolve in water, but only very slightly
o Because atoms have same electronegativity, molecules are non-polar
Can only form Van der Waal's forces with water
Which are much weaker than existing H-bonds
o EXCEPT: Fluorine which is such a strong oxidising agent, it reacts with water
Dissolve in non-polar solvents
o E.g.…read more

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OXIDISING ABILITY DECREASES:
o More shells
More shielding
Further from nucleus
Electrons less easily attracted
o This overcomes increasing nuclear charge
4. They all exhibit variable oxidation numbers
Cl2O7 / +7
NaClO4
ClO3 +6
NaClO3 +5
ClO2 +4
KClO2 +3
Cl2O / NaClO +1
Cl2 0
NaCl -1
o Fluorine is an exception (most powerful oxidising agent)
Only one oxidation number (-1)
CHEMICAL PROPERTIES
1.…read more

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H 2 (g) + Br2 (l)2HBr (g)
3. HALOGENS ACTING AS OXIDISING AGENTS:
o Displacement reactions
F2 Most powerful oxidising agent (most reactive)
Cl2
Br2
I2 Least powerful (least reactive)
o The more powerful oxidising agent will always get reduced
0 Cl2 + - 1 2K I - 1 2K Cl + 0 I 2 Br2 + NaCl
0 Br2 + - 1 2K I - 1 2K Br + 0 I 2 I 2 + KBr
4.…read more

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NaClO 2NaCl + N aClO3
REACTIONS OF HALIDE IONS:
o All halides are soluble
EXCEPT: Lead halides and AgCl / AgBr / AgI
o Precipitates form when solution of halide ions is treated with either Ag+ or Pb2+
N aCl (aq) + AgNO3 (aq)AgCl (s) + NaNO3(aq)
Cl-(aq) + Ag+(aq)AgCl (s)
o Identify presence of halide ions we use acidified solution of AgNO3 (aq) followed by reaction with ammonia
F- No precipitate -
Cl- Gives white precipitate Will dissolve in aqueous solution of ammonia
Br-…read more

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