Tuition - Group Study – II and VII
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Transcript of Tuition - Group Study – II and VII
Group Study – II and VII
Group II
• Alkali-earth metals• Oxidation state: +2– Removal (or sharing) or two valence shell
electrons ns2
– Therefore, 1st and 2nd IE usually low. 3rd IE usually jumps by about 6000kJ from 2nd due to removal of electron from an INNER quantum shell
Group II trends• Radii
– Atomic and ionic• INCREASES down group
– increasing number of quantum shells– Increasing shielding– Overall decrease in Effective Nuclear Charge despite increase in proton number due to increasing
shielding• Ionic radii < atomic radii
– 1 less quantum shell
• IE– DECREASES down group
• MORE quantum shells, valence electrons further, less strongly bound• 2nd IE > 1st IE, because outgoing 2nd electron attracted back by single positive charge on
ion after 1st electron left• M+ M2+ + e-
• Melting points– DECREASES down group– Related to atomic radii and IE– Lowered effective nuclear charge lower attraction for valence electrons
metallic bonds easier to break
Group II trends• Electronegativity
– DECREASES down group• Relate to atomic radii and IE• EN is like measure of attraction of electrons to itself• Therefore, more shell big radius lower eff. Nuc. Charge lower EN
• Conductivity– INCREASES down group
• Relate to atomic radii and IE also• Down group IE lower electrons escape easily
– So can move around and conduct electricity more easily!
• Standard Reduction Potential– DECREASES down group (become more negative)
• Relate to IE• Down group IE lower electrons escape easily don’t want them back
(don’t want get reduced) Eo more negative less easily reduced, more easily oxidised
• Meaning, down the group, reactivity INCREASES!
Beryllium• Some compounds are covalent e.g. BeCl2
• Some have partially covalent character e.g. BeO• Why?– Small size, high charge high charge density high
polarising power distort electron cloud of anion forms a “covalent” bond
– Diagonal relationship with Al• Note similarities between AlCl3 and BeCl2
• Revise how to draw the AlCl3 dimer and BeCl2 polymer!• Be and Al are frequently said to be electron deficient in
their covalent compounds. Dimer and polymerisation involves DATIVE BONDING
Beryllium
• BeCl2 is much more volatile than MgCl2
– Covalent molecule that is polymerised (not ionic)– Melting and boiling involves breaking of id-id
interactions• BeO and BeOH are amphoteric– BeO + H+ Be2+ + H2O– BeO + 2OH- + H2O Be(OH)4
2-
• Doesn’t this look like reaction of Al3+ in NaOH?
Group II – Rxn with H2O• Reacts readily to give H2 gas– Reactivity INCREASES down group– Products metal oxides; CaO onwards will
dissolve immediately to form hydroxidesBe Mg Ca Sr Ba
Rxn cond. Does not react
Slowly with water, fast with steam
Readily with water
Very readily Most reactive
Product none MgOSlightly soluble
Ca(OH)2 Sr(OH)2 Ba(OH)2
Solubility increases Basicity increases
Group II – Rxn with O2• Burns brilliantly when heated to give basic oxides– Reactivity INCREASES down group
• Sr and Ba almost spontaneously burns in air– Be gives amphoteric oxide (due to partially covalent
nature)Be Mg Ca Sr Ba
Product BeO MgO CaO SrO BaO
Behavior in water
NoneInsoluble due to partially covalent nature
Slightly soluble to form Mg(OH)2
Dissolves exothermically in waterGives hydroxides Solubility increases Basicity increases
Group II – Thermal stability
• Nitrates, carbonates and hydroxides decompose on heating– M(NO3)2 MO + 2NO2 + ½O2
– MCO3 MO + CO2
– M(OH)2 MO + H2O
• Thermal stability INCREASES down group– Fatter radii lower charge density less
polarisability polyatomic anion more stable
Group VII
• Halogens• Oxidation state: -1 (usually), Cl onwards can go as
high as +7• F is the most electronegative, will only be -1
because no one can steal its electrons.• F also cannot be more than -1 because it cannot
expand octet• Cl, Br, I can expand octet, share up to 7 of the
valence electrons.
Group VII trends• Atomic and Ionic radii
– INCREASES down the group• IE
– DECREASES down the group– But still relatively high, therefore they do not like to form cations, but
positive oxidation states in COVALENT compounds possible (esp if bonded to O)
• EN– DECREASES down the group– But most electronegative in respective periods
• Mp, bp– INCREASES down the group– Volatility decreases as a result– Larger size greater VDW interactions (id-id)
Group VII trends• Colour– INCREASES down the group• F2 – pale yellow gas
• Cl2 – pale green gas
• Br2 – red-brown liquid, red vapor
• I2 – black crystals, violet vapor
• Solubility– Good in organic solvents– Poor in water (fluorine oxidises water to O2)– Iodine solubility enhanced by I3
- formation
Group VII oxidising ability
• Readily oxidises, and gets reduced to -1 state• Oxidising ability DECREASES down group– (by now, you should realise that many trends are
quite opposite to group 2)• Oxidising ability high Eo very positive– Highest for F2 (+2.87V)– Lowest for I2 (+0.54V)– Why?• High EN of F, high tendency to accept electrons to form
octet
Group VII oxidising ability
• Cl2 and Br2 oxidise S2O32- to SO4
2-
– 4X2 + S2O32- + 5H2O 8X- + 2SO4
2- + 10H+
– S changes from +2 to +6
• I2 oxidises S2O32- to S4O6
2- only– I2 + S2O3
2- 2I- + S4O62-
Group VII displacements
• The more reactive Halogen will displace the less reactive one. – F2 will displace Cl-, Br-, I- to form F- and Cl2, Br2, I2
– Likewise, Cl2 will displace Br- and I-
– Br2 will displace I-
Group VII – Rxn with H2
• H2 + X2 2HX• As usual, fluorine is most reactive (wants to oxidise
H to H+ very very much)• Very high reactivity of fluorine can be explained by
weak F-F bond energy due to lone pair repulsion on the two F.
F2 Cl2 Br2 I2
Rxn Explosive in the dark
Explosive in sunlight, slow in dark
Requires heat and Pt catalyst
Slow, equilibrium mixture
Product HF HCl HBr HI
Group VII – Rxn with H2
• Stability of hydrogen halides– HF > HCl > HBr > HI– H-F bond length shortest due to small H and F atomic
radii bond strength highest– Evidence
• HI dissociates 33% into H2 and I2 at 1000*C• HI dissociates to form the strongest acid out, followed by
HBr, HCl and HF (weak acid)
• Acidity– HI > HBr > HCl >>> HF– HI dissociates the most completely in water
Group VII – Rxn with H2
• Reducing property– Ease of oxidation : HI > HBr > HCl >>>>>> HF– Easy to oxidise HI and HBr back to I2 and Br2 by
conc. H2SO4
– HCl oxidised only by very strong oxidising agents such as KMnO4 or MnO2
– Almost impossible to chemically oxidise HF
Group VII – Rxn with silver• Halide test– Add AgNO3(aq) and then dilute NH3 and then conc
NH3
Reagent F- Cl- Br- I-
AgNO3 No ppt White pptAgCl
Cream pptAgBr
Yellow pptAgI
Dil NH3
Conc NH3
n/a Soluble
soluble
Insoluble
soluble
Insoluble
insoluble
Ksp1.8 x 10-10 7.7 x 10-13 8.3 x 10-17
Group VII – Rxn with silver
• NH3 addition is to form the diamminesilver complex ion [Ag(NH3)2]+
• This decreases ionic product [Ag+][X-]• For AgI, Ksp is so low that even the decrease
of ionic product by NH3 is still higher than Ksp of AgI stay insoluble
Group VII – Rxn with conc H2SO4
• H2SO4 serves to force halide ions to accept back a H+ and form HX which escapes as white fumes
• It can also be an oxidising agent in the case of HBr and HI to give Br2 and I2
Group VII – Rxn with conc H2SO4
Reaction with H2SO4 Reaction with MnO2
Observation
F- No oxidation at all Cannot be oxidised White HF fumes
Cl- Cannot be oxidised with H2SO4 but possible with MnO2 added
4HCl + MnO2 Cl2 + MnCl2 + 2H2O
Cl2 gas evolved with white HCl fumes
Br- 2HBr + H2SO4 Br2 + SO2 + 2H2O Br2 gas evolved with some white HBr fumes
I- 8HI + H2SO4 4I2 + H2S + 4H2O I2 gas evolved, very little HI fumes
Group VII - Misc
• Chlorine with aq NaOH– Cold dilute NaOH• Disproportionation (1 ox state become 2 different ox
state)• Cl2 + 2OH- Cl- + ClO- + H2O
– Warming to 70*C, ClO- disproportionates further• 3ClO- 2Cl- + ClO3
-
• Chlorine with hot aq NaOH– 3Cl2 + 6OH- 5Cl- + ClO3
- + 3H2O