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