Lecture 6 Flashcards
(20 cards)
Learning objectives
6.1 Discuss periodic changes in structures and properties of simple compounds: oxides, halides and hydrides
6.2 Relate compound changes to fundamental trends in atomic properties
Describe cha
6.3 Changes in acidic/basic behaviour of oxides through the periodic table
6.4 Be able to rationalise the importance of hydrogen bonding on physical properties of certain hydrides
What type of oxides (in terms of structures) are common for each group?
Groups 1, 2 and bottom of 13,14 and 15 - ionic
“Staircase” going from top of 13 to botom of 16 and 17 - giant covelant/oligomeric
Top of 14,15,16,17 and all of 18 - molecular
What are the Oh and Td holes in a ccp for ionic like?
It is 1x Oh, and 1xTd hole per sphere - look at diagram to see how they form
Group 1 oxides
Li - forms Li2O with Li + and O 2-
It is a ccp array of O 2- ions, with Li + ions in all the tetrahedral holes (2 per O)
Each Li + is surrounded by 4O 2- ions and each oxide ion is surrounded by 8 Li + ions
Li coord number = 4, O coord no = 8
Na - Na2O2 with 2Na + and O2 2-) it is a peroxide
For K, Rb and Cs - form MO2 where M + and O2 - it is a superoxide
Goes from small metal and oxygen to big metal at big O2 - most stable
Group 2 oxides
Be - BeO ionic (different structure doesn’t matter)
Mg, Ca, Sr, Ba - all the same M 2+ and O 2- (ionic)
All ccp array of O 2- ions with M 2+ ions in all the octahedral holes
Each Mg 2+ is surrounded by 6O 2- and vice versa (coord no of 6)
Called the rock salt structure (NaCl)
Group 13 oxides
Down gp 13 EN starts high and then gets low.
Therefore B favours polar covelant bonding due to high EN forming B2O3 (each B is bonded to 3O) - giant/ infinite covalent structure (disordered/glassy)
Al, Ga, In, Tl are all ionic
Eg Al2O3 is ccp array of O 2- ions with Al 3+ ions in 2/3 octahedral holes
Group 14 oxides (I) - C, Si and Ge
High EN at top and low at bottom
C, Si, Ge all form MO2 (in +4 oxid state) - covelant
O=C=O (multiple bonding and sublimes at -78 *C) gas
CO (multiple bonding and also simple molecule)
SiO2 - s.b. Where each Si is bonded to 4 O ions forming infinite covalent network (disordered corner sharing SiO4 tetrahedra system) - it is a quartz/cristobalite structure
Each SiO4 tetrahedron shares 4 corners with other tetrahedra
mp is 1700 *C as got to break covelant bonds not vdwf
Forms this as more favourable to form s.b. inst. d.b. -
C-O bond distance leads to efficient pi overlap whereas longer Si-O bond leads to inefficient pi bonding
Si O also displays ‘extra pi bonding” (check diagram -16) due to overlap between empty dxz orbital on Si and lp on px O orbital forming a partial pi bond
Group 14 oxides (II) - Sn and Pb
Sn forms SnO2 but - ionic (eg rutile (TiO2) structure (diagram on slides))
Edge and corner sharing SnO6 octahedra
Pb - PbO with Pb in +2 oxide state as low EN (inert pair effect) - ionic
Group 15 - 17 oxides general notes
N,O,F, P,S,Cl As,Se,Br Sb,Te,I Bi,Po,At - how they group
- Predominantly covelant bonding
- Multiple bonds for small elements (N, O, P 5+, S 4+/6+ is when P and S are at high oxid states as orbitals contract a bit and so m.b. Is favoured)
- Single bonds lower down
- Simple molecules at the top whereas infinite/giant covalent structures lower down
- Tend to be less stable than gps 1-14 oxides as both elements in the oxide are high EN so would rather not share e-
- More ionic at the bottom eg Bi2O3
Common oxides of group 15
Loads of N ones: N20, .N=O (para), N203, NO2 (para), N2O4, N2O5 etc (look at diagrams but don’t need to memorise)
All molecular and unstable - note lots of mb going on
P4O6 is single bonding but P4O10 is mb
Common oxides of group 16
O=S=O, SO3 all db also
When get to solids eg SO3 sb solids they form these extended networks
Se and T form oxides form giant covelant
Oxides of group 17
There are 25 binary halide oxides
Range from explosive liquids to stable solids
ClxOy are all unstable
All are simple molecules
VSEPR shapes
Acidic oxides
Cl-O-Cl - brown-yellow explosive gas , Cl2O3, Cl2O7 ( 6 db and 1 sb) is a colourless oily liquid
Trends in oxides and their properties
- Ionic to covelant across the table
- Covelant to ionic down a group
- Mb for C, N, S 4+, S 6+, and P 5+ (top right)
- Simple molecules to infinite covalent down group
- Structures from ionic model /VSEPR in bottom left
- Oxides become more unstable across a period (more oxidising eg (N2O, BrO2)
- Oxides become more acidic across a period
What is a bronsted acid or base? What is the general eq.?
Give a specific example for both
bronsted acid - proton donor
H2O + HA ⇌ H3O+ + A-
Eg. SO3 + H2O ⇌ H2SO4 ⇌ H+ + HSO4 - ⇌ H+ + SO4 2-
bronsted base - proton acceptor / OH- generator
H2O + B ⇌ BH+ + OH-
Eg. CaO (s) + H2O (l) ⇌ Ca 2+ (aq) + 2OH - (aq)
Right of periodic table = acidic oxides, left of periodic table = basic oxides
What is the meaning of amphoteric?
Give an example
Amphoteric - can act as both an acid and base
These are in the middle of the periodic table - due to EN (check slide 23 for which are) values between 1.2 and 1.9)
As a base:
Al2O3 (s) + 6H + (aq) ⇌ 2Al 3+ (aq) + 3H2O (l)
As an acid:
Al2O3 (s) + 2OH - (aq) + 3H2O (l) ⇌ 2Al(OH)4 - (aq)
HCO3- is also amphoteric
What determines if an oxide is acidic or basic
Acidic oxides EN >1.9
Basic oxides EN<1.2
Look at E-O-H
E (element on LHS) is delta +ve and O is delta -ve - will form OH- (E-O will break) and E+
E (element on RHS) is delta -ve and O is delta -ve and H is delta +ve - will form H+
How do common fluorides relate to common oxide
Same pattern down periodic table:
Bottom left - ionic
Middle staircase - giant covalent
Top right - molecular
Examples of fluorides
NaF - rock salt structure
CaF2 - (same as LiO2 if you flip round the anions and cations)
In the middle form giant covelant fluorides
CF4, PF5 and SF6 are all molecular fluorides
How do hydrides relate to oxides?
Same pattern again:
Bottom left - ionic
Middle staircase - giant covalent
Top right - molecular
Don’t need to know the exact compounds that fit in which set of “lines” just need to understand general trend
Mp and bp of hydrides - group 14 and 16
REMEMBER: always consider vdw, permanent dipoles and h bonding
14 - CH4, SiH4, GeH4, SnH4
Only vdw interactions are important
Therefore bp increases as you go down gp 14 linearly
16- H2O, H2S, H2Se, H2Te
Vdwf increase down group
Dipole interactions decrease down group
HOWEVER H2O displays H bonding - giving it a higher bp
Pattern goes H2O (high), drop to H2S and then follows same linear trend before
