Transcript Periodicity (AHL) - slider-dpchemistry-11

Periodicity (AHL)
Year 11 DP Chemistry
Rob Slider
Oxides of Period 3
Na2O
MgO
Al2O3
SiO2
P4O10
SO3
Cl2O7
P4O6
SO2
Cl2O
What patterns are there in the following properties?
• Structure
• Melting/boiling points
• Electrical conductivity
• Acidity
Oxide structure
Oxides of Na,
Mg form ionic
bonds, so they
have ionic
lattices
Al2O3 forms
bonds that
have ionic and
covalent
character
SiO2 forms a
giant covalent
macromolecular
structure
Oxides of P, S,
Cl form simple
covalent
molecular
substances
Oxide melting/boiling points
Oxides of Na, Mg, Al
Ionic compounds
have high mp/bp
SiO2
The diamond-like
covalent
macromolecular
structure leads to a
high mp/bp
Oxides of P, S, Cl
Simple covalent
molecular substances
have weaker
intermolecular forces
and much lower
mp/bp
Oxide electrical conductivity
Oxides of Na, Mg, Al
Ionic compounds
form ions when
molten and will
conduct electricity
SiO2
The diamond-like
covalent
macromolecular
structure has fixed
electrons and will
not conduct
electricity as a liquid
or a solid
Oxides of P, S, Cl
Simple covalent
molecular substances
do not have any free
electrons and do not
conduct electricity
Oxide acidity
Oxides of Na, Mg
React with water to
form basic solutions
Oxides of Al
Acts as both an acid
and a base
Amphoteric
Oxides of Si, P, S, Cl
SiO2 reacts with
NaOH (i.e. acts as an
acid)
The other oxides
react with water to
form acidic solutions
Na2O(s) + H2O(l)  2Na+(aq) + 2OH-(aq)
SiO2(s) + 2NaOH(aq)  Na2SiO3(aq) + H2O(l)
Al2O3(s) + 6HCl(aq)  2AlCl3(aq) + H2O(l)
Al2O3(s) + 2NaOH(aq) +3 H2O(l)  2NaAl(OH)4(aq)
SO2(g) + H2O(l)  H2SO3(aq)
P4O10(s) + 6H2O(l)  4H3PO4(aq)
Cl2O7(s) + H2O(l)  2HClO4(aq)
Chlorides of period 3
NaCl
MgCl2
Al2Cl6
SiCl4
PCl3
PCl5
What patterns are there in the following properties?
• Structure
• Melting/boiling points
• Electrical conductivity
• Acidity
S2Cl2
Cl2
Chloride structure
Chlorides of Na, Mg,
form ionic bonds, so
they have ionic
lattices
Chlorides of Al form
bonds with ionic and
covalent character, so
they form lattices as
a solid and sublime to
a gas
Chlorides of Si,P, S
form simple covalent
molecular substances
Chloride melting/boiling points
Chlorides of Na, Mg
Ionic compounds
have high mp/bp
Chlorides of Al
Aluminium chloride
sublimes at 1780C to
form gaseous
molecules of Al2Cl6
Chlorides of Si,P, S
Simple covalent
molecular substances
have weaker
intermolecular forces
and much lower
mp/bp
Chloride electrical conductivity
Chlorides of Na, Mg
Ionic compounds
form ions when
molten and will
conduct electricity
Al
There is no ionic
character when solid
and molecules form
above 1780C, so no
electrical
conductivity
Chlorides of Si, P, S
Simple covalent
molecular substances
do not have any free
electrons and do not
conduct electricity
Chloride acidity
NaCl
This forms a
neutral solution
in water
Mg
Weakly acidic
NaCl(aq)  2Na+(aq) + 2Cl-(aq)
AlCl3(s) + 3H2O(l)  Al2O3(aq) + 6HCl(aq)
SiCl4(s) + 4H2O(l)  Si(OH)4(aq) + 4HCl(aq)
Chlorides of Al,
Si, P, S
These all react
vigourously with
water to form
acidic HCl
fumes
Cl2
Chlorine gas
reacts to a small
extent with
water to form
an acidic
solution
Cl2(g) + H2O(l)  HCl(aq) + HClO(aq)
First row d-block
The d-block elements are in the middle of the Periodic
Table and include the transition metals. Starting in
period 4 after the 4s fills, the 3d subshell begins to fill
with electrons
A transition metal (TM) is an
element that has at least one ion
with a partially filled d-subshell. Not
all d-block elements are TM. Sc
and Zn are not considered to be
TM (more later...)
Properties of TM
Due to the partially filled d-subshell, TM have unique
properties including:
•Multiple oxidation states
•Complex ion formation
•Formation of coloured compounds
•Catalytic properties
Electronic configurations
As we have seen previously, the
configurations of the first row d-block
mostly fill the 3d subshell in order.
The exceptions come from Cr and Cu
where we see more stable configurations
from the half-filled and filled 3d subshell.
This is possible because the 4s and 3d
subshells are so similar in energy
Sc
[Ar] 3d14s2
Ti
[Ar] 3d24s2
V
[Ar] 3d34s2
Cr
[Ar] 3d54s1
Mn
[Ar] 3d54s2
Fe
[Ar] 3d64s2
Co
[Ar] 3d74s2
Ni
[Ar] 3d84s2
Cu
[Ar] 3d104s1
Zn
[Ar] 3d104s2
Sc and Zn (not TM)
Sc forms Sc3+ which has the stable
configuration of Ar
Sc3+ has no 3d electrons, therefore it is not
considered to be a TM
Zn has a configuration of [Ar]3d104s2,
The Zn2+ ion ([Ar] 3d10), therefore is not a
typical TM ion
Variable oxidation states
+2
All transition metals can form the oxidation state of +2 due to
the loss of the two s-electrons. In the first row, the 4s.
This is because the 4s fills first, but when ions are being formed,
the 4s electrons are also lost first.
Examples:
To write the electronic structure for Co2+:
Co [Ar] 3d74s2
Co2+ [Ar] 3d7
The 2+ ion is formed by the loss of the two 4s electrons
To write the electronic structure for V3+:
V [Ar] 3d34s2
V3+ [Ar] 3d2
The 4s electrons are lost first, then one of the 3d electrons
Variable oxidation states
Due to the similar energy levels of the 4s and 3d, other oxidation states in
addition to +2 are also possible.
On the left, all of the electrons
from the 4s and 3d can be lost
forming ions such as Sc3+ and Ti4+.
This represents the largest possible
OS
On the right, the nucleus has a
stronger pull on the outer
electrons due to a greater positive
charge. This means that +2 is the
most stable as there is a greater
energy difference between the 3d
and 4s(Co, Ni, Cu)
Cu also forms +1 due to the formation of the stable [Ar]3d10
In the middle
V, Cr, Mg, Fe
It requires too much energy to remove all of the electrons from these
elements as the number of valence electrons and high nuclear charge
increases.
What often occurs is the formation of more stable oxyanions, such as
VO3-, vanadate(V). Some important ones to remember:
Oxidation
state
Chromium
+7
+6
Manganese
Iron
MnO4- permanganate
CrO42- chromate
Cr2O72- dichromate
+5
+4
+3
MnO2
Cr3+
Fe3+
Summary of oxidation states
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
+1
+3
+2
+2
+2
+2
+2
+2
+2
+2
+3
+3
+3
+3
+3
+3
+3
+3
+4
+4
+4
+5
+6
+6
+6
+7
Boxed states are
the important
ones to know
+2
Summary of 1st d-block OS
On the left, +2 state
highly reducing.
e.g. V2+(aq) , Cr2+(aq) are
strong reducing agents
(lose e- easily)
Higher OS’s become less
stable relative to lower ones
on moving from left to right
across the series(stronger +
nuclear force)
On the right, +2 is more
common; +3 state highly
oxidising.
E.g. Co3+ is a strong oxidising
agent (gain e- easily), Ni3+ &
Cu3+ do not exist in aqueous
solution.
Compounds
containing TM’s in
high OS’s tend to
be oxyanions and
oxidising agents
e.g. MnO4-
Complex ion formation
Complex ions have a metal ion at the centre with a number of other
molecules or ions surrounding it.
The bonds are coordinate bonds where a lone pair on a molecule/ion is
donated to a low energy, unfilled metal orbital such as a d-orbital. These
molecules/ions are called ligands.
These are some common
ligands found in complex ion
formation. Notice they all
have lone pairs of electrons
Ligands are neutral molecules or anions that contain a non-bonding pair
of electrons
Complex ions
Coordination number:
The most common complex ions
contain 4 or 6 ligands. These are
known as 4-coordinated and 6coordinated. 2 is also possible.
Water forms hexahydrated complex
ions (6-coordinated) with most
transition metals.
Example: [Fe(H2O)6]3+
Many complex ions form coloured
solutions
The charge on the complex is the
sum of the metal and the ligands.
The Cr is 2+ and the water is
neutral leading to a 2+ complex
ion charge.
You try:
•Fe(III) + CN- (6-coord)
•Cu (II) + Cl- (4-coord)
•Ag+ + NH3 (2-coord)
Complex ion geometry
2-coord complexes form
linear geometries
4-coord complexes form
tetrahedral or square planar
geometries
6-coord complexes tend to form
octahedral geometries
Complex compounds
Complex ions can be anions or
cations and will bond with oppositely
charged ions to make salts. Notice
how [Cu(NH3)4]2+ is formed:
(CuCl4)2- is an anion that can
form a compound with K+ to
form [K2(CuCl4)]
This complex ion can then bond with
Cl- to form [Cu(NH3)4]Cl2
Would you expect these
two compounds to be
soluble in water?
Yes, they are soluble in water.
How does the metal attract so many ligands??
You may be wondering why a metal ion will attract more ligands than it has
charges. +2 should attract -2 and +3 should attract -3, right??
Let’s look at an example:
Fe(H2O)6 3+
Fe: 1s22s22p63s23p63d64s2
Fe3+: 1s22s22p63s23p63d5
In Fe3+, the 4s is now empty and there are 5 unpaired e-.You might expect 5 ligands, but the ion uses six
orbitals from the 4s, 4p and 4d to accept lone pairs from six water molecules. It hybridises six new
orbitals all with the same energy.
Why not 4 or 8? Six is the maximum number of water molecules it is possible to fit around an iron ion
(and most other metal ions). By making the maximum number of bonds, it releases most energy and so
becomes most energetically stable.
Isomers (cis,trans)
Metal complexes sometimes have more than one type of ligand attached. This
leads to possible isomerism with complexes having different ligand
arrangements. These are called stereoisomers.
cis
When ligands are adjacent to each
other they are said to be cis-
cis-[CoCl2(NH3)4]+
trans
When ligands are opposite to each
other they are said to be trans-
trans-[CoCl2(NH3)4]+
Optical isomers
Some isomers are mirror images of one another. Therefore, they cannot be
superimposed on one another. These two mirror image compounds are known
as optical isomers or enantiomers.
Coloured complexes
Many d-complexes are coloured. These characteristic colours are specific to
individual ions and depend upon:
•Metal oxidation state
•Ligands attached
•Coordination number/shape
Same metal/different OS
Same metal/different ligand
Same metal/different coord
Why coloured? d,d transitions
The d-orbitals shown above, have various arrangements around the x, y and z axes.
When a 6-coord complex is formed with a d-block element, the ligands will approach
along the axes of an octahedral, to minimise repulsions of bonding e-.
The approach of the ligands raises
the energy level of the d-orbitals,
but the orbitals that lie on the
axes (4,5 above) will experience
more repulsion and thus will be a
slightly higher energy level (than
1,2,3).
This means the d orbitals are
split.
d,d transitions
Movement between the d-orbitals by
the e- represents an energy change, ΔE.
Remembering ΔE=hv, a transition
between d-orbitals represents a
specific frequency that is specific to a
complex.
Considering the four d-block elements above,
only 2 and 3 have possible transitions. They
are coloured due to the excitation of e- to
higher d-orbitals. This transition absorbs
specific frequencies and we perceive the
remaining frequencies.
Why are Sc3+ and Zn2+ colourless?
Hexa-aqua complex colours
This shows the colours of 6-coord aqua complexes of the first row d-block.
Exceptions are Cu(I) which only forms simple colourless compounds and
Cu(II) which forms a 4-coord aqua complex [Cu(H2O)4]2+ . Notice there are
no possible transitions for Sc3+ and Zn2+, so they are typically colourless.
Complex colours-examples
TM as catalysts
Transition metals and their compounds function as catalysts due to:
•their ability to change oxidation state
•In the metal’s ability to adsorb other substances on to their surface and
activate them in the process.
Iron in the Haber Process
The Haber Process combines
hydrogen and nitrogen to make
ammonia using an iron catalyst.
Nitrogen and hydrogen molecules are adsorbed on
to the metallic iron surface. The hydrogen almost
immediately splits into its component atoms by
sharing or exchanging electrons with the catalyst
surface
Catalyst examples
V2O5 in the Contact Process
This is the conversion of sulfur
dioxide to sulfur trioxide by passing
the gaseous reactants over a solid
vanadium (V) oxide
MnO2 in the decomposition of
hydrogen peroxide
This speeds up the spontaneous
decomposition of hydrogen peroxide
by manganese (IV) oxide
Nickel in the hydrogenation of
C=C bonds
This reaction see the conversion of
alkenes to alkanes
Enzymatic catalysis
Fe in haemoglobin for carrying oxygen
Co in vitamin B12 to help produce red
blood cells
See (Green, p92 for structures)
Catalytic converters
Pt and Pd are used to convert NOx
and CO to harmless gases
Economic significance of Contact Process
The Contact Process is used in the manufacture of sulfuric acid. Sulfuric acid
is used in many industrial processes such as the manufacture of polymers,
fertilisers and detergents. It is also used in mining and petrochemicals
industries.
Many experts point to the
amount of sulfuric acid
production as a good indication of
the health of a country’s chemical
industry. The more healthy the
chemical industry, the more
healthy industry is in general.
Economic significance of Haber Process
The Haber Process is the production of ammonia from it’s gaseous elements.
Ammonia is important to an economy for many reasons. This demonstrates a
country’s ability to take readily available raw materials and turn them into useful
products.
Importantly, it is used in
fertilisers which is vital in
helping to feed the
populations.
It is also used in the
manufacture of explosives.
Developed during WWI, this
helped Germany prolong the
war.
It is also used in the production
of polymers such as nylon,
which is used to make a variety
of materials from clothing to
toothbrushes to parachutes.