Transcript Group 2

Properties of Group 2
The alkaline earth metals
Electronic configurations
• All the alkaline earth metals have two
electrons in their outer shell;
• Be = 1s2 2s2
• Mg = 1s2 2s2 2p6 3s2
• Ca = 1s2 2s2 2p6 3s2 3p6 4s2
• Ba = 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p65s2
When they react these electrons are lost, forming an M2+ ion.
Emeralds are a beryllium
silicate.
Celestine is strontium
sulphate.
Group 2 elements are all
reactive and are found as
minerals. Eg;
Barytes is barium sulphate.
Barium meals
• Soft tissue normally
lets through X rays.
• But barium is opaque,
its heavy atom absorbs
X rays.
• A “barium meal”,
barium sulphate, can be
administered to
examine the gut.
• Although barium salts
are highly toxic the
sulphate is safe to use
as it is insoluble.
Testing for the presence
of sulphate ions.
• Barium chloride is used as a reagent to
test for the presence of sulphate ions.
• First dilute hydrochloric acid is added to
remove any carbonate ions present.
• CO32- + 2H+ → CO2 + H2O
• Otherwise white barium carbonate might
be precipitated.
• BaCl2 + SO42- → BaSO4 (s) + 2Cl• Barium sulphate forms a white ppt.
Bones and calcium
The adult skeleton contains
around 1 kg of calcium, as
calcium phosphate Ca3(PO4)2.
This explains why it is important to have a good
source of calcium in the diet.
Be
Strontium (Sr)
Strontium is in the same group of
the periodic table as calcium.
Mg
So it can replace
Ca in bone
minerals.
Ca
Ca
Sr
Sr
Ba
Sr
Ca
Including radioactive Strontium-90
from nuclear tests!
Strontium has four stable isotopes
84Sr
0.56%
86Sr
9.86%
87Sr
7%
88Sr
82.58%
Living organisms have no preferences between
the Sr isotopes in their food and drink.
87Sr
forms from
decay of 87Rb, which
has a half life of 4.75
x1010 years.
The 86Sr : 87Sr ratio
depends on the
underlying rocks.
These data can be used to
track down people’s origins
from their bones.
It is greatest, c0.720,
in ancient rocks of
highland Britain >100
million years old, eg;
granite and gneiss.
In the younger
rocks of lowland
Britain it is
c.704.
Case Study; Who were the “Beaker People who had
helped build the megalithic stone circles…?
And introduced metal into
Britain?
…buried their dead under round
barrows with their typical pots?
In 2003 Wessex
Archaeology excavated a
mass grave which
contained the remains of
two adult men, an
adolescent male and a
child at Boscombe Down,
near Stonehenge.
But are typical
of Cornwall,
Man, Lake
District and
Highland
Scotland.
The Boscombe burials had
very high ratios of 87Sr:86Sr
that are not matched in
Wessex.
Carnivores
←
Omnivorves
have intermediate ratii.
←
Animals preferentially
take up Ca rather than Sr
to form bone.
Strontium and the
diet.
ingest less Sr and have even
lower ratii.
↑
Herbivores
have a Sr:Ca ratio c 5x more
than the plants.
↑
Producers
Analysis of Gladiator bones show high Sr,
indicating vegetarian diets!
Radium
• Radium was discovered
by Madame Curie, who
noticed that the
uranium ore
pitchblende was 10x
more radioactive than
expected.
• From 10 tonnes of ore
she extracted 1g of
radium chloride, for
which she won the
Nobel Prize in 1903.
Physical properties
• Group 2 elements are
metals.
• They have relatively
high melting points
and boiling points.
• They are all good
conductors of heat
and electricity.
• They are grey, silvery
solids at 298K.
Reactivity
Group 2 are less
reactive than group 1.
Na vigorously reacts with
water.
The difference is due
to the amount of
energy required to
form the metal ion.
But Ca reacts very
slowly.
Energy
needed
Energy
needed
Li
519 kj/mol
Be
Na
494
Mg
2660
kj/mol
2189
K
418
Ca
1735
Rb
402
Sr
1614
Cs
376
Ba
1468
Energy
needed
Be
2660
kj/mol
Mg
2189
Ca
1735
Sr
1614
Ba
1468
• The combined
ionisation energies
decrease down the
group.
• This means that
reactivity increases
as it becomes
progressively easier
to form the M2+ ion.
Ionisation energies of Group II
5000
ionisation energy (Kj/mol)
4500
4000
3500
Combined ionisation
energies
2nd ionisation energy
3000
2500
2000
1st ionisation energy
1500
1000
500
0
Mg
Ca
Sr
element
Ba
Group 2
compounds
Group 2 compounds are
generally white ionic solids.
Which dissolve to
give colourless
solutions.
Flame tests
• If a wire loop is
dipped into a
solution or into the
solid compound a
flame will be
coloured.
• Calcium with a brick
red flame.
• Strontium with a red
flame.
• Barium with a green
flame
Oxidation numbers.
• All uncombined metals have an
oxidation number of O.
• In their compounds group 2 elements
have an oxidation number of +2.
• Forming an ion M2+
•M →
2+
M
+ 2e-
• As electrons are lost this is oxidation.
Reaction with air
• Group 2 elements
rapidly react with
air, forming an oxide
film that prevents
further reaction.
• They will burn in air,
giving flames of
characteristic
colours.
• Eg; Magnesium has
a white flame.
Oxidation of group 2 metals
Eg; Mg + ½ O2 → MgO
Oxidation No of
Magnesium
metal = 0.
Oxidation No of
magnesium in its
oxide = +2
As its oxidation number becomes more positive
magnesium has been oxidised.
Mg → Mg2+ + 2eOxygen is reduced;
½O2 + 2e- → O2-
Group 2 oxides
• Oxides are white,
ionic solids with
high MPs.
• They are prepared
by the
decomposition of
the carbonates or
nitrates.
• Due to its very high
MP magnesium
oxide is used as a
refractory lining in
furnaces.
General properties of oxides
• Generally metal oxides are basic
• They dissolve in water to form hydroxides;
• MO (s) + H2O → M(OH)2 (aq)
• Non-metal oxides are acidic.
• They dissolve to form acids;
• XO2
(s) + H2O → H2XO3 (aq)
• A few, such as aluminium, are amphoteric.
• These demonstrate both acidic and basic
properties.
Eg;Magnesium oxide
dissolves slowly in cold
water …
…giving an alkaline
solution.
MgO (s) + H2O → Mg(OH)2 (aq)
This is not a redox reaction;
MgO (s) + H2O → Mg(OH)2 (aq)
Mg oxidation
No = +2
Mg oxidation
No = +2
Hydrogen
oxidation No = +1
Oxygen
oxidation No =
-2
Hydrogen
oxidation No = +1
Oxygen
oxidation No =
-2
All oxidation numbers have remained unchanged.
Reactions of magnesium hydroxide
Mg(OH)2 neutralises
acids.
Mg(OH)2 + 2HCl→ MgCl2 + 2H2O
Treating indigestion.
HCl is
secreted in the
stomach to aid
digestion.
Excess acid
causes
indigestion.
“Milk of
Magnesia”
contains
magnesium
hydroxide so
neutralises
excess acid.
Some basic reactions;
• Acids react with many metals to give
hydrogen and a salt
• Eg; HCl
+ Na→ 1/2H2(g) + NaCl
• Acids neutralise alkalis to produce a salt and
water
• Eg; HCl
+ NaOH→ HOH + NaCl(aq)
• Acids react with carbonates to produce a
salt, carbon dioxide and water.
• 2HCl + CaCO3 → CaCl2 +CO2(g) + H2O
Reaction of magnesium with HCl.
Magnesium
reacts with
hydrochloric
acid to give
effervescence
of hydrogen.
Mg + 2HCl →MgCl2 + H2
This is a redox reaction;
Mg + 2HCl →MgCl2 + H2
Mg oxidation
No = 0
Mg oxidation
No = +2
Hydrogen
oxidation No = +1
Magnesium is oxidised;
Mg → Mg2+ + 2e-
Hydrogen
oxidation No =0
Hydrogen is reduced;
2H+ + 2e- → H2
Reactions of magnesium with water.
• Magnesium reacts slowly with cold
water;
• Mg + 2H2O → Mg(OH)2 + H2
• (The other members of its group react readily
with cold water to give the corresponding
sparingly soluble hydroxides.)
• Magnesium reacts more readily with
steam to form its oxide;
• Mg + H2O(g) → MgO + H2
Reactions of Ca, Sr and Ba with
water
• Calcium reacts rapidly with cold water
to give hydrogen and a white
percipitate of calcium hydroxide.
• Ca + 2H2O → Ca(OH)2 + H2
• Strontium and barium react in the same
way, but more vigorously.
Reactions of magnesium carbonate
Magnesium carbonate reacts with acids to give an
effervescence of carbon dioxide.
MgCO3 + 2HCl →MgCl2 + 2H2O + CO2