2.7The periodic table

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2.7 The periodic table Group 2

• Revision Notes

Crowe2012
2
Properties down group 2
a. explain the trend in the first ionization
energy down group 2
Both the table and graph above also shows a
specific trend in ionisation energies down the
group. What is the trend and can you explain
it?
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Properties down group 2
a. explain the trend in the first ionization
energy down group 2
Down the group Ionisation energies decrease
down the group There is an increasing effective
nuclear charge down the group. However, the
atomic radius increases too, so the outermost
electrons experience this less strongly. Also,
there is an increased shielding effect down the
group too.
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Properties down group 2
a. explain the trend in the first ionization
energy down group 2
Successive ionisation energies two electrons in
the outer shell. Losing two electrons leave them
with noble gas configurations. So readily form
2 ions. However, removing the third would mean
breaking into a new shell, and so requires great
energy.
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Properties down group 2
b. recall the reaction of the elements in group 2
with oxygen, chlorine and water
Reactivity increases down the group. So Barium
is the most reactive, and is often stored under
oil like the alkali metals, as a result!
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Properties down group 2
b. recall the reaction of the elements in group 2
with oxygen, chlorine and water
Reactions with oxygen
Metals burn in oxygen to form a simple metal
oxide.
CaO is called quick lime and is used in farming
to counteract soil acidity! It is made
commercially by the thermal decomposition of
limestone!
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Properties down group 2
b. recall the reaction of the elements in group 2
with oxygen, chlorine and water
Reaction with Chlorine
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Properties down group 2
b. recall the reaction of the elements in group 2
with oxygen, chlorine and water
Reactions with Water
Beryllium has no reaction with water or steam
even at red heat This is due to a thick
oxide layer on the surface
Magnesium has a very slight reaction with cold
water. The reaction soon stops because
magnesium hydroxide is almost insoluble in water
and forms a barrier preventing further
reaction. Magnesium will react with STEAM to
form the metal oxide and hydrogen gas.
Calcium, strontium and barium all react with cold
water with increasing vigour to give the metal
hydroxide and hydrogen.
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Properties down group 2
c. recall the reactions of the oxides of group 2
elements with water and dilute acid, and their
hydroxides with dilute acid
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Properties down group 2
c. recall the reactions of the oxides of group 2
elements with water and dilute acid, and their
hydroxides with dilute acid
Reaction of Oxides with Hydrochloric/Nitric Acid
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Properties down group 2
c. recall the reactions of the oxides of group 2
elements with water and dilute acid, and their
hydroxides with dilute acid
Reaction of Hydroxides with Hydrochloric/Nitric
Acid
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Properties down group 2
d. recall the trends in solubility of the
hydroxides and sulfates of group 2 elements
SULPHATE solubility DECREASES down the group
HYDROXIDE solubility INCREASES down the group
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Properties down group 2
d. recall the trends in solubility of the
hydroxides and sulfates of group 2 elements
Solubility is determined by two factors Lattice
dissociation enthalpy (energy needed to break up
a crystal lattice) Hydration enthalpy (energy
released when ions are hydrated) Ionic size has
an effect on these factors.

• Solubility of Hydroxides
• There is a decrease in lattice dissociation
  enthalpy down the group.
• This outweighs the change in enthalpy of
  hydration.
• As a result, there is an INCREASE in SOLUBILITY
  down the group.

insoluble
slightly soluble
soluble
soluble
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Properties down group 2
d. recall the trends in solubility of the
hydroxides and sulfates of group 2 elements
Solubility is determined by two factors Lattice
dissociation enthalpy (energy needed to break up
a crystal lattice) Hydration enthalpy (energy
released when ions are hydrated) Ionic size has
an effect on these factors.
Solubility of Sulphates Magnesium and calcium
sulphates are soluble. Anion gtgt Cation so
lattice enthalpy does not vary much down group as
cation size changes. However, hydration enthalpy
decreases down the group. Therefore, solubility
decreases down the group.
soluble
soluble
insoluble
insoluble
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Properties down group 2
e. recall the trends in thermal stability of the
nitrates and the carbonates of the elements in
groups 1 and 2 and explain these in terms of size
and charge of the cations involved
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Properties down group 2
e. recall the trends in thermal stability of the
nitrates and the carbonates of the elements in
groups 1 and 2 and explain these in terms of size
and charge of the cations involved
Thermal Stability of Carbonates
Carbonates of Group 1 are thermally
stable Exception is lithium carbonate which
decomposes to give the oxide All group 2
carbonates decompose to form stable oxides. E.g.
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Properties down group 2
e. recall the trends in thermal stability of the
nitrates and the carbonates of the elements in
groups 1 and 2 and explain these in terms of size
and charge of the cations involved
Explanation of stability of carbonates
Polarising the carbonate ion

• The positive ion attracts the delocalised
  electrons in the carbonate ion towards itself.
• The carbonate ion becomes polarised.
• If this is heated, the carbon dioxide breaks free
  to leave the metal oxide.

• The smaller the positive ion is, the higher the
  charge density, and the greater effect it will
  have on the carbonate ion.
• As the positive ions get bigger as you go down
  the Group, they have less effect on the carbonate
  ions near them.
• To compensate for that, you have to heat the
  compound more in order to persuade the carbon
  dioxide to break free and leave the metal oxide.
• In other words, as you go down the Group, the
  carbonates become more thermally stable.

(A similar explanation can be used for the
stability of nitrates.)
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Properties down group 2
f. recall the characteristic flame colours formed
by group 1 and 2 compounds and explain their
origin in terms of electron transitions

• Electrons occupy certain discrete energy levels.
• When an electron is promoted from its usual
  energy level to a higher one, the atom becomes
  excited.
• When the electron drops back down to its ground
  state (usual energy level), the atoms emits a
  photon of light in the visible range
• Meaning a certain amount of energy that has a
  wavelength corresponding to a certain colour.

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Properties down group 2
h. demonstrate an understanding of how to
minimise the sources of measurement uncertainty
in volumetric analysis and estimate the overall
uncertainty in the calculated result.
Accuracy Unless an activity instructs students
differently, they should assume that readings
from equipment and apparatus should be made with
the following precision.
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Properties down group 2
h. demonstrate an understanding of how to
minimise the sources of measurement uncertainty
in volumetric analysis and estimate the overall
uncertainty in the calculated result.
Errors Students should appreciate that any piece
of equipment (burette, pipette, thermometer,
balance) used in a quantitative exercise has an
uncertainty associated with its use. Even if the
equipment is used carefully, the uncertainty
leads to an error in the reading and in the final
result.