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Reaction with oxygen

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Flame tests
When group 2 metals are burned in oxygen, coloured
flames are produced. This is due to the presence of metal
ions. Flame tests exploit this fact.
magnesium – bright white
calcium – brick red/orange
strontium – red/crimson
barium – pale green/yellow-green
The presence of certain metal ions can be identified by
noting the characteristic flame colour that results from
burning. The colours for group 2 metal ions are:

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Explaining flame tests
When heated, some electrons in an atom or ion are excited
to higher energy levels. When they fall back to their initial
levels, energy is emitted; sometimes seen as visible light.
Electrons may be excited by
different amounts into
different energy levels and
drop back at different times.
The colour of the flame is a
combination of all these
energy emissions.
heat
light
energy

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Flame test colours

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Redox reaction with oxygen
When group 2 metals react with oxygen, they form the
metal oxide. For example:
2Mg(s) + O2(g) → 2MgO(s)
The oxidation state of magnesium has increased from 0 in
its elemental form to +2 when it is in magnesium oxide.
This means the magnesium has been oxidized.
The oxidation state of oxygen has decreased from 0 in its
elemental form to -2 when it is in magnesium oxide. This
means the oxygen has been reduced.
0 +20 -2
oxidation
states

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Redox reaction with chlorine
When group 2 metals react with chlorine, they form the
metal chloride. For example:
Ca(s) + Cl2(g) → CaCl2(s)
0 0 +2 -1
oxidation
states
The oxidation state of calcium has increased from 0 in its
elemental form to +2 when it is in calcium chloride. This
means the calcium has been oxidized.
The oxidation state of chlorine has decreased from 0 in its
elemental form to -1 when it is in calcium chloride. This
means the chlorine has been reduced.

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Reaction with water

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Redox reaction with water
When group 2 metals react with water they form the metal
hydroxide and hydrogen gas. For example:
Sr(s) + 2H2O(l) → Sr(OH)2(aq) + H2(g)
The oxidation state of strontium has increased from 0 in
its elemental form to +2 when it is in strontium hydroxide.
This means the strontium has been oxidized.
The oxidation state of hydrogen has decreased from +1 in
water to 0 when it is in its elemental form. The means the
hydrogen has been reduced.
0 +2+1 0
oxidation
states

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Explaining the trend in reactivity
The reactivity of the elements
down group 2 from beryllium to
barium increases.
Although increased shielding
cancels the increased nuclear
charge down the group, the
increase in atomic radius results
in a decrease in the attractive
force between the outer
electrons and the nucleus.
This is because it is
successively easier to remove
electrons to form the 2+ ion.
Mg
Ca
Sr
Ba

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Reaction of oxides with water
When group 2 metal oxides react with water they form the
metal hydroxide. For example:
SrO(s) + H2O(l) → Sr(OH)2(aq)
Similar to the reaction between the metal and water, the
resulting solution has high pH due to the hydroxide ions
from the metal hydroxide. Reactivity is as follows:
beryllium
magnesium
calcium
strontium, barium
does not react
reacts slowly to form
alkaline suspension
reacts to form alkaline suspension
react to form alkaline solutions
Oxide Reaction

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Decomposition of group 2 carbonates
When heated, the group 2 metal carbonates decompose
to form the metal oxide and carbon dioxide gas. Splitting
compounds using heat is called thermal decomposition.
magnesium carbonate: MgCO3
calcium carbonate: CaCO3
strontium carbonate: SrCO3
barium carbonate: BaCO3
increasing
stability
The group 2 carbonates become more stable to thermal
decomposition going down the group:
MCO3(s) → MO(s) + CO2(g)

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Decomposition of group 2 nitrates
Thermal decomposition of group 2 metal nitrates forms
the metal oxide, nitrogen dioxide and oxygen.
2M(NO3)2(s) → 2MO(s) + 4NO2(g) + O2(g)
Like the group 2 metal carbonates, the nitrates become
more stable to thermal decomposition down the group.
magnesium nitrate: Mg(NO3)2
calcium nitrate: Ca(NO3)2
strontium nitrate: Sr(NO3)2
barium nitrate: Ba(NO3)2
increasing
stability

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Explaining the trend in thermal stability
Metal ions become larger down group 2 but have the same
charge. This means their charge density is reduced.
A metal ion with a high
charge density has
strong polarizing power.
It can therefore polarize
the carbonate ion,
making it more likely to
split into O2-
and CO2
when heated. polarization
A metal ion with a low charge density has weak polarizing
power, meaning the carbonate ion is less polarized and
therefore more thermally stable.

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Equations for reactions

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Stability of group 2 carbonates