Trends in group 2 part 3 chemical properties

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Reaction with oxygen


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:


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.

heatlight

energ

y


Flame test colours


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 -2oxidation

states


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 -1oxidation

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.


Reaction with water


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 0oxidation

states


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


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 reactreacts slowly to form alkaline suspension

reacts to form alkaline suspension

react to form alkaline solutions

Oxide Reaction


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)


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


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.


Equations for reactions


Stability of group 2 carbonates

Technology

Trends in group 2 part 3 chemical properties