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Revision:Group 2

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TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Chemistry > Group 2

Group 2 is the second column of the periodic table, made up of beryllium, magnesium, calcium, strontium, and barium (And radium, but that's irrelevant)

Trends

Down group 2, the ionisation energy of the elements decrease, as the outer electrons are further away from the nucleus and are shielded more, which outweigh the effects of the greater nuclear charge.

The sulphates of group 2 elements decrease in solubility down the group. The hydroxides increase in solubility down the group.

Group 2 carbonates decompose upon heating, caused by the cation which polarises and weakens one of the C-O bonds in the carbonate. Upon heating, this bond breaks, and carbon dioxide is released:

\mathrm{MgCO_3 \longrightarrow MgO + CO_2}.

The polarising effect depends on on the charge density of the cation - the larger the cation, the lower this is, and the harder it becomes to decompose the carbonate. The group 1 carbonates, because they are larger and have a lower charge, do not decompose with heat, with the exception of lithium, which is similar to the magnesium ion:

\mathrm{Li_2CO_3 \longrightarrow Li_2O + CO_2}

Group 2 nitrates are also decomposed by heat:

\mathrm{2Mg(NO_3)_2 \longrightarrow 2MgO + 4NO_2 + O_2}

for similar reasons. They differ from group 1 nitrates, which also decompose, but do not release nitrogen dioxide:

\mathrm{2NaNO_3 \longrightarrow 2NaNO_2 + O_2}.

Lithium again behaves like magnesium:

\mathrm{4LiNO_3 \longrightarrow 2Li_2O + 4NO_2 + O_2}

Reactions

  • All group 2 elements react as expected with oxygen: \mathrm{2Mg + O_2 \longrightarrow 2MgO}
  • Group 2 elements react with chlorine (Just guessing here, not 100% sure on this) as expected: \mathrm{Mg + Cl_2 \longrightarrow MgCl_2}
  • With water, reactivity increases down the group. Beryllium does not react with water or steam. The rest of group 2 react as expected with water, forming hydroxides: \mathrm{Ca + 2H_2O_{(l)} \longrightarrow Ca(OH)_2 + H_2} (see solubility trends above for how soluble product is). Magnesium reacts with steam to form an oxide: \mathrm{Mg + H_2O_{(g)} \longrightarrow MgO + H_2}
  • Group 2 oxides react with water also to form hydroxides: \mathrm{MgO + H_2O \longrightarrow Mg(OH)_2}
  • Group 2 oxides behave as bases and react with dilute acids: \mathrm{MgO + 2HCl \longrightarrow MgCl_2 + H_2O}.
  • Group 2 hydroxides react with acids in neutralisation reactions: \mathrm{Mg(OH)_2 + 2HCl \longrightarrow MgCl_2 + 2H_2O}

Flame Colours

Both group 1 and group 2 metals and compounds burn with coloured flames. When given energy, electrons can move up to an orbital of higher energy. When they return to their original place, the excess energy is given out as a photon. The wavelength of this photon determines the colour of the flame:

  • Lithium - Red
  • Sodium - Orange
  • Potassium - Lilac
  • Calcium - Orange-Red
  • Strontium - Red (Crimson)
  • Barium - Green

Magnesium does not produce a flame colour in the visible range.

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