Group 2- Alkaline Earth Metals
- All of these elements are reactive metals. Magnesium and calcium are abundant in the earth's crust. The others are relatively rare.
Physical Properties
- Melting points and electrical conductivity:
- The melting points of the elements decrease down group II.
- Down the group, the size of the cations increases and the charge on the cations remains constant. The charge density thus decreases and the attraction between the cations and the delocalized electrons also decreases. The melting points and hardness therefore decrease.
- All group II elements will conduct electricity because the delocalized electrons are free to move throughout the solid.
- First & second ionization energies:
- The first & second ionization energies decrease down group II.
- Down the group, the number of shells increases. Therefore the outer shells are more shielded from the nucleus, the effective nuclear charge decreases , are less closely held and are easier to remove.
- Successive ionization energies:
- The second ionization energy of group 2 elements is higher than the first as the second electron is removed from an already positive ion. There is greater attraction to as the effective nuclear charge attracting the outer electron has increased.
- The third ionisation energy of group 2 elements is much higher(there is a jump) than the second. The third electron must be removed from a doubly positively charged ion and from a stable full energy level closer to the nucleus.
Reactions of group 2 elements
- Reactions with oxygen
Group 2 elements form the solid ionic oxide of formula MO when heated in oxygen.
- For example magnesium forms magnesium oxide:
2Mg(s) + O2(g) → 2MgO(s)
- Reactions with chlorine
All the group 2 metals react when heated in chlorine to form solid ionic chlorides.
- Magnesium for example:
Mg(s) + Cl2(g) → MgCl2(s)
- Reactions with water
- Beryllium does not react with water or steam.
- Magnesium burns when heated in steam to form magnesium oxide and hydrogen.
Mg (s) + H2O(g) → MgO (s) + H2(g)
- Calcium reacts steadily with cold water. The solution goes cloudy as the hydroxide is not very soluble:
Ca(s) + 2H2O(l) → Ca(OH)2(s) + H2(g)
- Strontium and barium react vigorously with cold water, barium even more vigorously than strontium to form clear solution of hydroxides & hydrogen.
Sr(s) + 2H2O(l) → Sr(OH)2(aq) + H2(g)
Ba(s) + 2H2O(l) → Ba(OH)2(aq) + H2(g)
- Since the ionisation energies decrease down a group, the electrons are more easily removed from the atoms and the reaction with water becomes increasingly vigorous down the group.
Reactions of the oxides of group 2 elements
- Reactions with water
- Group 2 oxides are basic in nature ,when dissolves in water, forms hydroxides which are alkaline in nature
- BeO is not attacked by water as it is covalent rather than ionic in nature
- The oxides of the other group 2 elements are ionic and react with water to form hydroxides.
MgO(s) + H2O(l) → Mg(OH)2(s)
CaO (s) + H2O (l) → Ca(OH)2 (s), this reaction is also called slaking of lime
- Reactions with dilute acid
- The oxides of group 2 are bases and so react with acids to form salt and water.
MgO(s) + H2SO4 (aq) → MgSO4(aq) + H2O (l)
CaO(s) + 2HNO3(aq) → Ca(NO3)2(aq) + H2O(l)
BaO (s) + 2HCl(aq) → BaCl2(aq) + H2O (l)
In AS/A-level Chemistry exam, you will be asked to write the equations above, remember to put the states of the reactants and products! 👨🏫
Reactions of the hydroxides of group 2 elements
- Hydroxides of group 2 are also bases and so react with acids to form salts and water:
Ca(OH)2(aq) + 2HNO3(aq) → Ca(NO3)2(aq) + 2H2O(l)
Solubility of group 2 sulphates
- The solubility of the sulphates decreases down group II.
MgSO4 is soluble, CaSO4 is sparingly soluble and SrSO4 and BaSO4 are insoluble.
- If sulphuric acid or sodium sulphate is added to aqueous solutions of calcium, strontium or barium ions, a white precipitate will be formed:
Ba2+(aq) + SO42-(aq) → BaSO4(s) thick white precipitate formed
Sr2+(aq) + SO42-(aq) → SrSO4(s) thick white precipitate formed
Ca2+(aq) + SO42-(aq) → CaSO4(s) faint white precipitate formed
- If dilute sulphuric acid or sodium sulphate is added to a solution of Mg2+ or Be2+, there is no reaction and no precipitate is formed.
- The thick white precipitate formed when barium ions are added to sulphate ions can be used as a test for sulphate ions:
- add 1 cm3 of aqueous barium chloride, and also1 cm3 of dilute hydrochloric acid, to 1 cm3 of the unknown solution. If a thick white precipitate is formed, and that precipitate does not dissolve in dilute HCl, then sulphate or hydrogensulphate ions are present.
- The hydrochloric acid removes any other ions such as carbonate or hydroxide ions which may affect the test.
Solubility of group 2 hydroxides
- The solubility of the group II hydroxides increases down the group.
Mg(OH)2 is insoluble, Ca(OH)2 is sparingly soluble and Sr(OH)2 and Ba(OH)2 are soluble.
- If dilute sodium hydroxide is added to a solution of Mg2+ ions, a white precipitate will be formed immediately:
Mg2+(aq) + 2OH-(aq) → Mg(OH)2(s)
- If dilute sodium hydroxide is added to a solution of Ca2+ ions, a faint white precipitate will be formed (excess NaOH must be added).
Ca2+(aq) + 2OH-(aq) → Ca(OH)2(s)
- If dilute sodium hydroxide is added to a solution of Sr2+or Ba2+, there is no reaction and no precipitate is formed.
Uses of sulphates and hydroxides
- Magnesium hydroxide is almost insoluble and is sold as a suspension in water. In this form it is known as “milk of magnesia”. It is taken to alleviate constipation.
- Calcium hydroxide in solid form is known as “slaked lime” and is used to neutralize acidic soil.
- Barium sulphate can be eaten as part of a “barium meal”. Barium is good at absorbing X-rays and so when the barium sulphate gets to the gut the outline of the gut can be located using X-rays. Although barium ions are very toxic, this technique is harmless because barium sulphate is completely insoluble.
Reference:
https://getrevising.co.uk/resources/group_2_notes
This is the end of the topic!
Drafted by Cherry (Chemistry)