Group 2

• The group 2 oxides, hydroxides, and carbonates have many uses related to their basic properties and ability to neutralise acids.
• Calcium hydroxide, Ca(OH)2, is added to fields as lime by farmers to increase the pH of acidic soils. It neutralises the acid in the soil forming neutral water:

  $Ca(OH)_2(s) +$$2H^+$$(aq) -> Ca^2+$$(aq) +$$2H_2O(l)$
• Group 2 bases are often used as antacids for treating acid indigestion.

  Many indigestion tablets use magnesium and calcium carbonates as the main ingredients.

  $CaCO_3(s) +$$2HCl(aq) -> CaCl_2(aq) +$$H_2O(l) +$$CO_2(g)$

  Milk of magnesia is a suspension of white magnesium hydroxide in water. Magnesium hydroxide is only very slightly soluble in water.

  $Mg(OH)_2(s) +$$2HCl(aq) -> MgCl_2(aq) +$$2H_2O(l)$
• The elements in group2 are metals, sometimes named the alkaline earth metals. The name comes from the alkaline properties of the metal hydroxides.
• The elements in group 2 are reactive metals and do not occur in the elemental form naturally. On Earth they are found in stable compounds like CaCO3
• Each group 2 element has two outer shell electrons, two more than the electron configuration of a noble gas. The two electrons are in the outer s sub-shell
• Redox reactions are the most common type of reaction of the Group 2 elements. Each metal atom is oxidised, losing two electrons to form a 2+ ion. Another species will gain these two electrons and be reduced. The group 2 element is called a reducing agent because it has reduced another species
• The group 2 elements react with water to form an alkaline hydroxide and hydrogen gas. Water and magnesium react very slowly, but the reaction becomes more vigorous with metals further down the group. Reactivity increases down the group
• All group 2 elements take part in redox reactions with dilute acids to from a salt and hydrogen gas
• When redox reactions are carried out with each group 2 element, the reactivity increases down group 2. This is because the ionisation energies decrease down the group due to the attraction between the nucleus and the outer electrons decreasing as a result of increasing atomic radius and increasing shielding.
• The group 2 elements become more reactive and stronger reducing agents down the group
• The oxides of Group 2 elements react with water, releasing hydroxide ions and forming alkaline solutions of the metal hydroxide:

  $CaO(s) +$$H_2O(l) -> Ca^2+$$(aq) +$$2OH^-(aq)$
• The group 2 hydroxides are only slightly soluble in water. when the solution becomes saturated, any further metal and hydroxide ions will form a solid precipitate:

  $Ca^2+$$(aq) +$$2OH^-(aq) -> Ca(OH)_2(s)$
• The solubility of the group 2 hydroxides in water increases down the group, so the resulting solutions contain more hydroxide ions and are more alkaline
• $Mg(OH)_2 (s)$is only very slightly soluble in water. The solution has a low OH- concentration and a pH ~ 10
• $Ba(OH)_2(s)$ is much more soluble in water. The solution has a greater OH- concentration and a pH ~ 13
• Testing alkalinity down group 2:

  1. add a spatula of each group 2 oxide to water in a test tube
  2. shake the mixture. On this scale, there is insufficient water to dissolve all of the metal hydroxide that forms. You will have a saturated solution of each metal hydroxide with some white solid undissolved at the bottom of the test tube
  3. measure the pH of each solution. The alkalinity will be seen to increase going down the group