Reactivity trends: Group 2

• Characteristic physical properties

  Elements in Group 2 are metals - alkaline earth metals. The name comes from the alkaline properties of the metal hydroxides. The elements are reactive metals and do not occur in their elemental form naturally. On Earth they are found in stable compounds, e.g. CaCO3
• REDOX reactions and Reactivity?

  REDOX reactions are most common for group 2 - each metal atom is oxidised, losing 2 electrons to form 2+ ion with electron configuration of a noble gas.

  Another species will gain these two electrons and be reduced

  Group 2 element= reducing agent because it has reduced another species

  Each group 2 element has 2 outer shell electrons, 2 more than electron configuration of a noble gas. These 2 electrons are in the outer s sub-shell.
• REDOX reactions with oxygen?

  Group 2 elements all react with oxygen to produce a metal oxide, with general formula MO, made up of m2+ and O2- ions
• REDOX reactions with water?

  React with water to form alkaline hydroxide with the general formula, M(OH)2, and hydrogen gas

  Water and magnesium react very slowly, reactions become more and more vigorous down group - reactivity increases down group
• In the reaction of Sr and water why aren't all the hydrogen atoms reduced?

  There is 1 Sr atom in the reaction and its oxidation state increases by +2 giving an overall increase of +2

  Whereas there are 4 H in 2H2O - For 2H their oxidation states decrease by 1 each forming H2 (g) giving a total decrease of -2

  The other 2H do not change and form Sr(OH)2
• REDOX with dilute acids?

  Many metals take part in redox reactions with dilute acids to form a salt and hydrogen gas:

  metal + acid= salt + hydrogen

  All group 2 metals react this way - reactivity increases down group
• Trend in Reactivity and ionisation energy?

  When the REDOX reactions are carried out on group 2 elements, reactivity increases down group 2

  Atoms of group 2 elements react by losing electrons to form +2 ions. The formation of +2 ions from gaseous atoms requires the input of 2 ionisation energies (1st and 2nd). Ionisation energies decrease down group 2
• Why do ionisation energies decrease down group 2?

  Because the attraction between the nucleus and outer electrons decreases as a result of increasing atomic radius and increasing shielding
• Although other energy changed take place when group 2 elements react, the first and second ionisation energies make up most of the energy input. The total energy input from ionisation energies to form 2+ ions decreases down the group
• Group 2 elements become more reactive and stronger reducing agents down the group
• Reactions with water: the oxides of group 2 elements react with water, releasing hydroxide ions, OH- and forming alkaline solutions of the metal hydroxide.

  CaO (s) +H2O (l) = Ca2+ (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:

  Ca2+ (aq) + 2OH- (aq) = Ca(OH)2 (s)
• The solubility of the hydroxides in water increases down the group, so the resulting solutions contain more OH- ions and are more alkaline
• Mg(OH)2 is only very slightly soluble in water - the solution has a low concentration of OH- (aq) and a pH of 10
• Ba(OH)2 (s) is much more soluble in water. The solution has a greater OH- (aq) concentration and a pH of 13
• to show the trend:

  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 each test tube
  3. measure the pH of each solution - alkalinity will be seen to decrease down the group
• The group 2 oxides, hydroxides and carbonates have many uses related to their basic properties and ability to neutralise acids
• Group 2 compounds in agriculture: Ca(OH)2 is added to fields as lime to increase the pH of acidic soils - neutralises acid in the soil, forming neutral water

  Ca(OH)2 (s) + 2H+ (aq) = Ca2+ (aq) + 2H20 (l)
• Group 2 compounds in medicine: group 2 bases are often used as antacids for treating acid indigestion. Many indigestion tablets use magnesium and calcium carbonates as the main ingredients, whilst 'milk of magnesia' is a suspension of white magnesium hydroxide in water - only very slightly soluble in water
• The acid in the stomach is mainly HCl:

  Mg(OH)2 (s) + 2HCl (aq) = MgCl2 (aq) + 2H20 (l)

  CaCo3 (s) + 2HCl (aq) = CaCl2 (aq) + H20 (l) + CO2 (g)