Ionisation energy values for Group 2 elements

1. Ionisation energy values for Group 2 elements 1st IE (kJmol-1) 2nd IE (kJmol-1) Be 900 1800 Mg 736 1450 Ca 590 1150 Sr 548 1060 Ba 502 970

2. Starter questions • Explain the trend in reactivity down group 2 using the 1st and 2nd ionisation energy values below: • Using the above trend, predict how barium would react with: (a) oxygen; and (b) water. Write balanced equations for each. • Use oxidation numbers to show that magnesium is oxidised when it reacts with oxygen and that oxygen is reduced. • Give a use for this reaction • Write half equations for the reaction between magnesium and water. 1st IE 2nd IE (kJmol-1) (kJmol-1) Be 900 1800 Mg 736 1450 Ca 590 1150 Sr 548 1060 Ba 502 970

3. Answers • Reactivity increases down the group because less energy is required to remove the two outer electrons. This is because as the number of shells increase, the distance between the nucleus and the outer electrons increases and there is a weaker force of attraction on outer electrons. Also, there are more inner shells which have a greater shielding effect. • 2Ba + O2 2BaO violent reaction Ba + 2H2O  Ba(OH)2 + H2 violent reaction producing colourless alkaline solution 2Mg + O2 2MgO 0 0 +2 -2 3) Magnesium 0 to +2 so has been oxidised. Oxygen 0 to -2 so has been reduced. 4) Magnesium is used in flares. Calcium is used in making high-grade steel and in the production of uranium. 5) Mg  Mg2+ + 2e– 2H2O + 2e–  2OH– + H2