Ionisation energy values for Group 2 elements

1st IE (kJ mol-1) 2nd IE (kJ mol-1)

Be 900 1800Mg 736 1450Ca 590 1150Sr 548 1060Ba 502 970

Starter questions

1) Explain the trend in reactivity down group 2 using the 1st and 2nd ionisation energy values below:

2) Using the above trend, predict how barium would react with: (a) oxygen; and (b) water. Write balanced equations for each.

3) Use oxidation numbers to show that magnesium is oxidised when it reacts with oxygen and that oxygen is reduced.

4) Give a use for this reaction

5) Write half equations for the reaction between magnesium and water.

1st IE 2nd IE (kJ mol-1) (kJ mol-1)

Be 900 1800Mg 736 1450Ca 590 1150Sr 548 1060Ba 502 970

Answers1) 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.

2) 2Ba + O2 2BaOviolent reaction

Ba + 2H2O Ba(OH)2 + H2violent 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