Transcript Ionisation energy values for Group 2 elements

Ionisation energy values for
Group 2 elements
1st IE (kJ mol-1)
Be
Mg
Ca
Sr
Ba
2nd IE (kJ mol-1)
900
736
590
548
502
1800
1450
1150
1060
970
Starter questions
1)
Explain the trend in reactivity down group 2 using the 1st and 2nd
ionisation energy values below:
Be
Mg
Ca
Sr
Ba
1st IE
(kJ mol-1)
900
736
590
548
502
2nd IE
(kJ mol-1)
1800
1450
1150
1060
970
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.
Answers
1) 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 
2BaO
violent reaction
Ba + 2H2O  Ba(OH)2 + H2
violent reaction producing colourless alkaline solution
2Mg
0
+
O2
0
2MgO
+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

2H2O + 2e–
Mg2+ + 2e–

2OH– + H2