Learning objective: WALT: recall the reaction of the elements in group 2 with oxygen, chlorine and water recall the reactions of the oxides of group 2 elements with water and dilute acid, and their hydroxides with dilute acid WILF: Equations for the reactions and be able to use trends to predict reactions 29/07/2018

Flame test slap Ba2+ Li+ K+ Sr2+ Ca2+ Na+ Ba2+ Li+ K+ Sr2+ Ca2+ Na+

Solubility trends A useful fact… Barium chloride and hydrochloric acid are the test for sulfate ions (SO42-) If sulfate is present a thick white precipitate of barium sulfate (BaSO4) forms The acid is present to remove any carbonates – these would give a false positive The ionic equation is: Ba2+ (aq) + SO42- (aq)  BaSO4 (s) You now know that barium sulfate is insoluble!

Solubility of group 2 compounds Hydroxides increasingly soluble down group M2+ (aq) + 2 OH- (aq)  M(OH)2 (s) Sulphates decreasingly soluble down group M2+ (aq) + SO42- (aq)  MSO4 (s) REMEMBER The test for SO42- is add Ba2+ to make… thick white precipitate i.e. BaSO4 is completely INSOLUBLE

Reactions with oxygen They burn! Watch the magnesium… All of the group 2 metals react in a similar way, though barium also forms substantial amounts of barium peroxide BaO2. Magnesium burns vigorously with a brilliant white flame - the one element in the s-block which does not show its flame test colour (none) when burning. 2 Mg (s) + O2(g)   2 MgO(s) Calcium burns with a brick-red or orange-red flame, strontium with a crimson flame, barium with a pale apple green flame.

Reactions with chlorine Watch the video clip All of the metals react similarly to give white, ionic chlorides. Mg(s) + Cl2(g)  MgCl2(s) Except beryllium which forms a covalent oxide Why is BeCl2 covalent? What shape is it? Why can it polymerise?

Reaction with water In reverse order: Barium: quite vigorous effervescence; colourless alkaline solution; goes cloudy/milky on standing Ba (s) + 2 H2O (l)  Ba(OH)2 (aq) + H2 (g) Ba(OH)2 (aq)  Ba2+ (aq) + 2 OH- (aq) Ba2+ (aq) + CO32- (aq, from dissolved CO2)  BaCO3 (s) (c.f. limewater) Calcium: Effervescence; milky suspension; mildly alkaline Ca (s) + 2 H2O (l)  Ca(OH)2 (aq) + H2 (g) Ca(OH)2 (aq) ⇋ Ca2+ (aq) + 2 OH- (aq)

Reaction with water Magnesium: Cold water – very slow reaction Mg (s) + 2 H2O (l)  Mg(OH)2 (aq) + H2 (g) Mg(OH)2 (aq) ⇋ Mg2+ (aq) + 2 OH- (aq) Mg less reactive Mg(OH)2 only very sparingly soluble Steam – vigorous reaction Mg(s) + H2O(g)   MgO(s) + H2(g)

A practical In separate beakers add a few drops of universal indicator to about 50 ml HCl and H2SO4 You are going to investigate what happens when you react these acids with the oxides (MO) and hydroxides (M(OH)2) of group 2           MgO(s) + 2 H+(aq)    Mg2+(aq) + H2O(l) All the group 2 oxides react similarly except with sulphuric acid. In this case magnesium oxide reacts as given, but the sulphates of Ca – Ba are insoluble. The oxide therefore reacts superficially but reaction then ceases.

Bases and alkalis All alkalis are bases… …but not all bases are alkalis An alkali is a soluble base Magnesium hydroxide is definitely a base: Mg(OH)2 (s) + 2 HCl (aq)  MgCl2 (aq) + H2O (l) But it is weak alkali Mg(OH)2 (s) + (aq) ⇋ Mg2+ (aq) + 2 OH- (aq)