Answers to Group 2 Practice Questions

Q1:    

Sr(s) + 2H2O(l)   → Sr(OH)2(aq) + H2(g)                   redox

Ba(s) + ½O2(g)  → BaO(s)                                             redox

Mg(s) + 2HNO3(aq)  → Mg(NO3)2(aq) + H2(g)        redox

MgO(s) + 2HNO3(aq)  → Mg(NO3)2(aq) + H2O(l)   not redox

SrO(s) + H2O(l)  → Sr(OH)2(aq)                                   not redox

 

Q2:      The equation is Ca(s) + 2H2O(l)  → Ca(OH)2(aq) + H2(g).  Looking at the state symbols gives a good start towards what would be observed:

  • Fizzing – the reaction produced hydrogen gas
  • Heat produced – the reaction is very exothermic
  • Calcium dissolves – the reaction forms aqueous calcium ions
  • Indicator turns from green to blue/purple – an alkaline solution of calcium hydroxide is formed
  • White precipitate/turns milky – calcium hydroxide is sparingly soluble, so once the solution is saturated, further calcium hydroxide will be formed as solid particles.

 

Q3: (i) The reactions all produce metal hydroxides, but the amounts of metal used are different. Pieces of the same size do not contain the same number of moles – this depends on the density and the relative atomic mass. Hence the number of moles of metal hydroxide formed in each case is different, and the solution concentrations formed in the same volume of water (500 cm3) will be different.

(ii) The solubility of the metal hydroxides increases going down Group 2, so the amount of metal hydroxide that dissolves in the same volume of water increases. The pH of the solution is determined by the concentration of hydroxide ions in solution, and is not affected by the remaining undissolved hydroxide ions. Hence, the calcium hydroxide solution will be least alkaline (lowest pH) and the barium hydroxide solution will be most alkaline (highest pH).

 

Q4:    Mg(s) + 2H2O(g) → MgO(s) + H2(g)