Group 2 Hydroxides & Sulfates (Edexcel International A Level Chemistry)

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Philippa

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Philippa

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Solubility Trends

Group 2 hydroxides

  • Going down the group, the solutions formed from the reaction of Group 2 oxides with water become more alkaline
  • When the oxides are dissolved in water, the following ionic reaction takes place:

O2- (aq) + H2O(l) → 2OH- (aq)

  • The higher the concentration of OH- ions formed, the more alkaline the solution
  • The alkalinity of the formed solution can therefore be explained by the solubility of the Group 2 hydroxides

Solubility of the Group 2 Hydroxides Table

Group 2 hydroxide Solubility at 298 K
(mol / 100 g of water)
Mg(OH)2 2.0 x 10–5
Ca(OH)2 1.5 x 10–3
Sr(OH)2 3.4 x 10–3
Ba(OH)2 1.5 x 10–2

  • The hydroxides dissolve in water as follows:

X(OH)2 (aq) → X(aq) + 2OH- (aq)

          Where X is the Group 2 element

  • When the metal oxides react with water, a Group 2 hydroxide is formed
  • Going down the group, the solubility of these hydroxides increases
  • This means that the concentration of OH- ions increases, increasing the pH of the solution
  • As a result, going down the group, the alkalinity of the solution formed increases when Group 2 oxides react with water

Group 2 - Trends Group 2 Hydroxide, downloadable AS & A Level Chemistry revision notes

Going down the group, the solubility of the hydroxides increases which means that the solutions formed from the reactions of the Group 2 metal oxides and water become more alkaline going down the group

Group 2 sulfates

  • The solubility of the Group 2 sulfates decreasing going down the group

 Group 2 - Trends Group 2 Sulfate, downloadable AS & A Level Chemistry revision notes

Going down the group, the solubility of the sulfates decreases

Examiner Tip

You may be wondering why there are no trends here for the solubility of Group 1 hydroxides and sulfates. You should recall from previous studies that Group 1 compounds are all soluble in water. They will therefore not produce any precipitates when testing for cations, so to identify them you need to use flame tests.

Group 1 hydroxides will be more soluble than Group 2 hydroxides. Even though we say the solubility of the Group 2 hydroxides increases down the group barium hydroxide is less soluble than a Group 1 hydroxide such as potassium hydroxide.

At 25 °C the solubility of Ba(OH)2 is 4.68 g / 100 cm3

At 25 °C the solubility of KOH is 121 g / 100 cm3

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Philippa

Author: Philippa

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.