The Hydroxides and Sulfates of Group 2
This lesson covers:
- Solubility trends of group 2 compounds
- Role of group 2 oxides as bases
- Applications of group 2 elements and compounds
Opposite solubility trends for group 2 hydroxides and sulfates
The solubility of group 2 metal hydroxides and sulfates follows opposite trends down the group:
- Group 2 metal hydroxides become more soluble down the group
- Group 2 metal sulfates become less soluble down the group
This is summarised in the table below:
| Group 2 element | Solubility of hydroxide | Solubility of sulfate |
|---|---|---|
| Magnesium | Insoluble | Soluble |
| Calcium | Sparingly soluble | Sparingly soluble |
| Strontium | Soluble | Insoluble |
| Barium | Soluble | Insoluble |
Group 2 oxides are bases
The oxides of group 2 elements (MO) are basic and alkaline:
They readily react with water to form metal hydroxides that dissolve in water:
- MO(s) + H2O(l) ➔ M(OH)2(aq)
This releases hydroxide ions (OH-) making the solutions strongly alkaline. For example:
BaO(s) + H2O(l) ➔ Ba2+(aq) + 2OH-(aq)
The pH of the resulting solutions increases down the group as the solublity of the hydroxides increases.
An exception is MgO which reacts slowly and has low solubility.
Applications of group 2 elements and compounds
Group 2 elements and compounds have important practical uses including:
Magnesium in titanium extraction
Magnesium metal plays a key role in extracting titanium from its ore titanium(IV) oxide (TiO2). The ore is first converted to titanium(IV) chloride (TiCl4) using chlorine gas and carbon. After purifying the TiCl4 through fractional distillation, magnesium acts as a reducing agent in this high temperature reaction:
TiCl4(l) + 2Mg(s) ➔ Ti(s) + 2MgCl2(s)
Calcium hydroxide for neutralising soil acidity
Calcium hydroxide (Ca(OH)2), also called slaked lime, counters high acidity in soils by neutralising hydrogen ions. It is spread over agricultural fields to raise pH levels optimal for healthy crop growth. The ionic equation for the neutralisation is:
H+(aq) + OH−(aq) ➔ H2O(l)
Magnesium hydroxide as an antacid
Magnesium hydroxide (Mg(OH)2) is an effective antacid, which is a base substance that neutralises stomach acid. It works by reacting with the hydrochloric acid in the stomach, neutralising the excess acid that causes the symptoms of indigestion. The ionic equation for the neutralisation is:
H+(aq) + OH−(aq) ➔ H2O(l)
Barium sulfate in medical imaging.
Barium sulfate is mixed into a liquid suspension (known as a barium meal) and swallowed by patients to highlight and image soft tissues like the digestive tract. Because barium sulfate absorbs X-rays, the organs in which it coats become visible in X-ray images, helping diagnose conditions of the oesophagus, stomach and intestines.
Calcium oxide and calcium carbonate in sulfur dioxide removal:
Sulfur dioxide gas produced by power stations is an atmospheric pollutant. It can be removed or "scrubbed" from flue gas emissions (emissions from industrial chimneys and exhuasts) by spraying a slurry containing calcium oxide or calcium carbonate which reacts with the SO2 to form a solid waste product, calcium sulfite (CaSO3):
CaO(s) + SO2(g) + H2O(l) ➔ CaSO3(s) + H2O(l)
CaCO3(s) + SO2(g)+ H2O(l) ➔ CaSO3(s) + H2O(l) + CO2(g)