Group 2, the alkaline earth metals (3.2.2)
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Group 2 elements exhibit metallic bonding in a giant lattice structure, with positive ions surrounded by a sea of delocalized electrons.
Melting points decrease down Group 2 because larger ions have a lower charge density leading to weaker metallic bonds, requiring less energy to break.
As you go down group 2, reactivity increases.
When explaining this trend reference these four key points.
1. Increasing atomic radius as an extra shell of electrons is added.
2. Increasing nuclear shielding effect from inner shells of electrons.
3. Reduced attraction of the outer electrons to the positive nucleus.
4. The outer electrons are therefore more easily lost as you go down the group.
The increased reactivity of Group 2 metals as you go down the group is explained as follows.
Increasing atomic radius. As you move down the group, each element has an additional electron shell compared to the one above it. This increased distance between the nucleus and the outermost electrons means the nucleus has less pull on these outer electrons.
Greater shielding effect. With more inner electron shells, there is increased shielding. The inner electrons create a barrier that lessens the attraction between the nucleus and the outermost electrons.
Although the positive nuclear charge increases with each successive element, this is offset by the larger atomic radius and increased shielding. Consequently, the effective nuclear charge felt by the outer electrons decreases down the group, contributing to the increased reactivity.
The reactions of Group 2 metals with water occur by the following general equation:
For example looking at the reaction of calcium with water:
In the reaction the following oxidation state changes occur:
Note: the state symbol for can be (aq) or (s).
The Group 2 metal hydroxides’ solubility increases as you go down Group 2 meaning that under standard conditions, will form as a solid precipitate, whereas will form an aqueous solution.
The solubility of Group 2 sulfates decreases down the group.
- Magnesium sulfate : soluble in water
- Calcium sulfate : only partially soluble
- Strontium sulfate : poorly soluble in water
- Barium sulfate : insoluble in water
The hydroxides formed, , have variable solubility in water, forming alkaline solutions.
The alkalinity of the resulting solution increases down the group in line with solubility.
Magnesium hydroxide, , is only sparingly soluble and therefore only a small number of hydroxide ions dissociate in water. is normally observed as a precipitate when it is formed in an aqueous environment.
Barium hydroxide, , is very soluble and therefore more hydroxide ions dissociate which leads to a higher in comparison.
Indigestion often results from excess stomach acid. In medicine, magnesium hydroxide and calcium carbonate serve as effective antacids for managing indigestion.
and act as bases, which neutralise excess acid, and provide relief from symptoms like heartburn.
These equations illustrate the use of magnesium hydroxide and calcium carbonate to treat indigestion:
Barium sulfate is an insoluble salt.
Barium meals are commonly used in X-ray imaging because barium is a dense contrast medium – meaning that it is an effective X-ray absorber – so it is an ideal substance to enhance the visibility of certain structures in the digestive system.
When ingested, barium coats the lining of the gastrointestinal tract due to its insolubility, providing a clear outline of the oesophagus, stomach, and intestines during X-ray imaging.
This contrast allows healthcare professionals to diagnose conditions such as gastrointestinal obstructions, ulcers and abnormalities with greater precision than an X-ray scan without the use of barium.
Calcium oxide , commonly known as quicklime, is a crucial agent in the process of desulfurization of flue gases, particularly in industries that burn fossil fuels.
The primary reaction involves the absorption of sulfur dioxide by calcium oxide to form solid calcium sulfite :
In a further oxidation reaction, calcium sulfite is converted into the environmentally benign calcium sulfate, , commonly known as gypsum.
Magnesium is used in industry to extract titanium from titanium(IV) chloride in the Kroll process.
Magnesium is a good reducing agent, effectively removing chlorine from to leave behind titanium.
Titanium(IV) chloride gas is exposed to molten magnesium in an inert argon atmosphere. High temperatures of around are required.
The product is a mixture of titanium and magnesium chloride. This can be crushed and made into a slurry with water, dissolving the magnesium chloride and allowing the pure titanium to be separated by filtration.
Sodium is also an effective reducing agent in this process.
Acidified barium chloride solution is used to test for the presence of sulfate ions in a solution.
1. Add ~of unknown solution to a clean test tube.
2. Add a few drops of dilute hydrochloric acid to the solution.
3. Add a few drops of barium chloride solution to the acidified test solution.
If sulfate ions are present, a white precipitate of barium sulfate will form.
Note that the acidification step is crucial to prevent interference from carbonate ions, which also form a white precipitate with barium.
Barium chloride is highly soluble whereas barium sulfate is insoluble in water. When ions mix with ions in solution, the formation of a white precipitate serves as a clear and reliable indicator of the presence of sulfate ions in the solution.
Dilute hydrochloric acid is added to the solution before introducing to ensure accurate results. Acidification prevents false positives by removing carbonate ions .
Carbonate ions, if present, would react with barium ions to form barium carbonate This is also insoluble in water, forming a white precipitate leading to a misidentification of sulfate ions.
The acid reacts with any carbonate ions, converting them to carbon dioxide gas and water, eliminating their interference in the test.