Alkaline earth metals for AQA A-level Chemistry
This page covers the following topics:
1. Trends of alkaline earth metals
2. Reactions with alkaline earth metals
3. Uses of alkaline earth metals
4. Solubility of group 2 hydroxides and sulphates
The elements in group 2 are known as the alkaline earth metals. As you move down the group of the periodic table, additional shells of electrons are added that increase the atomic radius.
As the atomic radius decreases, the nucleus has a tighter hold on its electrons, so more energy is required to remove them. Therefore, the ionisation energy of alkaline earth metals decreases going down a group.
Melting point decreases going down group 2 of the periodic table as metallic bonds are weaker for larger elements and less energy is required to break them. Magnesium is an exception, with an unusually low melting point.
Alkaline earth metals react with oxygen to form metal oxides:
2M + O₂ → 2MO
The colours of flames for this reaction are provided in the image. A thin layer of alkaline earth metal oxide commonly forms on the surface of a piece of metal inhibiting further reaction between the metal and oxygen in the air.
Calcium, strontium, barium and radium react with water to produce a metal hydroxide and hydrogen. When hydrogen is produced in this reaction, bubbles are observed in a solution.
M + 2H₂O → M(OH)₂ + H₂
Beryllium and magnesium react with water steam and produce a metal oxide and hydrogen instead.
M + H₂O → MO + H₂
Beryllium is used in radiology as X-rays can pass through it. Strontium has a bright red flame when reacting with oxygen; thus, it is used in fireworks, signal flares. Radium is radioactive and sometimes is used to treat cancer.
Mg(OH)₂ can be used to neutralise stomach acid as it is alkaline, helping to treat indigestion and heartburn. The reductive power of magnesium is also essential for the extraction of titanium metal from its ore:
2Mg + TiCl₄ → 2MgCl₂ + Ti
Ca(OH)₂ solution, that is usually called lime water, is used to detect CO₂. In the presence of carbon dioxide the lime water turns milky white, as CaCO₃ is formed. Calcium oxide is used to remove toxic gases, like sulphur dioxide, from flue columns:
CaO + SO₂ → CaSO₃
The insolubility of barium sulphate in water makes it safe for use in medical imaging in contrast to other alkaline metal sulphates. Barium salts can also be used to detect sulphates. Barium ions react with sulphate ions in solutions to form insoluble barium sulphate precipitate.
Solubility in water of alkaline earth metal hydroxides increases going down the group due to a corresponding decrease in lattice energy. A lower lattice energy means it's easier for a crystal to be split into metal and hydroxide ions. Thus, beryllium and magnesium hydroxides are insoluble in water.
The solubility of alkaline earth metal sulphates at the top of group 2 is the highest due to a smaller ionic size which results in a higher charge density. Going down group 2, metal sulphates need more energy to split into ions in water solution that reduces their solubility.
Another name for aqueous Ca(OH)₂ is lime water. Which compound is commonly detected using lime water?
Ca(OH)₂ solution, that is usually called lime water, is used to detect CO₂. In the presence of carbon dioxide the lime water turns milky white, as CaCO₃ is formed.
Which alkaline earth metal forms a hydroxide that is the most soluble in water?
Solubility in water of alkaline earth metal hydroxides increases going down the group due to a corresponding decrease in lattice energy. An alkaline earth metal that is in the lowest position in the periodic table is radium. Thus, radium is the most soluble hydroxide out of all group 2 metals.
Would calcium, strontium or barium have the lowest melting point out of these metals?
The melting point of alkaline earth metals decreases going down the group. Thus, the element that is in the lowest position of group 2 in the periodic table would have the lowest melting point, that is, barium.
Provide an example of beryllium usage in medicine.
Beryllium is used in radiology as X-rays can pass through it.
Would magnesium, calcium or strontium have the lowest ionisation energy out of these metals?
As the atomic radius decreases, the nucleus has a tighter hold on its electrons, so more energy is required to remove them. Therefore, the ionisation energy of alkaline earth metals decreases going down a group and alkaline earth metal that has the lowest ionisation energy out of the elements provided is strontium.
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