Topic 11: Group 2 Metals. Flashcards
What is another name for Group 2 metals?
Group 2 metals are also known as alkaline earth metals.
What is metallic radius?
The metallic radius refers to half the distance between the nuclei of two adjacent metal atoms in a metallic lattice.
How does metallic radius change as we go down Group 2?
The metallic radius increases as you go down Group 2. This occurs because the number of electron shells increases, making the metal atoms larger. Despite the increase in positive charge (more protons), the added electron shells outweigh the effect of increased nuclear charge, leading to a larger atomic size.
Why does the melting point of Group 2 elements decrease down the group?
The melting point of Group 2 elements decreases down the group because the metal atoms become larger, leading to weaker metallic bonding. The increased distance between the positively charged metal ions and the delocalized electrons reduces the strength of the metallic bonds, making it easier for the atoms to separate and lowering the melting point.
Why does the density of Group 2 elements generally increase down the group?
The density of Group 2 elements generally increases down the group because the metal atoms become larger and heavier.
Although the atomic volume increases, the increase in mass outweighs the increase in volume, leading to higher density.
Density = mass ÷ volume.
We can test for calcium, strontium and barium ions in compounds using flame tests.
Describe how to conduct a flame test and the results with each metal.
A nichrome wire, cleaned with hydrochloric acid is dipped into a sample of the compound to be tested and heated in a non-luminous Bunsen flame:
- Calcium compounds give a brick-red colour.
- Strontium compounds give a scarlet/red colour.
- Barium compounds give an apple-green colour.
Describe the reaction of Group 2 metals and oxygen.
Generally, the Group 2 metals burn in oxygen to form a white solid metal oxide.
The general equation for the reaction of a Group 2 metal with oxygen O₂ is:
2X + O₂ → 2XO (where X is a Group 2 metal).
- Beryllium is coated in a thin layer of beryllium oxide which inhibits the reaction meaning it only reacts in a powder form.
2Be + O₂ → 2BeO - Magnesium burns in oxygen with a bright white flame
2Mg + O₂ → 2MgO - Calcium burns with a bright white flame which is slightly red at the top.
2Ca + O₂ → 2CaO - Strontium is reluctant to start burning but burns intensely with a white flame.
2Sr + O₂ → 2SrO - Barium burns in oxygen with white flame.
2Ba + O₂ → 2BaO
Group 2 metals get more reactive with oxygen going down the group.
Why does the reactivity of Group 2 metals increase down the group?
In Group 2 metals, reactivity increases down the group because as you move down the group, each successive element has an additional electron shell meaning the outer electrons are further from the nucleus (increased atomic radius) and experience less attraction due to increased shielding from inner electrons. This results in lower ionisation energy, because the outermost electrons are further from the nucleus and experience less effective nuclear charge, it requires less energy and is easier for these metals to lose their two outer electrons to form +2 ions and react more readily.
Barium metal is so highly reactive that it must be stored under oil to keep it out of contact with air.
What is ionisation energy?
Ionisation energy is the amount of energy required to remove an electron from an atom or ion in its gaseous state to form a positively charged ion.
The higher the ionisation energy, the more energy is needed to remove an electron.
It’s a measure of how strongly an atom holds onto its electrons.
Generally, ionisation energy decreases as you move down a group in the periodic table and increases as you move across a period from left to right.
Describe the reaction of Group 2 metals and dilute hydrochloric acid.
All Group 2 metals react with dilute hydrochloric acid to produce a metal chloride and hydrogen gas. The reactions get more vigorous as you go down the group.
The general equation for this reaction is:
X + 2HCl → XCl₂ + H₂ (where X is a Group 2 metal).
For example:
- Ca + 2HCl → CaCl₂ + H₂.
- Ba + 2HCl → BaCl₂ + H₂.
Describe the reaction of Group 2 metals and dilute sulfuric acid.
Dilute sulfuric acid reacts with Group 2 metals to produce a metal sulfate and hydrogen.
The general equation for this reaction is:
X + H₂SO₄ → XSO₄ + H₂ (where X is a Group 2 metal).
For example:
Mg + H₂SO₄ → MgSO₄ + H₂.
Be + H₂SO₄ → BeSO₄ + H₂.
Sr + H₂SO₄ → SrSO₄ + H₂.
The reactions of Group 2 metals with dilute sulfuric acid do not get more vigourous down the group. Why?
Beryllium and magnesium produce soluble sulfates when reacted with dilute sulfuric acid.
However,the reaction of calcium, strontium and barium with dilute sulfuric acid is quickly stopped by the formation of an insoluble sulfate layer on the surface of the metals which slow down or stop the reaction.
Calcium gets completely covered by a layer of sparingly soluble calcium sulfate and the bubbles of hydrogen stop rising from the metals.
Strontium and barium produce insoluble sulfates so will quickly stop any reaction with sulfuric acid.
Describe the reaction of Group 2 metals and water.
- Beryllium reacts with steam only at very high temperatures (around 700°C or more).
Be + H₂O → BeO + H₂. -
Magnesium has a very slight reaction with cold water. The reaction stops due to the production of an insoluble coat of magnesium hydroxide.
Mg + 2H₂O → Mg(OH)₂ + H₂.
Magnesium burns in steam more readily than cold water:
Mg + H₂O → MgO + H₂. -
Calcium, strontium and barium all react in cold water to produce their hydroxide and hydrogen gas. The reactions become increasingly vigorous down the group.
Ca + 2H₂O → Ca(OH)₂ (limewater/slaked lime) + H₂.
Sr + 2H₂0 → Sr(OH)₂ + H₂.
Ba + 2H₂0 → Ba(OH)₂ + H₂.
Beryllium only reacts with steam at high temperatures but, going down Group 2, the metals react more readily and rapidly with cold water, with barium reacting the fastest. This shows that reactivity increases down the group.
Describe the reaction of Group 2 oxides and water.
Apart from beryllium, all Group 2 oxides react with water to produce hydroxides.
- Beryllium oxide is essentially unreactive toward water.
-
Magnesium oxide produces a solution that is around pH 10. This is because the magnesium hydroxide is only sparingly soluble so not many OH- ions are released into the solution.
MgO + H₂O → Mg(OH)₂. - Calcium oxide (quicklime) undergoes an exothermic reaction to produce calcium hydroxide (also known as slaked lime or lime water). Calcium hydroxide is partially soluble so the resulting solution is pH 11.
CaO + H₂O → Ca(OH)₂. - Strontium oxide and barium oxide produce hydroxides which are increasingly soluble.
They react in the same way as calcium but produce solutions with a higher pH as more OH- ions get released into the solution.
SrO + H₂O → Sr(OH)₂.
BaO + H₂O → Ba(OH)₂.
The solutions formed from the reactions of Group 2 metal oxides with water get more alkaline going down the group.
The solutions formed from the reactions of Group 2 metal oxides with water get more alkaline going down the group.
What is the reason for this?
As you move down Group 2, metal oxides react with water to form metal hydroxides that are more soluble and more alkaline/basic. This increased solubility means more hydroxide OH- ions are released, making the solution more alkaline.
