3.2.2 Group 2, the alkaline earth metals - Chemistry Alevel

Trend - Atomic radius - group

Increases down the group.

Explanation - Atomic radius - group

The nuclear charge increases BUT shielding increases and there are more e⁻ shells.

Trend - Comparing atomic radius vs cationic radius

The cationic radius is smaller than the atomic radius.

Explanation - Comparing atomic radius and cationic radius

If the s² electrons are lost there are fewer shells as the outermost shell is lost. The remaining electrons are attracted by the same number of protons as were present in the atom ∴ the proton to electron ratio increases & the remaining electrons are attracted more strongly.

Trend - First ionisation energy - group

The first ionisation energy decreases down the group.

Explanation - First ionisation energy G2

The outermost e⁻ is further from the nucleus/there are more shells so the atomic radius increases, the shielding of the outermost e⁻ by the inner e⁻s increases. These factors outway the increased nuclear charge. ∴ the outer e⁻ is less strongly attracted to the nucleus and less energy is required to remove them.

Reaction with water - Mg - description

Reaction is very slow with cold water and is stops very soon after starting as the magnesium hydroxide formed is almost insoluble in water and forms a. barrier preventing further reaction. Magnesium does however react with steam.

Reaction with water - equation - Mg

Mg(s) + 2H₂O(l) → Mg(OH)₂(s) + H₂(g)

Reaction with steam equation - Mg

Mg(s) + H₂O(g) → MgO(s) + H₂(g) (magnesium oxide is the dehydrated form of the product)

Reaction with water - Ca - description

Effervescence/fizzing, Ca moves up and down (due to the bubbles on the surface of the Ca reducing the overall density), the metal dissapears but is replaced by a white suspension.

Reaction with water GENERAL description - G2

They react with cold water with increasing vigour to form hydrogen gas and sparingly soluble hydroxides which make the water alkaline (OH⁻)

Reaction with water - equation - Ca

Ca(s) + 2H₂O(l) → Ca(OH)₂(aq/s) + H₂(g) (Turns limewater cloudy)

Reaction with water description - Sr

Effervescence/fizzing, metal disappears, colourless solution forms.

Reaction with water equation - Sr

Sr(s) + 2H₂O(l) → Sr(OH)₂(aq) + H₂(g)

Reaction with water - Ba - description

Effervescence/fizzing, metal disappears, colourless solution forms.

Reaction with water equation - Ba

Ba(s) + 2H₂O(l) → Ba(OH)₂(aq) + H₂(g)

Trend - reactivity - group (G2)

The reactivity increases down the group. ΔH₁ and ΔH₂ ↓ going down the group so the overall reactions become less endothermic/more exothermic.

Explanation - reactivity G2

When these metals react they loose their s² e⁻s (outermost). The atomic radii become bigger down the group, the outer e⁻s experience more shielding by the inner e⁻s. These factors out weigh the increased nuclear charge so the outer e⁻s are lost more easily requiring less energy to be removed (reactions become more exothermic)

Solubility of hydroxides - Mg(OH)₂

Insoluble

Solubility of hydroxides - Ca(OH)₂

Slightly soluble

Solubility of hydroxides - Sr(OH)₂

Soluble

Solubility of hydroxides - Ba(OH)₂

Soluble

Ionic equation for ppt formed when reacting with water - Mg

Mg²⁺(aq) + 2OH⁻(aq) → Mg(OH)₂(s)

Ionic equation for ppt formed when reacting with water - Ca

Ca²⁺(aq) + 2OH⁻(aq) → Ca(OH)₂(s)

Trend - Hydroxide solubility

The solubility increases down the group.

How is Mg(OH)₂ used in medicine?

It is used as an antacid because it is insoluble in water allowing it to not irritate the oesophagus but still neutralise excess stomach acid.

Mg(OH)₂ in medicine equation

2HCl(aq) + Mg(OH)₂(s) → MgCl₂(aq) + 2H₂O(l)

How is Ca(OH)₂ used in agriculture?

It is used to neutralise acidic soil for better crop growth.

Solubility of sulphates - MgSO₄

Soluble

Solubility of sulphates - CaSO₄

Slightly soluble

Solubility of sulphates - SrSO₄

Insoluble

Solubility of sulphates - BaSO₄

Insoluble

Trend - Solubility of sulphates

The group 2 sulphates become less soluble as you go down the group.

How can BaSO₄ used in medicine when toxic?

Ba²⁺ is toxic to humans but when used as a barium meal BaSO₄ doesn't react with stomach acid and is insoluble in water.

How is BaSO₄ used in medicine?

It is used as a barium meal to line the gut for x-rays as it absorbs x-rays (doesn't turn black).

Using BaCl₂ to test for sulphate ions - steps

Add HCl acid (aq) to the solution, add BaCl₂ (aq) to the solution being tested.

Using BaCl₂ to test for sulphate ions - Step 1 reason

This removes any other ions other than sulphate ions that could give a ppt with BaCl₂.

Using BaCl₂ to test for sulphate ions - Step 1 half equation

CO₃²⁻ + 2H⁺ → CO₂ + H₂O

Using BaCl₂ to test for sulphate ions - Step 2 reason

This produces a white ppt with SO₄²⁻ ions.

Using BaCl₂ to test for sulphate ions - Step 2 half equation

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

Reaction with HCl - Mg equation

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

Reaction with acid observations G2

Effervescence/fizzing, metal moves to the top of the water (supported by H₂(g) bubbles), metal disappears.

Trend - reaction with acid - group 2

The reactions become more vigorous and faster as you go down the group.

Calcium and H₂SO₄ reaction description

The CaSO₄ produced is solid so a layer forms acting as a barrier between the Ca and the H₂SO₄ as it is sparingly soluble.

Why do you need to neutralise soil?

Improves soil texture (seedlings), lets nitrogen fixing bacteria grow and the availability of phosphorus increases.

How are calcium oxide and calcium carbonate used

They are used to remove sulphur dioxide from flue gases, when it is produced as a by-product of the combustion of fossil fuels in order to prevent acid rain.

How is magnesium used?

Magnesium is used in the extraction of titanium . Titanium oxide (ore) is initially converted to TiCl₄ and then is reduced to titanium with magnesium.

Magnesium reducing Ti equation

2Mg + TiCl₄ → 2MgCl₂ + Ti