3.1.2 (d, e) Reaction of Group 2 Oxides with Water and Group 2 compounds as Bases

Syllabus

(d) the action of Water on Group 2 Oxides and the approximate pH of any resulting solutions, including the trend of increasing alkalinity

(e) uses of some Group 2 compounds as bases, including equations, for example (but not limited to):

(i) Ca(OH)₂ in agriculture to neutralise acid soils

(ii) Mg(OH)₂ and CaCO₃ as ‘antacids’ in treating indigestion.

What does this mean?

Teachers have been telling you that Metal Oxides are bases from Year 9.

They've also been telling you that if a base dissolves it forms an Alkali.

Nothing has changed.

Metal Oxide + Water → Metal Hydroxide solution

MO_(s) + H₂O_(l)→ M(OH)_2(aq)

The only problem is that not all Oxides are equally soluble.

You'll have seen Calcium Hydroxide solution as Limewater - it always has some bits floating in it, telling you that although it is quite soluble it isn't totally soluble.

Magnesium Hydroxide is less soluble.

Barium And Strontium Hydroxide are more soluble.

Learn the trend in solubility - you don't have to explain it.

Milk of Magnesia isn't a solution - the Magnesium hydroxide is insoluble. It's suspended in the water

The more soluble the Hydroxide the more OH^- ions available per mole of Metal Hydroxide dissolved.

So the more soluble Hydroxides are more alkaline because it is the concentration of OH^- ions that determines alkalinity.

So you can't give an actual pH until you cover that topic.

But you must guess somewhere between 8 - 14 for any alkali.

And since Calcium, Barium and Strontium Hydroxide are quite soluble 12-ish is likely to be acceptable.

But Magnesium Hydroxide isn't very soluble so you should guess somewhere closer to 8.

This explains why "Milk of Magnesia" can be used as an indigestion cure.

If it was very soluble the alkalinity of the Magnesium Hydroxide would corrode you mouth and throat before getting to your stomach to neutralise some of the acid.

But because its only sparingly soluble most of Milk of Magnesia is solid (and so not actually alkali) suspended in water.

It can't corrode your throat.

But it can still neutralise stomach acid as a solid base.

Calcium Hydroxide is soluble enough to be sprayed on acidic soil to change the pH.

Or it could be added as a solid, as insoluble Calcium Carbonate is.

And because Calcium Carbonate is an insoluble base it can be swallowed in indigestion tablets.

Acid-Base reactions are not Redox reactions because there are no changes in Oxidation number.

Mg(OH)_2(s) + 2 HCl_(aq) → MgCl_2(aq) + 2 H₂O_(l)

_{The Mg ions remain 2+}

The Oxygen is -2 in the OH^-ion, and -2 in water

The Hydrogen is +1 in the OH^-ion and HCl, and +1 in water

CaCO_3(s) + 2 HCl_(aq) → CaCl_2(aq) + H₂O_(l)+ CO_2(g)

_{The Ca ions remain 2+}

The Oxygen is -2 in the CO₃^2-ion, and -2 in water and CO₂

The Hydrogen is +1 in the HCl,and +1 in water

The Chlorine is -1 in the HCl, and -1 in CaCl₂

The Carbon is +4 in the CO₃^2-ion, and +4 in CO₂

Exam-style Questions

1. (a) State the trend in atomic radius down Group II from Mg to Ba and give a reason for this trend.

Trend ..................................................................................................................................................................

Reason ...............................................................................................................................................................

......................................................................................................................................................................(2)

(b) State and explain the trend in melting points of the elements down Group II from Mg to Ba.

Trend ...................................................................................................................................................................

Explanation ..........................................................................................................................................................

...........................................................................................................................................................................

.........................................................................................................................................................................(3)

Write an equation for the reaction of Magnesium with steam and an equation for the reaction of Strontium with water.

Trend ....................................................................................................................................................................

Equation for Magnesium ..........................................................................................................................................

Equation for Strontium.........................................................................................................................................(3)

(Total 8 marks)

Answers

1. (a) Trend: increases (1)

Wrong trend CE=0 and in (b)

Reason: More electron shells (1)

OR implies more shell / sub-shells / levels

(b) Trend: decreases (1)

Explanation: Metallic bonds weaker (1)

OR weaker attraction between ions (or nuclei) & delocalised electrons

Atoms (ions) larger (1)

This mark is only scored if previous mark given

CE if mention molecules, intermolecular forces ionic bonding

Equation for magnesium: Mg + H₂O --> MgO + H₂ (1)

Equation for strontium: Sr +2 H₂O --> Sr(OH)₂ + H₂ (1)

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