Notes · ChemistryUK · A-Levels

Group 2, the alkaline earth metals

The Group 2 metals - beryllium to barium - show clear, explicable trends as you go down the group. This chapter develops the trends in atomic radius, ionisation energy, melting point and reactivity, the reactions of the metals with water, the opposite solubility trends of the hydroxides and sulfates, and the everyday and medical uses of Group 2 compounds.

4 sections~12 min reading time3 competenciesLevel Foundation 1 · Standard 3

T·0999 / 18

Exam profile

AO1 · Describe the trends in the physical properties, reactions and uses of the Group 2 metals and their compoundsAO2 · Explain the reactivity and solubility trends from electron configuration and ionisation energyAO3 · Apply the solubility trends to identification tests and to the uses of the compounds

Operators:statedescribeexplainpredictdeduce

basic level

AS-Level requires the trends down Group 2, the reactions with water, the solubility trends of the hydroxides and sulfates, and the main uses.

higher level

The full A-Level expects these trends to be explained quantitatively (ionisation energy, lattice and hydration enthalpies) and applied to unfamiliar contexts and identification tests.

Depth

Text

Contents · 4 sections

Group 2, the alkaline earth metals

Trends in physical properties down Group 2#

StandardAQA 7405 3.2.2.1DfE GCE Chemistry - Group 2

First ionisation energy down Group 2

Fig. 1First ionisation energy falls down the group as the outer electrons become further from the nucleus and better shielded.

The Group 2 metals all have two electrons in their outer

s

sub-shell, and they react by losing both to form

2+

ions. Going down the group, the atomic radius increases, because each successive element has an additional occupied electron shell. The extra inner shells also increase the shielding of the outer electrons from the nuclear charge, so even though the nuclear charge rises, the outer electrons are held less tightly.

As a result, the first (and second) ionisation energies decrease down the group: the outer electrons are further from the nucleus and better shielded, so less energy is needed to remove them. This falling ionisation energy is the key to the reactivity trend - because the metals react by losing their two outer electrons, the easier those electrons are to remove, the more reactive the metal. Reactivity therefore increases down Group 2, from magnesium to barium.

The melting points generally decrease down the group (with magnesium a slight anomaly), because the metallic bonding weakens: the

2+

ions get larger down the group, so the attraction between the ions and the delocalised electrons is spread over a greater distance and is weaker. This is the same 'larger ions, weaker attraction' argument used for many trends, applied here to metallic bonding.

These physical trends are all consequences of one underlying change - the increasing atomic radius and shielding down the group. When you explain any Group 2 trend, anchor it in this: more shells means a larger radius and more shielding, so the outer electrons are held less tightly. That single idea accounts for the decreasing ionisation energy, the increasing reactivity, and the weakening metallic bonding together.

Worked example

Reactivity trend down the group

Explain why the reactivity of the Group 2 metals increases from magnesium to barium.

How they react
The metals react by losing their two outer electrons to form 2+ ions.
Trend in ionisation energy
Down the group the atoms are larger and their outer electrons more shielded, so the ionisation energies decrease.
Link to reactivity
The lower the ionisation energy, the more easily the outer electrons are lost, so the metals become more reactive down the group.

Result: Reactivity increases down Group 2 because the falling ionisation energies make the two outer electrons progressively easier to lose.

Typical mistakes

Saying reactivity increases down the group because of increasing nuclear charge - it is the easier loss of electrons (lower ionisation energy) that matters.
Forgetting that shielding, not just radius, contributes to the falling ionisation energy.

Active revision

Explain why barium is more reactive than calcium, referring to ionisation energy, atomic radius and shielding.

Active recall

Recall the key points — then reveal.

Sources: GCE AS and A level subject content for the sciences (Department for Education) · AQA A-level Chemistry 7405 specification (AQA)

Reactions with water and reactivity#

StandardAQA 7405 3.2.2.1DfE GCE Chemistry - reactions of Group 2

Reaction of a Group 2 metal with water

Fig. 2Group 2 metals reduce water to hydrogen, forming the metal hydroxide; the reaction is more vigorous down the group.

The Group 2 metals react with water to form a metal hydroxide and hydrogen gas, and the reaction becomes more vigorous down the group as reactivity increases. For calcium and below, the general reaction is

\text{M} + 2\text{H}_2\text{O} \rightarrow \text{M(OH)}_2 + \text{H}_2

. Calcium reacts steadily with cold water, releasing bubbles of hydrogen and forming slightly soluble calcium hydroxide; barium reacts more vigorously still.

Magnesium is a special case: it reacts only very slowly with cold water, but it reacts readily with steam. With steam the product is the oxide rather than the hydroxide, because at the high temperature the reaction is

\text{Mg} + \text{H}_2\text{O} \rightarrow \text{MgO} + \text{H}_2

, with the magnesium burning brightly. This distinction - slow with cold water, vigorous with steam giving the oxide - is frequently tested.

In every case the metal is oxidised (its oxidation state rises from

0

+2

) and hydrogen in water is reduced (from

+1

0

\text{H}_2

), so these are redox reactions. Writing the correct products, balancing the equation and identifying the oxidation-state changes together make a complete answer. The vigour of the reaction - how fast the bubbles form and how quickly the metal is consumed - is the visible measure of reactivity.

The trend links directly back to ionisation energy. Because the metals react by losing their two outer electrons, the metal that loses them most easily reacts fastest; ionisation energy falls down the group, so reactivity rises. This is why barium reacts far more vigorously with water than magnesium, and it is the reasoning examiners expect when they ask you to predict or explain the reactivity order.

\text{M} + 2\text{H}_2\text{O} \rightarrow \text{M(OH)}_2 + \text{H}_2

Group 2 metal with water

For calcium and below; magnesium reacts only slowly with cold water but with steam gives MgO + H2.

Worked example

Magnesium with steam versus water

Describe and give equations for the reaction of magnesium with (a) cold water and (b) steam.

Cold water
Magnesium reacts only very slowly: Mg + 2H2O -> Mg(OH)2 + H2, with few bubbles over a long time.
Steam
Magnesium reacts readily, burning with a bright white light: Mg + H2O -> MgO + H2.
Explain the products
The high temperature of steam gives the oxide rather than the hydroxide, and the greater vigour reflects the faster reaction.

Result: Cold water gives slow formation of Mg(OH)2; steam gives vigorous formation of MgO with a bright flame.

Typical mistakes

Writing that magnesium reacts vigorously with cold water - it is slow with cold water but fast with steam, giving MgO.
Forgetting hydrogen gas as a product, or giving the wrong formula for the hydroxide.

Active revision

Write balanced equations for (a) calcium reacting with cold water and (b) magnesium reacting with steam, and state one observable difference between the two reactions.

Active recall

Recall the key points — then reveal.

Sources: AQA A-level Chemistry 7405 specification (AQA)

Solubility trends of hydroxides and sulfates#

StandardAQA 7405 3.2.2.2DfE GCE Chemistry - solubility trends

Opposite solubility trends

Fig. 3Hydroxides become more soluble down the group; sulfates become less soluble.

The Group 2 hydroxides become more soluble down the group. Magnesium hydroxide is only very slightly soluble (its suspension is the mild alkali 'milk of magnesia'), calcium hydroxide is sparingly soluble (its solution is limewater), and barium hydroxide is reasonably soluble, giving a strongly alkaline solution. So a solution of a Group 2 hydroxide becomes more alkaline (higher pH) down the group, because more hydroxide ions are released into solution.

The Group 2 sulfates show the opposite trend: they become less soluble down the group. Magnesium sulfate is soluble, calcium sulfate is only slightly soluble, and barium sulfate is essentially insoluble. This decreasing solubility of the sulfates is the basis of the test for sulfate ions: adding barium chloride (or barium nitrate) solution to a solution containing sulfate ions gives a white precipitate of insoluble barium sulfate.

Because the sulfate test relies on barium sulfate being insoluble, the barium salt must be acidified first (usually with dilute hydrochloric or nitric acid) to remove carbonate ions, which would also give a white precipitate with barium and give a false positive. Recognising why the acid is added - to prevent interference from carbonate - is a common exam point that distinguishes a full answer.

These opposite trends are used to identify Group 2 compounds and appear in the uses of the compounds. The insolubility of barium sulfate makes it safe to use as a 'barium meal' in medical X-rays despite barium ions being toxic, because the insoluble compound is not absorbed by the body. When explaining or predicting solubility, remember: hydroxides more soluble down the group, sulfates less soluble down the group.

Worked example

Testing for a sulfate ion

A solution is thought to contain sulfate ions. Describe a test to confirm this and give the expected observation.

Add acidified barium chloride
Acidify the sample with dilute hydrochloric acid (to remove carbonate), then add barium chloride solution.
Expected observation
A white precipitate of barium sulfate forms if sulfate ions are present: Ba2+ + SO4^2- -> BaSO4.
Why acidify
The acid removes carbonate ions, which would otherwise also give a white precipitate with barium and a false positive.

Result: A white precipitate with acidified barium chloride confirms sulfate ions; the acid prevents a false positive from carbonate.

Typical mistakes

Getting the two trends the wrong way round (they are opposite).
Forgetting to acidify the barium chloride in the sulfate test, allowing carbonate to give a false positive.

Active revision

Describe how you would test a solution to confirm the presence of sulfate ions, and explain why the reagent is acidified first.

Active recall

Recall the key points — then reveal.

Sources: AQA A-level Chemistry 7405 specification (AQA)

Uses of Group 2 compounds#

FoundationAQA 7405 3.2.2.3DfE GCE Chemistry - uses of Group 2 compounds

Uses of Group 2 compounds

Fig. 4Each use exploits a specific property: alkalinity, insolubility or reducing power.

Many uses of Group 2 compounds exploit their basic (alkaline) character to neutralise acids. Calcium hydroxide (slaked lime) is added to fields by farmers to raise the pH of acidic soils, improving conditions for crops; the reaction neutralises the excess acid in the soil. Because it is only sparingly soluble, it releases its alkalinity gradually and is safe to handle in bulk.

In medicine, the mild bases magnesium hydroxide and calcium carbonate are used as antacids to neutralise excess hydrochloric acid in the stomach and relieve indigestion. They are chosen because they are only weakly or sparingly soluble, so they neutralise the acid without making the stomach dangerously alkaline. The reaction, for example

\text{Mg(OH)}_2 + 2\text{HCl} \rightarrow \text{MgCl}_2 + 2\text{H}_2\text{O}

, removes the excess acid.

Barium sulfate is used as a 'barium meal' in medical imaging. Although barium ions are toxic, barium sulfate is so insoluble that essentially no barium ions dissolve and are absorbed by the body, so it is safe to swallow. Because barium is a heavy element it absorbs X-rays strongly, so it coats and shows up the soft tissue of the gut, which would otherwise be transparent to X-rays - a direct use of the sulfate's insolubility trend.

Magnesium has an important use as a reducing agent in the extraction of titanium. Titanium is extracted from its ore by first converting it to titanium(IV) chloride, then reducing this with magnesium:

\text{TiCl}_4 + 2\text{Mg} \rightarrow \text{Ti} + 2\text{MgCl}_2

. Magnesium is used because it is reactive enough to displace titanium (it is a strong reducing agent), and this reaction is carried out in an inert argon atmosphere to prevent the hot metals reacting with air.

\text{TiCl}_4 + 2\text{Mg} \rightarrow \text{Ti} + 2\text{MgCl}_2

Extraction of titanium

Magnesium reduces titanium(IV) chloride to titanium in an inert argon atmosphere.

Worked example

Why barium sulfate is used in medicine

Explain why barium sulfate is used as a barium meal in X-ray imaging of the gut, despite barium ions being toxic.

Toxicity of barium ions
Soluble barium ions are toxic, so a soluble barium compound could not be swallowed safely.
Insolubility of the sulfate
Barium sulfate is essentially insoluble, so almost no barium ions dissolve into the body and are absorbed.
Why it shows on X-rays
Barium is a heavy element that absorbs X-rays strongly, so the coated gut shows up clearly where the surrounding tissue would be transparent.

Result: Barium sulfate is safe because its insolubility stops the toxic ions being absorbed, while its X-ray absorption reveals the gut.

Typical mistakes

Saying barium sulfate is safe because barium is non-toxic - it is toxic, but the sulfate is so insoluble it is not absorbed.
Confusing which compound is used for which purpose.

Active revision

Explain why barium sulfate can be safely swallowed as a barium meal, even though barium ions are toxic.

Active recall

Recall the key points — then reveal.

Sources: AQA A-level Chemistry 7405 specification (AQA)

References & sources

Sources

Department for Education

GCE AS and A level subject content for the sciences

AQA

AQA A-level Chemistry 7405 specification

Group 2, the alkaline earth metals

4 sections~12 min reading time3 competenciesLevel Foundation 1 · Standard 3

Group 2, the alkaline earth metals

Key points

Reactivity trend down the group

Key points

Magnesium with steam versus water

Key points

Testing for a sulfate ion

Key points

Why barium sulfate is used in medicine

Sources

Group 2, the alkaline earth metals

Key points

Reactivity trend down the group

Key points

Magnesium with steam versus water

Key points

Testing for a sulfate ion

Key points

Why barium sulfate is used in medicine

Sources