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3.2Periodic Table and Energy

Group 2 — OCR A Level Chemistry A H432 Revision Notes & Topical Questions

5 min read 10 parts Lesson 4 of 21

📚 OCR A Level Chemistry A H432 Hub / Topical Revision
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Complete PDF with figures and original page layoutPage 1 OCR A Level Chemistry A Your notes Group 2 Contents Group 2 Elements Group 2 Compounds Page 2 Group 2 Elements …

Part 1 of 8Structure and ions of Group 2

Structure and ions of Group 2
Electron configuration
  • Group 2 elements have two electrons in the outer s subshell (s2)
Redox behaviour
  • Redox behaviour each metal atom is oxidised, losing two valence electrons to form a 2+ ion with a noble gas configuration
  • Ca → Ca2+ + 2e− changes [Ar]4s2 to [Ar].
  • Another species gains the electrons and is reduced, so the Group 2 metal is a reducing agent.

Part 2 of 8Reactivity trend

  • Trend
    • reactivity increases down the group
    • Atomic radius and shielding increase.
    • Nuclear attraction for the outer electrons decreases, so they are removed more easily and cations form more easily.

Part 3 of 8Reactions with oxygen and water

With oxygen
  • 2M(s) + O2(g) → 2MO(s)
  • M goes 0 to +2 (oxidised) and O goes 0 to −2 (reduced).
With water
  • M(s) + 2H2O(l) → M(OH)2(s) + H2(g)
  • M is oxidised from 0 to +2, hydrogen is reduced from +1 to 0 and oxygen is unchanged.
  • The solubility of the hydroxides increases down the group.

Part 4 of 8Reactions with dilute acids

Hydrochloric acid
  • M(s) + 2HCl(aq) → MCl2(aq) + H2(g)
  • M is oxidised from 0 to +2, hydrogen is reduced from +1 to 0 and chlorine is unchanged.
Sulfuric acid
  • M(s) + H2SO4(aq) → MSO4(aq) + H2(g)
  • Some metals form an insoluble sulfate.
  • Sulfate solubility decreases down the groupcalcium sulfate is sparingly soluble, while strontium and barium sulfates are insoluble.

Part 5 of 8Ionisation energies and reactivity

Forming 2+ ions
  • Forming 2+ ions it needs two ionisation energies
  • M(g) → M+(g) + e−.
  • M+(g) → M2+(g) + e−.
Trend
  • Trend both the first and second ionisation energies decrease down the group
  • The atomic radius and shielding increase, so the attraction between nucleus and outer electrons falls.
Link to reactivity
  • Link to reactivity the first and second ionisation energies make up most of the energy input in these reactions
  • As the total energy needed falls down the group, the elements become more reactive and stronger reducing agents.

Part 6 of 8Group 2 oxides and hydroxides in water

Group 2 oxides and hydroxides in water
Oxides
  • Oxides they react with water to release hydroxide ions, giving alkaline solutions
  • CaO(s) + H2O(l) → Ca2+(aq) + 2OH−(aq).
Trend
  • Trend solutions get more alkaline down the group because solubility increases and more OH− is present
  • Mg(OH)2 is only slightly soluble, giving a low OH− concentration and pH about 10.
  • Ba(OH)2 is much more soluble, giving a higher OH− concentration and pH about 13.
Saturation
  • Saturation once saturated, extra ions precipitate: Ca2+(aq) + 2OH−(aq) → Ca(OH)2(s)

Part 7 of 8Group 2 compounds in agriculture

Group 2 compounds in agriculture
Purpose
  • Purpose soil that is too acidic can be neutralised by adding a Group 2 base
  • The amount must be controlled because too much makes the soil too alkaline for crops.
Calcium hydroxide (slaked lime)
  • Ca(OH)2(s) + 2H+(aq) → Ca2+(aq) + 2H2O(l)
  • It is reasonably soluble and acts relatively quickly.
Calcium carbonate (limestone)
  • CaCO3(s) + 2H+(aq) → Ca2+(aq) + H2O(l) + CO2(g)
  • It is cheaper and safer to handle but largely insoluble, so it acts more slowly.

Part 8 of 8Group 2 compounds in medicine

Group 2 compounds in medicine
Antacid (milk of magnesia)
  • Antacid (milk of magnesia) a suspension of Mg(OH)2 neutralises excess stomach hydrochloric acid
  • Mg(OH)2(s) + 2HCl(aq) → MgCl2(aq) + 2H2O(l).
  • It is safe because its very low solubility keeps OH− low, so the solution is only weakly alkaline (pH about 10).
Barium meal
  • Barium meal a patient drinks a suspension of BaSO4 before an X-ray of the digestive system
  • Barium is a large atom that absorbs X-rays well, so the coated gut shows clearly.
  • Soluble barium compounds are toxic, but BaSO4 is insoluble and not absorbed into the bloodstream.

Key terms

TermMeaning
Reducing agentA species that donates electrons and is oxidised
Ionisation energyEnergy needed to remove one mole of electrons from one mole of gaseous atoms or ions
Slaked limeCalcium hydroxide, used to neutralise acidic soil
Barium mealA suspension of barium sulfate swallowed before an X-ray of the gut
AntacidA substance that neutralises excess stomach acid

Avoid these mistakes

1
Saying reactivity increases down the group because ionisation energy increases — it decreases
2
Forgetting that Group 2 metals are oxidised and act as reducing agents
3
Stating barium sulfate is toxic — soluble barium compounds are toxic but insoluble BaSO4 is safe
4
Saying Mg(OH)2 is strongly alkaline — its low solubility gives only pH about 10

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