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AS Level Chemistry 9701
10. Group 2
Written by: Pranav I
Formatted by: Pranav I
Index
10.1 Physical properties of Group 2 elements
- Referred to as alkaline earth metals
- Have electronic configurations with 2 electrons in their outer shell (in the s sub-shell)
✅ Metallic radius (definition)
Half the distance between nuclei in a giant metallic lattice
- The metallic radius of Group 2 elements increases down the group
- 2 outer electrons occupy principal quantum shells further from the nucleus (more shielding)
- Also have general trends in other physical properties, such as melting point and density
10.2 Reactions of Group 2 elements
- Form ionic compounds with a 2+ charge (lose their outermost electrons)
- Act as reducing agents
- Donate electrons
- Oxidized themselves (oxidation state increases from 0 to +2)
- The reactivity of Group 2 elements increases down the group
- Ionization energy decreases down the group (easier to remove outermost electrons)
- Increase in the number of shells, shielding effect, and the distance of the outermost electrons from the nucleus
- Example: the reaction of Group 2 metals with HCl(aq) becomes more vigorous as we go down the group
- The solubility of Group 2 sulfates decreases down the group
- Mg + H2SO4 is similar to Mg + HCl in terms of reaction rate
- Ca, Sr, and Ba are quickly stopped by the formation of an insoluble sulfate layer on the surface of the metals
10.3 Reactions with oxygen
- Group 2 metals burn in air (oxygen) to form white solid oxides
- 2Mg(s) + O2 → 2MgO(s)
- The oxides are basic in character
- Get more reactive with oxygen going down the group
- Burn with characteristic flame colours
Flame tests
- Used to test for the presence of Group 2 ions in compounds
- Procedure
- Clean a nichrome wire with concentrated hydrochloric acid
- Dip it in a sample of the compound to be tested
- Heat in a non-luminous Bunsen flame
- Mg2+ → brilliant white flame (not characteristic; observed with other metals too)
- Ca2+ → brick-red flame
- Sr2+ → scarlet/red flame
- Ba2+ → apple-green flame
Reactions of oxides with dilute acid
- Type of reaction: neutralization
- General reaction: basic oxide + dilute acid → salt + water
- With HCl(aq), all Group 2 metals show similar results
- With H2SO4(aq), insoluble sulfates interfere with the reaction (except for Mg)
- Significant neutralization can occur when a powdered form of the metal oxide is used
10.4 Reactions with water
- Mg reacts very slowly with cold water to form a weakly alkaline solution
- Mg(s) + 2H2O (aq) → Mg(OH)2(aq) + H2(g)
- Hot Mg reacts vigorously with steam
- Mg(s) + H2O(g) → MgO(s) + H2(g)
- Ca reacts more vigorously with water at room temperature
- Steady stream of hydrogen gas
- Cloudy white suspension of slightly soluble Ca(OH)2
- Down the group, hydrogen gas is released more rapidly
Reactions of Group 2 oxides and water
- General reaction: metal oxide + water → metal hydroxide
- A saturated solution of Mg(OH)2 has a pH of 10
- A saturated solution of Ca(OH)2 has a pH of 11
- Has a higher concentration of OH–(aq) compared to Mg(OH)2
- The solutions formed from the reaction of the Group 2 metal oxides with water get more alkaline (increasing pH) going down the group
- The solubility of the Group 2 hydroxides increases down the group
Reactions of hydroxides with dilute acids
- Type of reaction: neutralization
- General reaction: hydroxide + dilute acid → salt + water
- Mg(OH)2(s) + 2HCl(aq) → MgCl2(aq) + 2H2O(l)
- Ba(OH)2(aq) + H2SO4(aq) → BaSO4(s) + 2H2O(l)
Reactions of Group 2 carbonates
- General formula: XCO3
- Insoluble in water
- All react in dilute acid
- General reaction: carbonate + dilute acid → salt + water + carbon dioxide
- MgCO3(s) + H2SO4(aq) → MgSO4(aq) + H2O(l) + CO2(g)
- BaCO3(s) + H2SO4(aq) → BaSO4(s) + H2O(l) + CO2(g)
Thermal decomposition of Group 2 carbonates and nitrates
- Decompose when heated
- Thermal decomposition of Group 2 carbonates
- General reaction: carbonate → oxide + carbon dioxide
- MgCO3(s) → MgO(s) + CO2(g)
- Thermal decomposition of Group 2 nitrates
- General reaction: carbonate → oxide + nitrogen dioxide + oxygen
- Mg(NO3)2(s) → MgO(s) + 2NO2(g) + O2(g)
- Nitrogen dioxide is a toxic, brown gas
- The thermal stability of Group 2 carbonates and nitrates increases down the group (a higher temperature is needed for thermal decomposition down the group)
