Lecture Ch11 Group 2

AS CHEMISTRY 9701Ch.11: Group 2MCKL CHEMISTRY PANEL

TERRENCE LAI

MCKL CHEMISTRY PANEL / TERRENCE LAI / AY 2015 CHAPTER 11: GROUP 2

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Contents (from Syllabus 2015)I Similarities and trends in the properties of the Group 2 metals

magnesium to barium and their compounds(i) reactions with oxygen, water and dilute acids(ii) behaviour of oxides, hydroxides and carbonates with

water and dilute acids(iii) thermal decomposition of nitrates and carbonates(iv) Interpret and make predictions from trends in physical

and chemical properties of elements and their compounds

2 Some uses of Group 2 compounds(i) Use of magnesium oxide as a refractory lining material(ii) Use of lime in agriculture


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Group 2 metals


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Group 2 metals (2)


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Physical properties of Group 2


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Reactions of Group 2 elements


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Reaction with oxygen (Flame test)


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2[NOT in syllabus]Why different colours? These are due to photons emitted when electrons return to their initial energy level after being excited by heating; since distribution of energy levels differs for elements, each element has its own characteristic emission spectrum.

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Reaction with water


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(easier to ionises)2

Reaction with dilute acid


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(easier to ionises)

Reaction of oxides with water and acids


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Reactions of hydroxides with water and acids, solubility of sulfatesGroup 2 hydroxides have increasing solubility down the group. Therefore the resulting solution is more alkaline.

General equationM(OH)2 (s) + aq M2+ (aq) + 2OH- (aq)

Hydroxides react with acid because they are basic:M(OH)2 (s) + 2H+ (aq) M2+ (aq) + 2H2O (l)

However the Group 2 sulfates have decreasing solubility down the group (BaSO4 is very insoluble – test for sulfate ion by adding BaCl2)

For reasons why is this please look at A2 chapter lattice energy


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Reactions of carbonates with water and dilute acidsCarbonates of Group 2 elements are not soluble in water, i.e. no reaction

However they react with dilute acid to form salt, water and carbon dioxide

Eg. With dilute sulfuric acidMCO3 (s) + H2SO4 (aq) MSO4 (aq) + H2O (l) + CO2 (g)

Reactivity decreases down the group because of the decreasing solubility of MSO4 (BaSO4 is insoluble), preventing further reaction beneath the surface layer of MSO4, so less CO2 given out

Reaction with nitric acid produces soluble nitrate salts:MCO3 (s) + 2HNO3 (aq) M(NO3)2 (aq) + H2O (l) + CO2 (g)

Reaction with hydrochloric acid produces soluble chloride salts:MCO3 (s) + 2HCl (aq) MCl2 (aq) + H2O (l) + CO2 (g)


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Thermal decomposition of Group 2 nitrates


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Toxic Nitrogen(IV)

Oxide

Thermal decomposition of Group 2 carbonates


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(Read A2 chapter lattice energy to find out more)

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Uses of Group 2 oxides


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(High charge density means closely packed in ionic lattice, so strong ionic bondings)


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Cement factoryCaCO3 can be converted into CaO by heating in a rotating kilnCaCO3 CaO + CO2

lime/quick limecement + sand + small rocks concreteConcrete is an important building material, can be reinforced by setting the concrete with iron rods running through it.

Liming in agricultureCa(OH)2 (slaked lime) is basic, so it is used to neutralize soil aciditySource of soil acidity may come from acid rain, or leaching of H+ as a result of hydrolysis by Al3+ cations:2Al3+ + 3H2O AlOH2+ + Al(OH)2

+ + 3H+

SummaryGroup 2 elements (Mg to Ba) have increasing atomic radii, increasing ionic radii, decreasing first IE, decreasing melting points and decreasing hardness down the group

Group 2 elements react with water, oxygen to form hydroxides and oxides respectively; both of these are basic so some dissolve in water to form alkaline solution; all oxides and hydroxides also react with acids to form soluble salts and water

Group 2 carbonates react with dilute acid to form salt, water and CO2

Group 2 carbonates and nitrates decompose on heating, and releases NO2+O2(for nitrates) and CO2 (for carbonates) and oxides

Reactivity of Group 2 increases down the group as the first two IE decrease, however the solubility of sulfates and thermal decomposition of carbonates/nitrates decrease down the group due to increasing charge density of the cation (more on A2 lattice energy)

Limestone is used as building material, MgO used as refractory lining, Ca(OH)2 used to neutralize acids in acidic soil


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Lecture Ch11 Group 2

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