5.3 & 5.4 Group 2

What is the outcome from syllabus?

Candidates should be able to:

(a) describe the reactions of the elements with oxygen and water

(b) describe the behaviour of the oxides with water

(c) describe the thermal decomposition of the nitrates and carbonates

(d) interpret, and make predictions from, the trends in physical and chemical properties of the elements and their compounds

(e) explain the use of magnesium oxide as a refractory lining material and calcium carbonate as a building material

(f) describe the use of lime in agriculture

5.3 Group 2

Sr

Ba

Ra

Ca

Be

Mg

Alkaline metal: ns2

12+

Mg

BERYLLIUM

MAGNESIUM

CALCIUM

STRONTIUM

BARIUM

RADIUM

5.3 Group 2

5.3 Group 2

Element Color Element Color

Li Scarlet Be －Na Yellow Mg －K Lilac Ca Brick-red

Rb Red Sr Crimson

Cs Blue Ba Apple-green

The Flame Color:

5.3 Group 2

I II III IV

Li Be B C Period 2

Na Mg Al Si Period 3

Increasing electronegativity

Increasing electronegativity

The untypically large electronegativities of Period 2 elements (caused by their small size) mean that they are in some ways more typical of elements one group to the right than to elements of their own group.

Diagonal relationships ( 角对 线规则 )

For example: Be(OH)2 + 2H+ → Be2+ + 2H2O Be(OH)2 + 2OHˉ→ [Be(OH)4]2ˉ

5.3 Group 2

Electron arrangement

Metallic radius/nm

First + Second IE/ kJ mol-1

Tm/K Tb/K Density/g cm-3

MgMagnisum

[Ne]3s2 0.160 2189 922 1380 1.74

CaCalcium

[Ar]4s2 0.197 1735 1112 1757 1.54

SrStrontium

[Kr]5s2 0.215 1614 1042 1657 2.60

BaBarium

[Xe]6s2 0.224 1468 998 1913 3.51

The physical properties of Group 2:

Magnesium oxide

2Mg (s) + O2 (g) 2MgO (s)

① burns very vigorously ② bright white flame ③ white solid produced

a. in the air b. in oxygen

5.3 Group 2

5.3 Group 2

The reason that there are different types of oxides is related to the sizes of the ions:

O2ˉ > O2

2ˉ > O2ˉ

If the cation is too small, it is not easy for enough peroxide or superoxide ions to cluster round it to form a stable crystal lattice. For example, Lithium can only forms the ‘normal’ oxide.

..¨OO.. ..

oxygen ion

¨ ¨O—OO—O¨.. ....

superoxide ionperoxide ion

¨ ¨O—OO—O¨ ¨.... ....22 ..22....

5.3 Group 2

The ‘normal’ oxide, MO(M2+ + O2ˉ), is formed when the metals are heated in oxygen. Strontium and Barium also form peroxides. As the M2+ ions are smaller than the M+ ions in Group I, peroxides do not form until lower down the group II than in Group I.

The closer the anions with cations, the more stable the ionic compounds crystal.

5.3 Group 2

Mg (s) + 2H2O (l) Mg(OH)2 (aq) + H2 (g)

♦ Reaction with water

Mg (s) + H2O (g)

steam

MgO (s) + H2 (g)

slowly

rapidly

Beryllium does not react directly with water all. The rest of the Group II metals react with increasing rapidity on descending the group.

5.3 Group 2

MgO (s) + H2O (l)

♦ Oxide reaction with water

Partially soluble

The rest of the Group II oxides react with increasing rapidity on descending the group.

Mg(OH)2 (aq)

In the saturated solution, pH(Mg(OH)2) = 10

♦ Reaction with acids

5.3 Group 2

Mg (s) + 2HCl (aq) MgCl2 (aq) + H2 (g)

Mg (s) + H2SO4(aq) MgSO4 (aq) + H2 (g)

The reaction is more vigorous as we go down the group.

5.3 Group 2

Carbonates, CO3

2ˉ

Mg Ca SrBa

MgCO3 → MgO + CO2

Group IIGroup II

Same pattern but higher temperatures needed for decomposition

Nitrates, NO3

ˉ

MgCaSrBa

M(NO3)2 → MO + 2NO2 + 1/2O2

Thermal stability describes how easily or otherwise a compound will decompose on heating. Increased thermal stability means a higher temperature is needed to decompose the compound.

The charge density (Z/r) of the cations (polarization) will affect the decomposition temperature.

The larger value of Z/r,The easier breaking up of distorted anions:

CO32- → CO2 + O2-

NO32- → NO2 + O2-

Same pattern but higher temperatures needed for decomposition

5.3 Group 2

Which one of the following equations represents the reaction that occurs when calcium nitrate is heated strongly?

A. Ca(NO3)2 → Ca(NO2)2 + O2

B. 2Ca(NO3)2 → 2CaO + 4NO2 + O2

C. Ca(NO3)2 → CaO + N2O + 2O2

D. 3Ca(NO3)2 → Ca3N2 + 4NO2 + 5O2

E. Ca(NO3)2 → CaO2 + 2NO2

√

5.3 Group 2

Which one of the following elements is likely to have an electronegativity similar to that of aluminium?

A. Barium

B. Beryllium

C. Calcium

D. Magnesium

E. Strontium

√ diagonal relationship

5.3 Group 2

Which one of the following statements is true?

A. All nitrates of Group II metals are decomposed by heat to give the oxide NO2

B. Aqueous sodium nitrate in acidic to litmus.

C. Aqueous ammonium nitrate is alkaline to litmus.

D. The alkali metal nitrites are insoluble in water.

E. Metals dissolve in concentrated nitric acid to give hydrogen.

√

5.4 Compounds of Group II Elements

coins cosmetics

pipeShip

5.4 Compounds of Group II Elements

Oxide Melting point/℃MgO 2852

CaO 2614

SrO 2430

BaO 1918

Table 1: the melting points of the oxides of the Group II elements.

As M2+ cationic size increases down the Group, the ionic bonds become weaker, hence, less energy is needed to break the bonds and a low melting point is expected.

refractory material

Magnesium oxide is used to line industrial furnaces because it has a very high melting point. Which type of bond needs to be broken for magnesium oxide to melt?

A. co-ordinate

B. covalent

C. ionic

D. metallic

5.4 Compounds of Group II Elements

√

5.4 Compounds of Group II Elements

CaCO3(limestone)

CaO(lime)Ca(OH)2(slaked lime)

Δ

+ H2O

+ CO2

5.4 Compounds of Group II Elements

Acid + Base → Salt + Water

Ca(OH)2 (s) + 2HNO3(aq) → Ca(NO3)2(aq) + 2H2O(l)

This is a base and is used in agriculture to treat acidic soil.

A farmer spreads lime on land which has already been treated with a nitrogenous fertilizer. Which reactions will occur over a period of time?

1. Ca(OH)2 + CO2 → CaCO3 + H2O

2. Ca(OH)2 + 2H+ (aq) → Ca2+(aq) + 2H2O

3. Ca(OH)2 + 2NH4+(aq) → Ca2+(aq) + 2NH3 + 2H2O

5.4 Compounds of Group II Elements

√

√

√

5.4 Compounds of Group II Elements

When decomposing in water, organic refuse is oxidised to form carboxylic acids. The water becomes acidic and aquatic life is destroyed.

Which additives are suitable to remove this acid pollution?

1. calcium carbonate

2. calcium hydroxide

3. potassium nitrate

√

√

5.4 Compounds of Group II Elements

Hard water: Ca2+, Mg2+, SO42-, Cl-

Soft water: Ca2+, Mg2+, HCO32-

Ca2+(aq) + SO42-(aq) → CaSO4(s)

“temporary hardness”

“permanent hardness”

Ca2+ (aq) + 2HCO3ˉ(aq) → CaCO3(s) + CO2(g) + H2O(l)

Mg2+ (aq) + 2HCO3ˉ(aq) → MgCO3(s) + CO2(g) + H2O(l)

Δ

Δ

5.4 Compounds of Group II Elements

scum

Ca2+ (aq) + 2C17H35COOˉ(aq) → Ca(C17H35COO)2 (s)

Mg2+ (aq) + 2C17H35COOˉ(aq) → Mg(C17H35COO)2 (s)

calcium stearate

magnesium stearate

stearate

stearate

5.4 Compounds of Group II Elements

A number of methods can be used for softening water:

♦ Boiling removes temporary hardness, but is expensive.

♦ Calcium hydroxide is cheap and can be added to precipitate out temporary hardness as calcium carbonate. Ca(HCO3)2(aq) + Ca(OH)2(s) → 2CaCO3(s) + 2H2O(l)

♦ Sodium carbonate may be added to precipitate out calcium or magnesium ions. Mg2+ (aq) + Na2CO3(aq) → MgCO3(s) + 2Na+(aq)

♦ Use ion exchange resins: plastic beads which contain sodium ions.

River water in a chalky agricultural area may contain Ca2+, Mg2+, CO3

2-, HCO3-, Cl- and NO3

- ions. In a waterworks, such water is treated by adding a calculated quantity of calcium hydroxide.

Which will be precipitated following the addition of calcium hydroxide?

A. CaCl2

B. CaCO3

C. MgCO3

D. Mg(NO3)2

5.4 Compounds of Group II Elements

√

Table 2: Active Ingredients in Commercial Antacid Tablets

Chemical Name Chemical Formula

Chemical Reaction

Magnesium Hydroxide

Mg(OH)2

Mg(OH)2 + 2H+ → Mg2+ + 2H2O

Calcium Carbonate CaCO3 CaCO3 + 2H+ → Ca2+ + H2O + CO2 (g)

Sodium Bicarbonate NaHCO3 NaHCO3 + H+ → Na+ + H2O + CO2 (g)

Aluminum Hydroxide

Al(OH)3 Al(OH)3 + 3H+ → Al3+ + 3H2O

Dihydroxyaluminum Sodium Carbonate

NaAl(OH)2CO3 NaAl(OH)2CO3 + 4H+ → Na++ Al3++ 3H2O + CO2(g)

5.4 Compounds of Group II Elements

5.4 Compounds of Group II Elements

The metals of Group II react readily with oxygen to from compounds of general formula MO. When each of these oxides is added to water, which forms the most alkaline solution?

A. MgO

B. CaO

C. SrO

D. BaO√

5.4 Compounds of Group II Elements

The solubility of some Group II metal compounds in mmol·dm-3

Mg2+

Ca2+

Sr2+

Ba2+

CO32- SO4

2- CrO42- C2O4

2-

1.5 1830 8500 5.7

0.13 47 870 0.05

0.07 0.71 5.9 0.29

0.09 0.009 0.01 0.52

decreasesdown the

group

NaCl(s)(g)Clˉ+(g)Na+ →

781 kJ•mol-1－=latt

∆ H

定义 : 由无限 的气 正 子远离 态 负离 , 在 准状 下形成标 态

1mol 子晶体 的离 时 焓变 , 叫 晶体 的该种 晶格能 Hlatt 。

The enthalpy change when 1 mole of an ionic compound is

formed from its gaseous ions under standard conditions

(298K ， 100 kPa)

5.4 Compounds of Group II Elements

5.4 Compounds of Group II Elements

Hydration Enthalpy(∆Hhyd , 水合热 ): The amount of

energy relaeased when one mole of aqueous ions is

formed from its gaseous ions.

Na+ (aq)(g) + aqNa+ →

∆Hhyd = － 406 kJ·mol-1

5.4 Compounds of Group II Elements

When an ionic solid is dissolved in water, two processes are taking place. They are the breakdown of the ionic solid, and subsequent stabilization of the ions by water molecules (hydration).

Na+(g) + Clˉ(g)

∆Hlatt =

－ 776 kJ·mol-1

∆Hhyd = － 772 kJ·mol-1

∆Hsolu = ∆Hhyd － Hlatt

NaCl (s) Na+(aq) + Clˉ(aq)∆Hsolu

5.4 Compounds of Group II Elements

For MSO4, SO42- is quite large compared with M2+.Going

down the group II, the increase in size of the cations ∆Hlatt does not cause a significant change in the but the ∆Hhyd become less and less negative down the group. As a result, the dissolution process becomes less and less exothermic and the solubility of the sulphates(VI) of Group II metals decreases down the group.For M(OH)2, OH- and M2+ are of the same order of magnitude, Going down the group II, the increase in size of the cations ∆Hhyd does not cause a significant change in the but the ∆Hlatt

become less and less negative down the group. As a result, the dissolution process becomes less and less exothermic and the solubility of the hydroxides of Group II metals increases down the group.