1. Chapter 5 The Periodic Table: Group2
5.1 Redox Reactions
5.2 Using Oxidation Numbers
5.3 Group2
5.4 Compounds of Group 2 Element

2. 5.1 & 5.2 What is the outcome from syllabus?
Candidates should be able to:
Describe and explain redox processes in terms of electron transfer and/or of changes in oxidation number (oxidation state)

3. 5.1 Redox Reactions Reduction – oxidation happens everywhere!
Because both reduction and oxidation are going on side-by-side, this is known as a redox reaction.

4. 5.1 Redox Reactions
♣ Oxidation and reduction in terms of oxygen transfer
Definitions:
Oxidation is gain of oxygen
Reduction is loss of oxygen
(oxidising agent)
(reducing agent )

5. 5.1 Redox Reactions
♣ Oxidation and reduction in terms of hydrogen transfer
Definitions:
Oxidation is loss of hydrogen
Reduction is gain of hydrogen
Oxidising agent : acidified potassium dichromate(VI)
Reducing agent : sodium tetrahydridoborate, NaBH4

6. 5.1 Redox Reactions
♣ Oxidation and reduction in terms of electron transfer
Definitions:
Oxidation is loss of electrons.
Reduction is gain of electrons.

7. 5.1 Redox Reactions Oxidation Number/State (Ox/ON):
The number of electrons lost or gained by an atom in a compound compared to the uncombined atom. It forms the basis of a way of keeping track of redox (electron transfer) reactions.
♣ Every element in its uncombined state has an oxidation number of zero.
♣ A positive number shows that the element has lost electrons and has therefore been oxidised.
♣ A negative number shows that the element has gained electrons and has therefore been reduced.
♣ The more positive the number, the more the element has been oxidised. The more negative, the more it has been reduced.
♣ The numbes are written in Roman numberals and awlways have a sign unless they are zero, e.g. +II.

8. 5.1 Redox Reactions
Some elements almost always have the same oxidation states in their compounds:
element
usual oxidation state
exceptions
Group 1 metals
always +I
Group 2 metals
always +II
Oxygen
usually -II
except in peroxides (H2O2) , superoxide (NaO2) and F2O
Hydrogen
usually +I
except in metal hydrides where it is -1 (NaH)
Fluorine
always -I
Chlorine
usually -I
except in compounds with O or F (HClO)

9. 5.2 Using Oxidation Numbers
Oxidation numbers are used:
■ In the systematic naming system of inorganic compounds (ones not based on carbon chain)
■ To work out in a redox reaction which elements have been oxidised and which reduced.

10. ① Ionic compounds: “-ide” ② Molecular compounds:
5.2 Using Oxidation Numbers
Naming inorganic Compounds (the sum of ON = 0):
♦ Compounds with two elements
① Ionic compounds: “-ide”
② Molecular compounds:
CuO
Copper (II) oxide
Cu2O
Copper (I) oxide
FeCl2
Iron (II) chloride
FeCl3
Iron (III) chloride
NaH
Sodium Hydride
PCl3
Phosphorus trichloride
Phosphorus (III) chloride
PCl5
Phosphorus pentachloride
Phosphorus (V) chloride
CO2
Carbon dioxide

11. ♦ Compounds with three elements
5.2 Using Oxidation Numbers
♦ Compounds with three elements
SO42-
sulphate
NO3-
nitrate
NH4+
ammonium
PO43-
phosphate
MnO4-
manganate
CO32-
carbonate
K2SO4
Potassium sulphate
Potassium sulphate (VI)
K2SO3
Potassium sulphite
Potassium sulphate (IV)

12. 5.2 Using Oxidation Numbers
Oxidation involves an increase in oxidation number
Reduction involves a decrease in oxidation number

13. 5.2 Using Oxidation Numbers
The chlorine is the only thing to have changed oxidation state.
This is a good example of a disproportionation (歧化)reaction. A disproportionation reaction is one in which the same element is both oxidised and reduced.

14. 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

15. 5.3 Group 2
Alkaline metal: ns2
12+ Mg Be Mg Ca Sr Ba Ra

16. 5.3 Group 2
BERYLLIUM
MAGNESIUM
CALCIUM
STRONTIUM
BARIUM
RADIUM

17. 5.3 Group 2 The Flame Color: Element Color Li Scarlet Be － Na YellowMg K
Lilac Ca
Brick-red Rb
Red Sr
Crimson Cs
Blue Ba
Apple-green

18. 5.3 Group 2 The second period:Li Be B C N O F Ne
The second period:
It is generally true that elements in the same group of the Periodic Table have similar properties. However, the elements in the second period (Li to Ne) show many properties which are not typically of their groups.
The reasons for these anomalies are:
Unusually high ionisation energy
♦ These atoms are particularly small
Unusually electronegatitive
High charge density to polarise anions

19. 5.3 Group 2
♦ The elements of Period 2 can form bonds only through their 2s and 2p orbitals.
1370 kJ mol-1 required to promote a paired electron to the next empty orbital (3d)
nitron
Energy
phosphorus
Energy
2900 kJ mol-1 required to promote a paired electron to the next empty orbital (3s)
Nitrogen only forms NCl3
Phosphorus can forms PCl3 and PCl5

20. Diagonal relationships (对角线规则)
5.3 Group 2
Increasing electronegativity
I II III IV
Li Be B C Period 2
Na Mg Al Si Period 3
Increasing electronegativity
Diagonal relationships (对角线规则)
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.
For example: Be(OH)2 + 2H+ → Be2+ + 2H2O
Be(OH)2 + 2OHˉ→ [Be(OH)4]2ˉ

21. The physical properties of Group 2:
Electron arrangement
Metallic radius/nm
First + Second IE/ kJ mol-1
Tm/K
Tb/K
Density/g cm-3 Mg
Magnisum
[Ne]3s2
0.160
2189
922
1380
1.74 Ca
Calcium
[Ar]4s2
0.197
1735
1112
1757
1.54 Sr
Strontium
[Kr]5s2
0.215
1614
1042
1657
2.60 Ba
Barium
[Xe]6s2
0.224
1468
998
1913
3.51

22. 5.3 Group 2 Magnesium oxide 2Mg (s) + O2 (g) 2MgO (s)
a. in the air
b. in oxygen
① burns very vigorously
② bright white flame
③ white solid produced

23. 5.3 Group 2 . O
oxygen ion
O—O
superoxide ion
peroxide ion 2
The reason that there are different types of oxides is related to the sizes of the ions:
O2ˉ > O22ˉ > 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.

24. 5.3 Group 2
The closer the anions with cations, the more stable the ionic compounds crystal.
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.

25. 5.3 Group 2 ♦ Reaction with water
Mg (s) + 2H2O (l) Mg(OH)2 (aq) + H2 (g)
slowly
Mg (s) + H2O (g)
steam
MgO (s) + H2 (g)
rapidly
Beryllium does not react directly with water all. The rest of the Group II metals react with increasing rapidity on descending the group.

26. In the saturated solution, pH(Mg(OH)2) = 10
5.3 Group 2
♦ Oxide reaction with water
MgO (s) + H2O (l)
Mg(OH)2 (aq)
Partially soluble
In the saturated solution, pH(Mg(OH)2) = 10
The rest of the Group II oxides react with increasing rapidity on descending the group.

27. The reaction is more vigorous as we go down the group.
♦ Reaction with acids
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.

28. 5.3 Group 2
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.
Group II Mg Ca Sr Ba
MgCO3 → MgO + CO2
The charge density (Z/r) of the cations (polarization) will affect the decomposition temperature.
Carbonates,
CO32ˉ
Same pattern but higher temperatures needed for decomposition
The larger value of Z/r,
The easier breaking up of distorted anions:
CO32- → CO2 + O2-
NO32- → NO2 + O2-
M(NO3)2 → MO +
2NO2 + 1/2O2 Mg Ca Sr Ba
Nitrates,
NO3ˉ
Same pattern but higher temperatures needed for decomposition

29. √ 5.3 Group 2 Which one of the following equations represents the
reaction that occurs when calcium nitrate is heated
strongly?
Ca(NO3)2 → Ca(NO2)2 + O2
2Ca(NO3)2 → 2CaO + 4NO2 + O2
Ca(NO3)2 → CaO + N2O + 2O2
3Ca(NO3)2 → Ca3N2 + 4NO2 + 5O2
Ca(NO3)2 → CaO2 + 2NO2 √

30. diagonal relationship
5.3 Group 2
Which one of the following elements is likely to have
an electronegativity similar to that of aluminium?
Barium
B. Beryllium
C. Calcium
D. Magnesium
E. Strontium
diagonal relationship √

31. √ 5.3 Group 2 Which one of the following statements is true?
All nitrates of Group II metals are decomposed by heat to give the oxide NO2
Aqueous sodium nitrate in acidic to litmus.
Aqueous ammonium nitrate is alkaline to litmus.
The alkali metal nitrites are insoluble in water.
Metals dissolve in concentrated nitric acid to give hydrogen. √

32. 5.4 Compounds of Group II Elements
coins
cosmetics
Ship
pipe

33. 5.4 Compounds of Group II Elements
Table 1: the melting points of the oxides of the Group II elements.
Oxide
Melting point/℃
MgO
2852
CaO
2614
SrO
2430
BaO
1918
refractory material
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.

34. √ 5.4 Compounds of Group II Elements
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 √

35. 5.4 Compounds of Group II Elements
CaCO3(limestone) Δ
+ CO2
+ H2O
Ca(OH)2(slaked lime)
CaO(lime)

36. Ca(OH)2 (s) + 2HNO3(aq) → Ca(NO3)2(aq) + 2H2O(l)
5.4 Compounds of Group II Elements
Ca(OH)2 (s) + 2HNO3(aq) → Ca(NO3)2(aq) + 2H2O(l)
Acid + Base → Salt + Water
This is a base and is used in agriculture to treat acidic soil.

37. √ √ √ 5.4 Compounds of Group II Elements
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 √ √ √

38. √ √ 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 √ √

39. 5.4 Compounds of Group II Elements
Soft water: Ca2+, Mg2+, HCO32-
“temporary hardness”
Ca2+ (aq) + 2HCO3ˉ(aq) → CaCO3(s) + CO2(g) + H2O(l)
        Mg2+ (aq) + 2HCO3ˉ(aq) → MgCO3(s) + CO2(g) + H2O(l) Δ
Hard water: Ca2+, Mg2+, SO42-, Cl-
“permanent hardness”
Ca2+(aq) + SO42-(aq) → CaSO4(s)

40. 5.4 Compounds of Group II Elements
     Ca2+ (aq) + 2C17H35COOˉ(aq) → Ca(C17H35COO)2 (s)
        Mg2+ (aq) + 2C17H35COOˉ(aq) → Mg(C17H35COO)2 (s)
calcium stearate
magnesium stearate
stearate
scum

41. 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.

42. √ 5.4 Compounds of Group II Elements
River water in a chalky agricultural area may contain Ca2+, Mg2+, CO32-, 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 √

43. Table 2: Active Ingredients in Commercial Antacid Tablets
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)

44. √ 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 √

45. The solubility of some Group II metal compounds in mmol·dm-3
5.4 Compounds of Group II Elements
The solubility of some Group II metal compounds in mmol·dm-3
CO SO CrO C2O42-
Mg2+
Ca2+
Sr2+
Ba2+
decreases
down the group

46. 5.4 Compounds of Group II Elements
定义: 由无限远离的气态正负离子, 在标准状态下形成1mol 离子晶体时的焓变, 叫该种晶体 的晶格能Hlatt 。
The enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions under standard conditions (298K，100 kPa)
NaCl(s)
(g)
Clˉ +
Na+
781 kJ•mol-1 － = 
latt D H

47. 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) + aq
Na+
Hhyd = －406 kJ·mol-1

48. 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).
NaCl (s)
Na+(aq) + Clˉ(aq)
Hsolu
Hlatt =
－776 kJ·mol-1
Hhyd =
－772 kJ·mol-1
Na+(g) + Clˉ(g)
Hsolu = Hhyd －Hlatt

49. 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.