5.3 & 5.4 Group 2

1. 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

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

3. BERYLLIUM STRONTIUM MAGNESIUM BARIUM CALCIUM RADIUM 5.3 Group 2

4. 5.3 Group 2 The Flame Color:

5. 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

6. 5.3 Group 2 The physical properties of Group 2:

7. 2Mg (s) + O2 (g) 2MgO(s) b. in oxygen a. in the air 5.3 Group 2 Magnesium oxide ① burns very vigorously ② bright white flame ③ white solid produced

8. . . . . . . . . . ¨ . . . . ¨ ¨ ¨ O O—O O—O ¨ ¨ ¨ ¨ ¨ oxygen ion peroxide ion superoxide ion 2 2 5.3 Group 2 Thereason 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.

9. 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.

10. Mg (s) + 2H2O (l) Mg(OH)2 (aq) + H2 (g) Mg (s) + H2O (g) steam 5.3 Group 2 ♦ Reaction with water slowly 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.

11. Mg(OH)2 (aq) MgO (s) + H2O (l) 5.3 Group 2 ♦ Oxide reaction with water 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.

12. Mg (s) + 2HCl (aq) MgCl2 (aq) + H2 (g) Mg (s) + H2SO4(aq) MgSO4 (aq) + H2 (g) 5.3 Group 2 ♦ Reaction with acids The reaction is more vigorous as we go down the group.

13. Group II Carbonates, CO32ˉ Nitrates, NO3ˉ 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. Mg Ca Sr Ba MgCO3 → MgO + CO2 The charge density (Z/r) of the cations (polarization) will affect the decomposition temperature. 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 Same pattern but higher temperatures needed for decomposition

14. 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 √

15. 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 √

16. 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. √

17. 5.4 Compounds of Group II Elements cosmetics coins pipe Ship

18. 5.4 Compounds of Group II Elements Table 1:the melting points of the oxides of the Group II elements. 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.

19. 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 √

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

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

22. 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 √ √ √

23. 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 √ √

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

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

26. 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.

27. 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 √

28. 5.4 Compounds of Group II Elements Table 2: Active Ingredients in Commercial Antacid Tablets

29. 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 √

30. decreases down the group 5.4 Compounds of Group II Elements The solubility of some Group II metal compounds in mmol·dm-3 CO32- SO42- CrO42- C2O42- Mg2+ Ca2+ Sr2+ Ba2+ 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

31. ¾ ® NaCl(s) (g) + Clˉ (g) Na+ = D  H 781kJ•mol-1 － latt 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)

32. Hydration Enthalpy(Hhyd,水合热): The amount of energy relaeased when one mole of aqueous ions is formed from its gaseous ions. (g) + aq ¾ ® Na+ Na+ (aq) Hhyd = －406 kJ·mol-1 5.4 Compounds of Group II Elements

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

34. 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.