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 Alkaline metal: ns2 Be Mg Ca Sr Ba Ra 12+ Mg 5.3 Group 2 BERYLLIUM STRONTIUM MAGNESIUM BARIUM CALCIUM RADIUM 5.3 Group 2 The Flame Color: Element Color Element Color Li Scarlet Be － Na Yellow Mg － K Lilac Ca Brick-red Rb Red Sr Crimson Cs Blue Ba Apple-green 5.3 Group 2 Increasing electronegativity Increasing electronegativity I II III IV Li Be B C Period 2 Na Mg Al Si Period 3 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 5.3 Group 2 The physical properties of Group 2: Electron Metallic First + Tm/K arrangement radius/nm Second IE/ kJ mol-1 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 5.3 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 . ..¨ O. ¨ . ..¨ ¨ O—O . ¨¨ . .. ..O—O ¨ ¨¨ oxygen ion peroxide ion superoxide ion 2 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. 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. 5.3 Group 2 ♦ Reaction with water Mg (s) + 2H2O (l) Mg(OH)2 (aq) + H2 (g) slowly Mg (s) + H2O (g) MgO (s) + H2 (g) steam 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 ♦ 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. 5.3 Group 2 ♦ 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. 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 Carbonates, CO32ˉ Nitrates, NO3ˉ Mg Ca Sr Ba Mg Ca Sr Ba MgCO3 → MgO + CO2 Same pattern but higher temperatures needed for decomposition The charge density (Z/r) of the cations (polarization) will affect the decomposition temperature. M(NO3)2 → MO + 2NO2 + 1/2O2 The larger value of Z/r, The easier breaking up of distorted anions: CO32- → CO2 + O2NO32- → 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 diagonal relationship √ C. Calcium D. Magnesium E. Strontium 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 Ship cosmetics pipe 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. 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 5.4 Compounds of Group II Elements CaCO3(limestone) Δ + CO2 Ca(OH)2(slaked lime) + H2 O CaO(lime) 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. 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 √ +(aq) → Ca2+(aq) + 2NH + 2H O 3. Ca(OH) + 2NH 2 4 3 2 √ 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 Soft water: Ca2+, Mg2+, HCO32- “temporary hardness” Δ Ca2+ (aq) + 2HCO3ˉ(aq) → CaCO3(s) + CO2(g) + H2O(l) Δ Mg2+ (aq) + 2HCO3ˉ(aq) → MgCO3(s) + CO2(g) + H2O(l) Hard water: Ca2+, Mg2+, SO42-, ClCa2+(aq) + SO42-(aq) → CaSO4(s) “permanent hardness” 5.4 Compounds of Group II Elements Ca2+ (aq) + 2C17H35COOˉ(aq) → Ca(C17H35COO)2 (s) stearate calcium stearate Mg2+ (aq) + 2C17H35COOˉ(aq) → Mg(C17H35COO)2 (s) stearate magnesium stearate scum 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. 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 5.4 Compounds of Group II Elements Table 2: Active Ingredients in Commercial Antacid Tablets Chemical Name Magnesium Hydroxide Calcium Carbonate Sodium Bicarbonate Aluminum Hydroxide Dihydroxyaluminum Sodium Carbonate Chemical Formula Chemical Reaction Mg(OH)2 Mg(OH)2 + 2H+ → Mg2+ + 2H2O CaCO3 CaCO3 + 2H+ → Ca2+ + H2O + CO2 (g) NaHCO3 NaHCO3 + H+ → Na+ + H2O + CO2 (g) Al(OH)3 Al(OH)3 + 3H+ → Al3+ + 3H2O NaAl(OH)2CO3 NaAl(OH)2CO3 + 4H+ → Na++ Al3++ 3H2O + CO2(g) 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 CO32- SO42- CrO42- C2O42- Mg2+ 1.5 1830 8500 5.7 Ca2+ 0.13 47 870 0.05 Sr2+ 0.07 0.71 5.9 0.29 Ba2+ 0.09 0.009 0.01 0.52 decreases down the group 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DHlatt 。 The enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions under standard conditions (298K，100 kPa) Na+ (g) + Clˉ (g) NaCl(s) D H latt = －781 kJ•mol-1 5.4 Compounds of Group II Elements Hydration Enthalpy(DHhyd ,水合热): The amount of energy relaeased when one mole of aqueous ions is formed from its gaseous ions. Na+ (g) + aq Na+ (aq) DHhyd = －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). NaCl (s) DHsolu Na+(aq) + Clˉ(aq) DHlatt = －776 kJ·mol-1 DHhyd = －772 kJ·mol-1 Na+(g) + Clˉ(g) DHsolu = DHhyd －DHlatt 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 DHlatt does not cause a significant change in the but the DHhyd 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 DHhyd does not cause a significant change in the but the DHlatt 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.