inorganic chemistry : group 2

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  1. 1. PRE-UNIVERSITY SEMESTER 2 CHEMISTRY CHAPTER 4 : GROUP 2
  2. 2. 4.1 Physical Properties of Group 2 Group 2 are also known as alkali earth metal. The elements of Group 2 and some basic physical properties are described as below Name , symbol Z Atomic radius/ nm Melting point (oC) 1st ionisation energy (kJ/mol) Electronic configuration Beryllium Be 4 0.112 1287 900 1s2 2s2 Magnesium Mg 12 0.160 650 738 1s2 2s2 2p6 3s2 Calcium Ca 20 0.197 842 590 1s2 2s2 2p6 3s2 3p6 4s2 Strontium Sr 38 0.215 777 550 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 5s2 Barium, Ba 56 0.218 727 503 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s2
  3. 3. 4.1.1 Atomic radius Atomic radius depend on 2 factors Nuclear charge Screening effect When going down to Group 2, both screening effect and nuclear charge increase. However, the increase in screening effect is more significant, as more shell is used to filling in the electrons. This will cause the effective nuclear charge to decrease, resulting the electron cloud to be further away from the nucleus. Hence atomic radius increase. 4.1.2 Melting point The melting point of the Group 2 generally decrease when goes down to group. All the elements occur as hexagonal closed-packed structures with the exception of barium and radium, which adopt the more open body-centred cubic structure. The density decreases from Be to Mg to Ca as a result of very strong metallic bonding in the Group 2 elements, which leads to short metalmetal distances in the lighter elements (225 pm in beryllium, for instance) and as a result small unit cells.
  4. 4. 4.1.3 Ionisation Energy. The 1st ionisation energy decrease when goes down to Group 2. Atomic size when goes down to Group 2 which contribute the decrease in ionisation energy. Furthermore, with the increase in atomic size, the number of shell also increase thus causing the screening effect to increase. This may also affected the effective nuclear charge as the distance between the electron and the nucleus is getting further. The third ionisation energy of Group 2 elements are extremely high, which suggested that the 3rd electron the withdrawn from an inner shell. Thus Group 2 elements only goes through 2nd ionisation energy and form a stable M2+. First Ionisation energy : M (g) M+ (g) + e- Second Ionisation energy : M+ (g) M2+ (g) + e- Element Be Mg Ca Sr Ba 1st IE (kJ/mol) 900 740 590 550 500 2nd IE (kJ/mol) 2700 2190 1740 1610 1470
  5. 5. 4.2 Chemical Properties of Group 2 Table below shows the E0 value of Group 2 elements, When going down to Group 2, E0 value become more negative, indicates the reducing ability increase when going down to group, hence stronger reducing agent. This may also indicates the reactivity increase when going down to Group 2. Henceforth, we shall discuss the reactivity of Group 2 elements with air (oxygen) and water. Element Be Mg Ca Sr Ba Eo / V - 1.85 -2.37 -2.87 -2.89 - 2.90 Trend of reducing agent Reducing strength increased
  6. 6. 4.2.1 Reaction of Group 2 elements with oxygen (air). The Group 2 elements react with O2 to form the oxides. All the elements except Be also form unstable peroxides (MO2). The oxides of Mg to Ra react with water to form the basic hydroxides while BeO and Be(OH)2 are amphoteric When BeO act as base : BeO + 2 H+ Be2+ + H2O When BeO act as acid : BeO + 2 OH- + H2O Be(OH)4 2- When Be(OH)2 act as base : Be(OH)2 + 2 H+ Be2+ + 2 H2O When Be(OH)2 act as acid : Be(OH)2 + 2 OH- Be(OH)4 2- On its nature, Beryllium is inert in air as its surface is passivated by the formation of a thin layer of BeO. Magnesium and calcium metals also tarnish in air with the formation of an oxide layer, but will burn completely to their oxides when heated. Strontium and barium, especially in powdered forms, ignite in air and are stored under hydrocarbon oils
  7. 7. The oxides of the other Group 2 elements can be obtained by direct combination of the elements (except Ba, which forms the peroxide) Their melting points decrease down the group as the lattice enthalpies decrease with increasing cation radius. Magnesium oxide is a high-melting-point solid (as is BeO) and is used as a refractory lining in industrial furnaces. Like BeO, MgO has a high thermal conductivity coupled with a low electrical conductivity. This combination of properties leads to its use as an electrically insulating material around the heating elements of domestic appliances and in electrical cables Element Reaction with oxygen Reactivity Melting point of oxide Be 2 Be + O2 2 BeO Mg 2 Mg + O2 2 MgO Ca 2 Ca + O2 2 CaO Sr Sr + O2 SrO Ba Ba + O2 BaO2 INCREASE DECREASE
  8. 8. The peroxides of Mg, Ca, Sr, and Ba are prepared by a variety of routes; only SrO2 and BaO2 can be made by direct reaction of the elements. All the peroxides are strong oxidizing agents and decompose to the oxide: 2 MO2 (s) 2 MO(s) + O2 (g) Special note : *The thermal stability of the peroxides increases down the group as the radius of the cation increases. This trend is explained by considering the lattice enthalpies of the peroxide and the oxide, and their dependence on the relative radii of the cations and anions. As O2 is smaller than O2 2, the lattice enthalpy of the oxide is greater than that of the corresponding peroxide. The difference between the two lattice enthalpies decreases down the group as both values become smaller with increasing cation radius, therefore the tendency to decompose decreases. Magnesium peroxide, MgO2, is consequently the least stable peroxide
  9. 9. 4.2.1.1 Reaction of Group 2 oxide with water : Properties of Group 2 hydroxide Beryllium oxide, BeO, is a white solid, which is insoluble in water, with coordination number of 4, as expected for the small Be2+ ion. The oxides of the other Group 2 elements all adopt coordination number of 6. This is due to Beryllium does not have empty d- orbital available to coordinate more than 8 electrons at its center, while other Group 2 elements have. Magnesium oxide is insoluble but reacts slowly with water to form Mg(OH)2; likewise CaO reacts with water to form the partially soluble Ca(OH)2. The oxides of Sr and Ba, SrO and BaO, dissolve in water to form the strongly basic hydroxide solutions: BaO(s) + H2O (l) Ba2+ (aq) + 2OH- (aq)
  10. 10. Element Reaction of metal oxide with water Rate of formation of base Be no reaction / does not dissolve Mg MgO + H2O Mg(OH)2 Ca CaO + H2O Ca(OH)2 Sr SrO + H2O Sr(OH)2 Ba BaO + H2O Ba(OH)2 INCREASE
  11. 11. 4.2.1 Reaction of Group 2 elements with water. All Group 2 react with water to from metal (II) hydroxide, M(OH)2, with hydrogen gas liberated The reactivity of Group 2 with water increase (as suggested by their E0 value). Beryllium react slowly under hot steam to form a white precipitate of beryllium hydroxide. Magnesium reacts similarly as beryllium does, however, compare to Be, the rate of reaction is higher. Magnesium hydroxide, Mg(OH)2, is basic but only very sparingly soluble; beryllium hydroxide, Be(OH)2, is amphoteric and in strongly basic solutions it forms the tetrahydroxyberyllate ion, Be(OH)4 Calcium react slowly with water under room condition, to form a cloudy calcium hydroxide (also known as lime water). Limewater is well known to test the presence of carbon dioxide, where CO2 will turn limewater chalky and form white precipitate of calcium carbonate, which then dissolved when on further reaction with CO2 to form the hydrogencarbonate (also known as bicarbonate) ion Ca(OH)2 (aq) + CO2 (g) CaCO3 (s) + H2O (l) CaCO3 (s) + H2O (l) + CO2 (g) Ca(HCO3)2 (aq) Strontium and barium can react even in cold water to form a water soluble strong base of strontium hydroxide and barium hydroxide respectively. However, rate of reaction of barium is greater than strontium, hence more vigorous
  12. 12. Element Condition of water Reaction equation Rate of reaction Ksp (mol3 dm-9) Solubility Be Hot steam Be + 2 H2O Be(OH)2 + H2 6.92 x 10-22 Mg Hot steam Mg + 2H2O Mg(OH)2 + H2 5.61 x 10-12 Ca Water at room temperature Ca + 2 H2O Ca(OH)2 + H2 5.50 x 10-6 Sr Cold water Sr + 2 H2O Sr(OH)2 + H2 7.24 x 10-6 Ba Cold water Ba + 2H2O Ba(OH)2 + H2 2.54 x 10-4INCREASE INCREASE
  13. 13. Element Group 2 carbonate Group 2 Nitrate Formula Decomposition temperature Stability Formula Stability Be BeCO3 1590C Be(NO3)2 Mg MgCO3 3500C Mg(NO3)2 Ca CaCO3 8320C Ca(NO3)2 Sr SrCO3 13400C Sr(NO3)2 Ba BaCO3 14500C Ba(NO3)2 INCREASE INCREASE 4.3 Thermal Decomposition of Nitrates and Carbonates All nitrates of the Group 2 elements are decomposed by heat to form metal oxides, nitrogen dioxide and oxygen gases. 2 M(NO3)2 (s) 2 MO (s) + 4 NO2 (g) + O2 (g) All carbonates of the alkaline-earth metals also decompose on heating, producing metal oxides and releasing carbon dioxide gas. MCO3 (s) MO (s) + CO2 (g)
  14. 14. The thermal stabilities of Group 2 nitrates and carbonates increase down the group from beryllium to barium. This means that the temperature needed to decompose the nitrates and carbonates increases down the group. The trend of decomposition for Group 2 nitrate and carbonate can be explained below Magnesium nitrate and magnesium carbonate decompose easily at low temperatures. This shows that the metal oxide is more stable than the nitrate and carbonate. This can be explained by the fact that the size of the oxide ion, O2-, is smaller than that of the nitrate, NO3 -, and carbonate, CO3 2- ions. As such, the oxide ion can approach closer to the Mg2+ cation forming a shorter and stronger bond Magnesium nitrate, MgNO3 Magnesium nitrate, MgCO3
  15. 15. Besides, magnesium ion has a high charge density ratio giving the ion a high polarisation power To polarise the electron clouds of the nitrate and carbonate ions. The electron clouds of the NO3 - ion and CO3 2- ion are easily distorted, rendering the nitrogen-oxygen bonds in the NO3 - ions and t

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