The S-Block Elements

The s-Block Elements 1) What are the common physical features of alkali metals? Solution- i) Common physical features of alkali metals: i) Alkali metals are silvery white, soft and light metals. ii) They have low density , which increases from Li to Cs down the group. iii) The general outer electronic configuration of all alkali metals in ns 1 (n= 2 to 7). iv) The atoms of alkali metal have large atomic size in a particular period. The atomic size increases from Li to Cs down the group due to addition of extra shell from element to the next. v) They have low ,melting and boiling points due to weak metallic bonding. vi) They have low ionisation enthalpy due to large atomic size. The ionization enthalpy decreases down the group from Li to Cs as atomic size increases. vii) The hydration enthalpies of alkali metal ions decreases with increases in ionic size. Li + > Na + > K + >Rb + > Cs + ii) Common chemical features of alkali metals: i) Alkali metals are highly reactive due to low ionisation enthalpy and large atomic size. The reactivity decreases down the group as ionization enthalpy decreases. ii) Alkali metal tarnish in dry air due to the formation of oxides, which, in turn, reacts with moisture to form hydroxides. iii) Alkali metals burn vigorously in oxygen forming oxides. Lithium forms monoxide, sodium forms peroxide and other form superoxides. iv) They react with water to form metal hydroxides and liberate hydrogen gas. 2M + 2H 2O →2MOH + H 2 (M= Li, Na, K, Rb, Cs) iv) Alkali metals react vigorously with halogens to form ionic halides of type MX (except for lithium , which forms covalent halides) 2M + X 2 →2MX (M= Li, Na, K, Rb, Cs) 2) Discuss the general characteristics and gradation in properties of alkaline earth metals. Solution- General characteristics of alkaline earth metals and gradation in properties: i) All the alkaline earth metals have a general outer electronic configuration of ns 2 (n= 2 to 7). ii) The atomic and ionic radii of the alkaline earth metals are smaller than those of the corresponding alkali metals belonging to the same period due to an increase in the nuclear charge. The atomic and ionic radii increase from Be to Ra due to the addition of shells from one element to the next, as the atomic number increases down the group. iii) The alkaline earth metals have low ionization enthalpies due to to large atomic size of their atoms. Their ionisation enthalpies decrease down the group due to an increases in atomic size. iv) The hydration enthalpies of alkaline earth metal ions decrease with an increase in ionic size down the group. Be 2+ > Mg 2+ > Ca 2+ >Sr 2+ > Ba 2+ 3) Why are alkali metals not found in nature? Solution Alkali metals are not found in a free state in nature due to their high reactivity. They always exist in a combined state with another element in the form of mineral s and ores in the earth’s crust. 4) Find out the oxidation state of sodium in Na 2O2. Solution In Na 2O2 (sodium peroxide) each oxygen has -1 oxidation state due to peroxy bond. ∴ The oxidation state of sodium is 1 in Na 2O2 . Let the oxidation state of Na be x . Then 2x -2 = 0 2x = 2 x= 2/2 = +1 Oxidation state of Na in Na 2O2 is +1. 5) Explain why is sodium less reactive than potassium. Solution Sodium is less reactive than potassium, due to its bigger size and low ionisation enthalpy. Potassium has a greater tendency to lose an electron and get oxidised than sodium. 6) Compare the alkali metals and alkaline earth metals with respect to ionization enthalpy. Solution i) Ionization enthalpy: Alkaline metals Alkaline earth metals The ionisation enthalpy of The ionisation enthalpy of alkali metals is low due to alkaline earth metals is also their large atomic size. low, but they are higher than Example : alkaline earth metals Li -520 kJ mol -1 corresponding to the alkali Na – 496 kJ mol -1 metals of the same period due to large nuclear change and their small size compared to alkali metals. Example: Be- 899 kJ mol -1 Mg- 737 kJ mol -1 ii) Basicity of oxides: Alkaline metals Alkaline earth metals The oxides of alkali metals The oxides of alkaline earth are strongly basic and form metals are basic, but not as very strong bases when strongly basic as alkali metal dissolved in water. oxides. The oxides of Example: alkaline earth metals are less Na 2O + H 2O →2NaOH basic than alkaline metal K2O + H 2O → 2KOH oxides. Example : CaO + H 2O →Ca (OH) 2 BaO + H 2O →Ba (OH) 2 iii) Solubility of hydroxides: Alkaline metals Alkaline earth metals The hydroxides of alkali The hydroxides of alkaline metals are highly soluble in earth metals are less or water: sparingly soluble in water. Example : The solubility increases NaOH , KOH down the group as the atomic number of the elements increases. Example: Ca(OH) 2 , Mg (OH) 2 7) In what ways lithium shows similarities to magnesium in its chemical behaviour? Solution i) Li and Mg, both react slowly with water. ii) The oxides and hydroxides of Li and Mg are less soluble, and the hydroxides decompose on heating. iii) Li and Mg, both combine with nitrogen directly to form nitrides, Li 3N and Mg3N2. iv) Li and Mg , do not form superoxides on combining with excess oxygen. v) The carbonates of Li and Mg are less stable and decompose easily on heating to form oxide with the liberation of CO 2 gas. vi) Li and Mg do not form solid hydrogen carbonates. vii) LiCl and MgCl 2 are deliquescent and crystallise, from their aqueous solution as hydrates, LiCl.2H 2O and MgCl 2.8H 2O. 8) Explain why can alkali and alkaline earth metals not be obtained by chemical reduction methods? Solution Alkali and alkaline earth metals cannot be obtained by chemical reduction methods as they are strong reducing agents and other reducing agents cannot reduce them . this is due to the low ionization potential of these elements and their tendency to lose electrons easily. This is evident from their large negative reduction potential value. 9) Why are potassium and caesium, rather than lithium used in photoelectric cells? Solution The atomic size of lithium is small, because of which the electron requires more energy to be removed from its valence shell than potassium and caesium. Therefore, potassium and caesium which can lose valence electrons by absorbing light of less energy, are used in photoelectric cells. 10) When an alkali metal dissolves in liquid ammonia the solution can acquire different colours. Explain the reasons for this type of colour change. Solution When an alkali metal dissolves in liquid ammonia, the solution acquires deep blue colour, because the ammoniated electron absorb energy in the visible region of light. In a concentrated solution , the blue colour changes to bronze. + - M + (x+y) NH 3 →[M(NH 3)x] + [e(NH 3)y] Ammoniated electron 11) Beryllium and magnesium do not give colour to flame where as other alkaline earth metals do so why? Solution The electrons in Be and Mg atoms are strongly bound and cannot be excited by the flame due to their small size compared to other elements. Therefore , Be and Mg do not give colour to the flame. 12) Discuss the various reactions that occur in the Solvay process. Solution The Solvay process is used to prepare sodium carbonate (washing soda). The principal reactions that take place in the Solvay ammonia process are: i) 2NH 3 +H 2O+CO 2→(NH 4)2CO 3 ii) →(NH 4)2CO 3+ H2O+CO 2→2NH 4HCO 3 iii) NH 4HCO 3 +NaCl → NH 4Cl + NaHCO 3 iv) 2 NaHCO 3→ Na 2CO 3 + H2O+CO 2 13) Potassium carbonate cannot be prepared by Solvay process. Why? Solution Potassium carbonate cannot be prepared by the Solvay process, as potassium hydrogen carbonate is soluble in water , and cannot b precipitated by adding of ammonium hydrogen carbonate to a saturated solution of potassium chloride. 14) Why is Li 2CO 3 decomposed at a lesser temperature whereas Na 2CO 3 at higher temperature . Solution + + Li 2CO 3 decomposes at a lower temperature due to the small size of the Li ion . The small size of Li , makes the lattice of Li 2O more stable than that of Li 2CO 3 . This results in the decomposition of Li 2CO 3 to Li 2O and CO 2 . Therefore, Li 2CO3 is not thermally stable. On the other hand, Na 2CO3, due to its larger size, forms a thermally stable lattice. 15) Compare the solubility and thermal stability of the compounds of the alkali metal with those of the alkaline earth metal nitrates. Solution i) Alkaline metals Alkaline earth metals Solubility : The nitrates of alkaline earth The nitrates of alkali metals metal decompose on heating are soluble in water. The to form metal oxide with the water solubility increases liberation of nitrogen dioxide down the group as the lattice and oxygen. enthalpy decrease rapidly than the hydration enthalpy. 2M(NO 3)2 → 2MO Thermal stability: + 4NO 2 + O 2 The nitrates of alkali metals, Alkali metal nitrate Alkali except lithium, decompose metal oxide on heating , to form the corresponding nitrites with the evolution of oxygen gas. (M=Be, Mg, Ca, Sr, Ba) However, lithium nitrate, on heating, decomposes to form lithium oxide, nitrogen dioxide and oxygen. 2MNO 3 → 2MNO 2 + O 2 Alkali metal nitrite Alkali metal nitrate ii) Carbonates: Alkaline metals Alkaline earth metals Solubility : Solubility : Alkali metal carbonates are Alkaline earth metal soluble in water.

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