The s-block elements: Alkali and alkaline earth metals

Transcription

1 The s-block elements: Alkali and alkaline earth metals

2 Alkali metals The s-block elements:alkali and alkaline earth metals othe alkali family is found in the first column of the periodic table include; Li, Na, K, Rb, Cs, Fr. o Metallic elements. o Among the alkali metals; sodium and potassium are abundant and lithium, rubidium and caesium have much lower abundances. Francium is highly radioactive; its longestlived isotope 223 Fr has a half-life of only 21 minutes.

3 Alkali metals The s-block elements:alkali and alkaline earth metals oatoms of the alkali metals have a single electron in valence shell (1 valence electron) and electron configuration of 1s 1. osoft metals (easily cut with a knife). othey are the most reactive metals. othey react violently with water. oalkali metals are never found as free elements in nature. They are always bonded with another element.

4 Alkali metals The s-block elements:alkali and alkaline earth metals The metals are all very reactive* when pure. strongly reducing (very powerful reducing agents) In nature, these elements only ever appear as their +1 cations. Their cations (with the exception of lithium) have low charge densities. often leads to good solubility and an ability to stabilize anions that can not be formed along with any other cation. Cations occur in wide variety of important chemicals. Form ionic compounds with highly electronegative (nonmetals) found in Groups 6 (O, S, Se and 7 (F, Cl, Br, I, At), as well as P and N from Group 5. *Most alkali metals are stored under oil. But lithium is so light that it floats on oil. So a coat of petroleum jelly is applied to lithium before it is stored.

5 Physical Properties Lowest ionization energies and electronegativities in periodic table - easily ionized to form ions with +1 charge/+1 oxidation state.

6 Physical Properties They are good conductors of electricity. Their ability to conduct electricity is due to the availability of their outer electrons. Low densities. Low melting points and boiling points. Due to the presence of a single valence electron.

7 Flame tests o Alkali metal ions are identied by flame tests. o In flame ions in excited states return to their ground states emit light. o The emission spectrum depends on a number of factors; - the flame temperature; - but is most strongly is the energy levels of the emitter, leading to characteristic colours. Element Li Na K Rb Cs Flame colour crimson yellow lilac purple blue

8 Production of alkali metals o Lithium sodium and potassium metal are produced by electrolysis of the molten chlorides. o Overall reactions: LiCl (l) Li (l) + ½ Cl 2(g) NaCl (l) Na (l) + ½ Cl 2(g)

9 Chemical reactions The s-block elements:alkali and alkaline earth metals Reactivity towards Bronsted acids The alkali metals react even with weak Bronsted acids such as water The rate of reaction depends upon the acidity of the medium 2Na + 2H 2 O 2NaOH + H 2 2Na + 2EtOH 2NaOEt +H 2 2Na + 2NH 3 2NaNH 2 +H 2 very slow reaction in absence of catalyst

10 Oxides of the alkali metals oalkali metal on combustion in Oxygen M + O 2 depends on the alkali metal Li 2 O (oxide) Na 2 O 2 (peroxide) KO 2, RbO 2, CsO 2 (superoxide) o On combustion in excess of air, lithium forms mainly the oxide, Li 2 O (plus some peroxide Li 2 O 2 ), sodium forms the peroxide, Na 2 O 2 (and some superoxide NaO 2 ) whilst potassium, rubidium and caesium form the superoxides, MO 2. The s-block elements:alkali and alkaline earth metals othe increasing stability of the peroxide or superoxide, as the size of the metal ion increases, is due to the stabilisation of large anions by larger cations through lattice energy effects. The oxidation state of O?

11 Oxides of the alkali metals o These oxides and peroxides are colourless when pure, but the superoxides are yellow or orange in colour. The superoxides are also paramagnetic. Sodium peroxide is widely used as an oxidising agent in inorganic chemistry. oall oxides compounds react violently with H 2 O (hydrolysis) under formation of hydroxide Li 2 O (s) + H 2 O (l) 2 LiOH (s) Na 2 O 2(s) + 2 H 2 O (l) 2 NaOH (s) + H 2 O 2(l) 2 KO 2(s) + 2 H 2 O(l) 2 KOH (s) + H 2 O 2(l) + O 2(g)

12 Oxides of the alkali metals othe oxides are all basic. They react readily with acids (even weak acids) Na 2 O + H 2 O 2NaOH (the water acts as an acid) Na 2 O + HCl 2NaCl The alkali metal hydroxides are the strongest of all bases. othey react with CO 2 to form carbonates: Li 2 O (s) + CO 2(g) 2 Li 2 CO 3(s) 2 Na 2 O 2(s) + 2 CO 2(g) 2 Na 2 CO 3(s) + O 2(g) 4 KO 2(s) + 2 CO 2(g) 2 K 2 CO 3(s) + 3 O 2(g) KO 2 is used as CO 2 and H 2 O absorber in diving gear.

13 Reactivity of Alkali Metals with Group 5, Elements (N, P) o With Phosphorus 12 M + P 4 4 M 3 P (phosphides) o With Nitrogen 6 Li + N 2 2 Li 3 N (nitride) Only Li forms a nitride. The lattice energy for nitrides with larger alkali metal cations is not sufficient to make ΔG rxn <0.

14 Reactivity of Alkali Metals with Group 7 Elements Halogens o The reactivity of alkali metals increases as the ionization energy decreases Cs > Rb > K> Na > Li o Reactivity increases moving down the group. oexample: The s-block elements:alkali and alkaline earth metals o The resulting compounds are colorless, crystalline ionic salts called halides.

15 Reactivity of Alkali Metals with Group 7 Elements Halogens o The melting and boiling points always follow the trend: ofluoride > chloride > bromide > iodide. oall these halides are soluble in water. othe low solubility of LiF in water is due to its high lattice enthalpy. othe low solubility of CsI is due to smaller hydration enthalpy of its two ions. Other halides of lithium are soluble in ethanol and acetone; LiCl is soluble in pyridine also.

16 Some typical reactions Reaction with water: All alkali metals react with water to produce hydrogen and the metal hydroxide. The s-block elements:alkali and alkaline earth metals M (s) + H 2 O (l) M + (aq) + OH - (aq) + ½ H 2(g) The metals are called alkali metals because the resulting solution is alkaline owing to the presence of the hydroxide ion. Reaction of lithium with oxygen: 2Li (s) + ½ O 2(g) Li 2 O (s) Alkali metal compounds are almost ionic.

17 Some typical reactions Reaction with halogens (group 17: F 2, Cl 2, Br 2, I 2 ) All alkali metals react with halogens to form ionic halides. M (s) + 1/2 X 2 MX (s) Ex. 2Na (s) + Cl 2(g) 2NaCl (s) The most vigorous reaction occurs between the elements which are furthest apart on the Periodic Table: - the most reactive alkali metal, francium (Fr), at the bottom of Group 1, with the most reactive halogen, fluorine, at the top of Group 7.

18 Activity Series for Alkali Metals The most active metals are the most easily oxidized. Consequently, they are the best reducing agents. Activity decreases going down the group; ease of oxidation.

19 Basicity of oxides, sulfides nitrides Salts containing high charge density anions like O 2-, S 2-, N 3-, P 3- with low charge density cations (alkali metal and most alkaline earth metals) are basic S 2- + H 2 O HS - + OH - N H 2 O NH OH -

20 Hydration o An ion in solution is surrounded by water molecules. o Hydration enthalpy ( hydrh): Enthalpy change for the transfer of an ion from the gas phase to solution M + (g) M + (aq)

21 Solubility of alkali metal compounds o Alkali metals have relatively large, negative enthalpies of hydration. o Because they carry a single charge, the forces holding their crystals together, are less strong than those holding together crystals of more highly charged ions. o As a consequence, almost all alkali metal compounds are extremely soluble in water. (solubilities often reaching several hundred grams per litre). o Exception: some lithium compounds with highly charged anions, example, Lithium phosphate: 0.39 g L -1

22 Chemical reactions Summery of the general reactions of alkali metals

23 Lithium Lithium is the most reducing of all the alkali metals but not the most reactive. Unlike the other alkali metals; it reacts with nitrogen to form a nitride.

24 ANOMALOUS PROPERTIES OF LITHIUM The anomalous behaviour of lithium is due to the: (i) (ii) exceptionally small size of its atom and ion, and high polarising power (i.e., charge/ radius ratio). As a result, i. there is increased covalent character of lithium compounds which is responsible for their solubility in organic solvents. ii. Further, lithium shows diagonal relationship to magnesium which has been discussed later.

25 Points of Difference between Lithium and other Alkali Metals (i) Lithium is much harder. Its m.p. and b.p. are higher than the other alkali metals. (ii) Lithium is least reactive but the strongest reducing agent among all the alkali metals. On combustion in air it forms mainly monoxide, Li 2 O and the nitride, Li 3 N unlike other alkali metals. (iii) LiCl is deliquescent and crystallises as a hydrate, LiCl.2H 2 O whereas other alkali metal chlorides do not form hydrates.

26 (iv) Lithium hydrogencarbonate (LiHCO 3 ) is not obtained in the solid form while all other elements form solid hydrogencarbonates. (v) Lithium nitrate when heated gives lithium oxide, Li 2 O, whereas other alkali metal nitrates decompose to give the corresponding nitrite. 4LiNO 3 2Li 2 O + 4 NO 2 + O 2 2 NaNO 3 2NaNO 2 + O 2 (vi) LiF and Li 2 O are comparatively much less soluble in water than the corresponding compounds of other alkali metals.

27 Points of Similarities between Li and Mg o The similarity between Li and Mg arises because of their similar sizes: atomic radii, Li = 152 pm, Mg = 160 pm; ionic radii : Li + = 60 pm, Mg 2+ = 65 pm.

28 Points of Similarities between Lithium and Magnesium othe main points of similarity are: (i) Both lithium and magnesium are harder and lighter than other elements in the respective groups. (ii) Lithium and magnesium react slowly with water. -Their oxides and hydroxides are much less soluble and their hydroxides decompose on heating. -Both form a nitride, Li 3 N and Mg 3 N 2, by direct combination with nitrogen. (iii) The oxides, Li 2 O and MgO do not combine with excess oxygen to give any superoxide.

29 Points of Similarities between Lithium and Magnesium (iv) The carbonates of Li and Mg decompose easily on heating to form the oxides and CO 2. -Solid hydrogencarbonates are not formed by Li and Mg. (v) Both LiCl and MgCl 2 are soluble in ethanol. (vi) Both LiCl and MgCl 2 are deliquescent and crystallise from aqueous solution as hydrates, LiCl 2H 2 O and MgCl 2 8H 2 O.

30 Uses The most important use of lithium is in rechargeable batteries for mobile phones, laptops, digital cameras and electric vehicles. Lithium metal is used to make useful alloys, for example with lead to make white metal bearings for motor engines, with aluminium to make aircraft parts. Used in anti-depression medication. Organometallics of lithium are important.

31 Li- bipolar disorder About two million people in the US experience the extreme mood swings of bipolar disorder. During a manic phase, they think they can conquer the world. During a depression, they may feel hopeless. The ionic compound lithium carbonate often is used to control these symptoms. Lithium ions probably have an effect on the transmission of messages between brain cells.

32 In 1991, Sony Corporation released the first commercial lithium ion battery with a carbon anode. Nowadays, many devices use rechargeable lithium ion batteries and several cathode chemistries have been developed. Why Li?? Lithium is the lightest of all metals; has the greatest electrochemical potential and provides the largest specific energy per weight.

33 Li- ion battery Charge Anode Electrolyte Cathode (Carbon / graphite) (LiPF 6 in EC:DMC) LiCoO 2 Anode: Cathode: Li Li + + e - Co 4+ + e - Co 3+ The cathode reactions are solid state redox reactions.