Alkali Metals Lecture 3 PDF

Alkali metals ( group IA) Li Lithium [He] 2s1 Na Sodium [Ne] 3s1 K Potassium [Ar] 4s1 Rb Rabidium [Kr] 5s1 Cs Cesium [Xe] 6s1 Fr Francium [Rn] 7s1 1  Group I elements: Lithium...

Alkali metals ( group IA) Li Lithium [He] 2s1 Na Sodium [Ne] 3s1 K Potassium [Ar] 4s1 Rb Rabidium [Kr] 5s1 Cs Cesium [Xe] 6s1 Fr Francium [Rn] 7s1 1  Group I elements: Lithium Sodium Potassium Rubidium Caesium 2 2 New Way Chemistry for Hong Kong A-Level Book 4 40.1 Characteristic Properties of the s-Block Elements (SB p.39) Some information about Group I elements Melting Boiling Group Atomic Ionic Crystal point point Density Abundance I metal radius radius structure (g cm–3) on earth (nm) (nm) (C) (C) (%) Li 0.152 0.060 b 180.5 1330 0.53 0.0020 Na 0.186 0.095 b 97.8 890 0.97 2.36 K 0.231 0.133 b 63.7 774 0.86 2.09 Rb 0.244 0.148 b 39.1 688 1.53 0.009 0 Cs 0.262 0.169 b 28.4 690 1.87 0.000 10 Fr 0.270 0.176 — 27 680 — Trace “b” denotes body-centred cubic structure 3 3 New Way Chemistry for Hong Kong A-Level Book 4  The name of group IA “Alkali metals” comes from the fact that their hydroxides are strong alkalis when they are dissolved in water. 2 M (s) +H2O 2 MOH (aq)+ H2(g)  Each alkali metal atom has a single electron in its outermost shell.  They tend to form singly charged positive ions (cations) when they react with nonmetals and form colorless crystalline ionic compounds that have high melting points. 4  Chemical properties  Alkali metals are very reactive, they are generally found in nature combined with other elements.  Their high reactivity is due to their larger atomic radii and low ionization energies (only one valence e -), thus they form ionic compounds easily.  Alkali metals are the most electropositive (the least electronegative) of elements.  In its chemical reactivity, lithium is less reactive than the other alkali metals with water, oxygen, and halogens and more reactive with nitrogen, carbon, and hydrogen. 5  a) Reaction with Nitrogen  Only lithium reacts with nitrogen to give lithium nitride 6 Li + N2 2 Li3N Li3N+ 3H2O 3 LiOH + NH3 https://www.youtube.com/watch?v=P4Vu9VJZFJE 6 40.1 Characteristic Properties of the s-Block Elements (SB p.41, notes p. 14) Formationof Formation ofBasic BasicOxides Oxides Group I elements Produce more than one type of oxides (except Li) All are ionic Three types of oxides: normal oxides (monoxides), peroxides, superoxides 7 7 New Way Chemistry for Hong Kong A-Level Book 4  b) Reaction with Oxygen 2Li + O2 2 Li2O normal oxide 6Na + 2 O2 2 Na2O + Na2O2 normal oxide peroxide 7K + 3 O2 2K2O + K2O2+ KO2 normal oxide peroxide superoxide normal oxide O2- Peroxide O22- Superoxide O2- These oxides are unstable in water. Li2O + H2O → 2LiOH Na2O2 + 2H2O → 2NaOH + H2O2 2KO2 + 2H2O → 2KOH + H2O2 + O2 8  c) Reaction with water 2 M (s)+2 H2O(l)⟶2 MOH (aq) +H2 (g)  All Alkali metals react with water to produce heat, hydrogen gas, and the corresponding metal hydroxide.  The heat produced by this reaction may ignite the hydrogen or the metal itself, resulting in a fire or an explosion.  The heavier alkali metals will react more violently with water. 9  d) Reaction with hydrogen 2 M (s)+ H2⟶2 (MH) (aq) They form ionic hydride e) Reaction with sulphur With sulphur, they form sulphides (M2S) and polysulphides (M2Sn). n = 2,3,4,5, or 6. 10  f) Reaction with carbon  Li is the only metal that reacts directly with carbon to give lithium carbide (dilithium acetylide).  Na and K can react with acetylene to give carbides (acetylides). 2Li + 2C Li2C2 ∆ Na + C2H2 NaC2H + 1/2H2 ∆ Na + NaC2H Na2C2 11  The metallic bond A metallic bond is the sharing of many detached electrons between many positive ions, where the electrons act as a "glue" giving the substance a definite structure. It is unlike covalent or ionic bonding. Metals have low ionization energy. Therefore, the valence electrons can be delocalized throughout the metals. Delocalized electrons are not associated with a particular nucleus of a metal, instead, they are free to move throughout the whole crystalline structure forming a "sea" of electrons. The sea of electrons is a negative charge cloud that attracts all of the positive ions. The metal ions would repel each other without the electron charge cloud, however the force of electrostatic attraction between the electrons and the positive ions holds the whole structure together. That’s why, metals often have a high melting or boiling points. The strength of metallic bonding is a function of the number of electrons provided by the atoms and the consequent charge on the metal ions. The ionic radius also plays a part, as smaller ions exert a greater force of attraction on the negative charge cloud. Increasing ionic charge = stronger metallic bonding Decreasing ionic radius = stronger metallic bonding 12  The effect of these two factors can be seen by comparing the melting points (the temperature needed to overcome the forces within the metal structure) down group 1 Group 1 metals Li Na K Rb Cs ionic radius / nm 0.068 0.098 0.133 0.148 0.167 melting point / K 454 371 337 312 302 It is clearly seen that as the ionic radius increases so the metallic bond strength and melting point decreases. Cesium would be a liquid on a warm summer day. 13  Differences between lithium and other group IA elements 1. Li has higher melting and boiling points compared to other group IA elements 2. Li is much harder. 3. Li is much less electropositive. 4. Li forms normal oxide while higher oxides are unstable. All the other alkali metal atoms form peroxides and superoxides. 5. Li is like group IIA elements reacts with nitrogen forming nitride, unlike group IA. 14 6. The ion (Li+) and its salts are heavily hydrated since its size is much smaller (higher charge density), which leads to lower electrical conductance in aqueous solutions. 7. Li has more tendency to form complexes because it is smaller. Complex formation decreases with increasing radius. 8. Due to Li small size and high ionization energy and electronegativity , it is harder for it to lose its valence electron and the covalent character of its compounds is greater than expected. 9. Li-halides and alkyls are soluble in organic solvents due to its covalent nature. 10. Li2CO3 ,Li3PO4, LiF are insoluble in water unlike same salts of the rest of alkali metals. 15 11. lithium is less reactive than the other alkali metals with water, oxygen, and halogens and more reactive with nitrogen, carbon, and hydrogen. 16  Industrial use  Sodium is the most important alkali metal in terms of industrial use. It is used in the reduction of organic compounds and in the preparation of many commercial compounds, including common Salt (NaCl), baking soda (NaHCO3), sodium carbonate (Na2CO3), and caustic soda (NaOH).  Potassium Potassium salts are consumed in considerable tonnages in the manufacture of fertilizers.  Lithium Li metal is used in certain light-metal alloys and as catalyst in organic syntheses. An important use of lithium is in the construction of lightweight batteries. 17 40.1 Characteristic Properties of the s-Block Elements (SB p.43) CharacteristicFlame Characteristic FlameColours Coloursof ofSalts Salts The outermost shell electrons of Group I & II elements are weakly held  The electrons can be excited to higher energy levels on heating  When electrons return to ground state, radiations are emitted  The radiations fall into the visible light region  The flame colour is a characteristic property of the element 18 18 New Way Chemistry for Hong Kong A-Level Book 4  Flame test  A flame test is an analytical procedure used to detect the presence of certain elements, primarily metal ions, based on each element's characteristic emission spectrum. https://www.youtube.com/watch?v=NEUbBAGw14k If you excite an atom by very strong heating, electrons can be promoted from their ground state to excited states. As they fall back down to the ground state (either in one go or in several steps), energy is released as light. The wavelength of the emitted light is in the visible region. 19 Flame test Li deep red Ca brick red Na yellow Sr blood red K lilac Ba apple green Rb bluish red Cs blue HCl(aq) sample