Inorganic Chemistry - Lecture 6 (PDF)

Inorganic Chemistry CHEM 205 – CHEM 208 Lecture 6 Group II The Alkaline Earth Metals This Lecture will demonstrate: ❖ General properties of the alkaline earth metals ❖ Extraction of the alkaline earth metals ❖ Chemical properties of the alkaline earth metals ❖ Anomalous behaviour of...

Inorganic Chemistry CHEM 205 – CHEM 208 Lecture 6 Group II The Alkaline Earth Metals This Lecture will demonstrate: ❖ General properties of the alkaline earth metals ❖ Extraction of the alkaline earth metals ❖ Chemical properties of the alkaline earth metals ❖ Anomalous behaviour of beryllium ❖ Important compounds and their reactions ❖ Uses of the alkaline earth metals  General properties of the alkaline earth metals Electronic Atomic radius Elements Symbol Ionic radius A⁰ Density g/cm3 structure A⁰ Beryllium Be [He]2s2 0.89 0.31 1.8 Magnesium Mg [Ne]3s2 1.36 0.65 1.7 Calcium Ca [Ar]4s2 1.74 0.99 1.6 Strontium Sr [Kr]5s2 1.91 1.13 2.6 Barium Ba [Xe]6s2 1.98 1.35 3.5 Radium Ra [Rn]7s2 1.50 5.0 General properties of the alkaline earth metals ❖ Reducing Properties All Group II metals have two electrons in their s valence orbital. They have poor control over their valence electrons so they are strong reducing agents but less than the corresponding Group I metals. Indeed Ca is often used in the laboratory and industry as powerful reducing agents for chemical reactions in non-aqueous solvents, such as liquid ammonia.  General properties of the alkaline earth metals ❖ Size The atoms are large, but are smaller than the corresponding Group I element since the extra charge on the nucleus draws the orbital electrons in. Similarly the ions are large, but are smaller than those of Group I, especially because removal of two orbital electrons increases the effective nuclear charge even further. Thus, these elements are smaller than and denser than Group I elements. ❖ Hardness Group II metals have two valency electrons which may participate in metallic bonding, hence the elements are harder than Group I, have higher cohesive energy, and have much higher melting points.  General properties of the alkaline earth metals ❖ Ionization energies The compounds are always divalent and ionic. Since the atoms are smaller than those in Group I, the electrons are more tightly held so that the energy needed to remove the first electron (first ionization energy) is greater than for Group I. Once one electron has been removed, the ratio of charges on the nucleus to orbital electrons is increased, so that the remaining electrons are more tightly held, and hence the energy needed to remove a second electron is nearly double that required for the first.  General properties of the alkaline earth metals ❖ Colour and magnetism of Group II compounds Since the divalent ions have an inert gas structure with no unpaired electrons, their compounds are diamagnetic and colourless. ❖ Flame tests  Extraction of the alkaline earth metals ❖ Extraction of Be Be elements can be obtained by electrolysis of molten chlorides , sodium chloride is added to lower the temperature. ❖ Extraction of Mg Mg elements can be obtained by electrolysis of sea water. ❖ Extraction of other elements Ca, Sr, Ba The production of Ca is by electrolysis of fused CaCl2 and CaF2; Sr and Ba are extracted by reduction of the corresponding oxides by Al, or by electrolysis of MCl2 (M = Sr, Ba) ❖ Extraction of other elements Ra It is produced as a result of the radioactive decay of heavier elements such as U-238.  Chemical properties of the alkaline earth metals ❖ Reaction with Water Group II metals are less reactive than those in Group I. Be is not typical and there is some doubt whether it reacts with steam water to form the oxide BeO, or fails to react at all. Magnesium can react slowly with hot water and the other metals Ca, Sr, and Ba react rapidly with cold water to form metal hydroxides and liberate hydrogen: Beryllium hydroxide Be(OH)2 is insoluble in water and is amphoteric. Amphoteric means, it reacts with acids and bases. For example, it dissolves in sodium hydroxide solution to give a colourless solution of sodium beryllate: With acids, beryllium hydroxide acts as a typical base, and beryllium salts are formed: The other group 2 hydroxides increase in basic strength in the order  Chemical properties of the alkaline earth metals ❖ Reaction with Oxygen Normal oxides All the elements in this group burn in oxygen to form a simple metal oxide MO: Beryllium oxide is amphoteric. It dissolves in acids to give salts, and in alkalis to give beryllium hydroxide. Magnesium oxide reacts with water forming magnesium hydroxide Mg(OH)2 which is weak base. Calcium oxide reacts very readily with water, and calcium hydroxide Ca(OH)2 is a moderately strong base, while strontium and barium hydroxides are even stronger bases. Peroxides There is an increase in the tendency to form the peroxide as you go down the Group. The ease with which the peroxides are formed increases with size. Beryllium, magnesium and calcium don’t form peroxides when heated in oxygen, but strontium and barium do. SrO2, BaO2.  Chemical properties of the alkaline earth metals ❖ Reaction with hydrogen All the elements except beryllium form hydrides MH2 by direct combination with hydrogen. They are all reducing agents that react with water and liberate hydrogen. Magnesium, Calcium, strontium and barium hydrides are ionic, and contain the hydrides ion H-. Ba + H2 = BaH2 Beryllium hydride is covalent and polymeric (BeH2 )n. ❖ Reaction with halogens All group II metals combine directly with the halogens at an appropriate temperature forming halides MX2. Ca + Cl2= CaCl2 Beryllium halide is covalent and polymeric (BeCl2 )n.  Chemical properties of the alkaline earth metals ❖ Complexes The most important complex formed by magnesium is chlorophyll, the green plant pigment that produces sugars and on which all life ultimately depends: The magnesium is at the centre of a flat organic ring system called a porphyrin in which four hetero cyclic nitrogen atoms are bonded to the magnesium.  Anomalous behaviour of beryllium ❖ Differences from group II elements Beryllium differs from the rest of the group because of its extremely small size and its high electronegativity. The following are examples of these differences: 1- Be forms complexes not typical of Group I and II. 2- Be is amphoteric. 3- Beryllium salts are acidic in pure water. The other Group II salts do not interact so strongly with water. 4- Be salts are among the most soluble known. 5- The halides and hydrides are polymeric, with multi centre bonding associated with electron deficiency. 6- Beryllium compounds have lower melting point (BeF2 m.p.800 ⁰C rest of group about 1300 ⁰C) and are soluble in organic solvents.  Anomalous behaviour of beryllium ❖ Diagonal relationship between beryllium and aluminium in group III Beryllium shows a diagonal relationship to aluminium in Group III because they have similar polarizing power: 1- Be like Al is rendered passive by nitric acid. 2- Be(OH)2 like Al(OH)3 is amphoteric. 3- Be salts like Al salts are extensively hydrolysed in water. 4- BeCl2 usually forms chains but also exists as the dimmer. AlCl3 is dimeric. = polymeric 5- Be2C, Like Al4C3, yields methane on hydrolysis.  Anomalous behaviour of beryllium ❖ Why are beryllium salts acidic when dissolved in water 4 The Be-O bond is strong, and this weakens the O-H bonds, hence there is a tendency to lose protons: Important compounds and their reactions Important compounds and their reactions  Uses of the alkaline earth metals ❖ Due to the light weight and strength of the beryllium, it has multi-structural uses in rockets and spacecraft. ❖ Magnesium is used as reducing agent in the extraction of silicon, titanium and beryllium, and in the manufacture of light-weight alloys for aircraft body. ❖ Calcium is used as reducing agent and, calcium carbonate is used as a mild cleanser in toothpaste - household detergents - acid treatment. ❖ Barium sulphate is medically used to diagnose intestinal disorders by x-rays (rays do not penetrate). Although barium salts are very toxic, they are poorly soluble and very few concentrations are used.  Assignment 5 ❖ The reduction potential of alkaline earth metals is ___________ alkali metals. a) may be equal to b) greater than c) less than d) equal to ❖ How many electrons do group 2 elements have in their S orbital of the valence shell? a) 1 b) 2 c) 3 d) 4  Assignment 5 ❖ Which of the following order is correct with respect to the Ionization energy? a) Ba > Mg > Ca < Sr > Be b) Be > Mg > Ca > Sr > Ba c) Be > Mg < Ca > Sr > Ba d) Be > Mg > Ca > Sr < Ba ❖ Amphoteric hydroxides react with both bases and acids. Which of the following Group II metal hydroxides is soluble in NaOH? a) Be(OH)2 b) Mg(OH)2 c) Ca(OH)2 d) Ba(OH)2