Presentation on theme: "Group 2: Alkaline Earth Metals (Be, Mg, Ca, Sr, Ba, Ra ) Most are metals that react with water to give X(OH) 2 and with oxygen to give XO. Reactivity increases."— Presentation transcript:
Group 2: Alkaline Earth Metals (Be, Mg, Ca, Sr, Ba, Ra ) Most are metals that react with water to give X(OH) 2 and with oxygen to give XO. Reactivity increases with atomic mass. Beryllium does not react with water (Highly toxic) Do not exist in pure form in nature due to high reactivity Readily lose 2 electrons to make cations with +2 charge
Alkaline Earths? Elements of Group 2 i) Form stable imsoluble oxides and hydroxides hence “earths” ii) Harder, denser, higher boiling and higher melting than the alkali metals. 2 Melting Point Boiling Point Density (at 20 °C) Lithium180.5 °C1347 °C0.534 g/cm 3 Beryllium1278 °C>3000 °C1.85 g/cm 3 Magnesium648.8 °C1090 °C1.74 g/cm 3 Calcium839 °C1484 °C1.55 g/cm 3 Strontium769 °C1384 °C2.54 g/cm 3 Barium729 °C1637 °C3.60 g/cm 3 Cesium28.4 °C678.5 °C1.873 g/cm 3 Images from
Ions of Alkaline Earths First Ionization Energy (kJ/mol) Second Ionization Energy (kJ/mol) Standard Reduction Potential (V = J/C) Lithium Beryllium Magnesium Calcium Strontium Barium Cesium Only forms +2 cations i) Has two valence electrons: [N.G.] ns 2 ii) Low first and second ionization energies iii) Excellent reducing agents.
Testing For Alkaline Earths Flame ColourStrength of Reaction with Water BerylliumNoneNo reaction MagnesiumNone (Magnesium metal heated in a flame burns bright white, but this is a combustion reaction not a flame test.) Calcium Strontium Barium 4 Flame test. Some of the alkaline earth metals give positive flame tests. Reactivity with water. Most of the alkaline earth metals react exothermically with water to give the corresponding hydroxide and hydrogen gas.
Reactions with Alkaline Earth Reaction with oxygen: Reaction with nitrogen: For the alkali metals only lithium reacts with N 2. Reaction with halogens: Reactivity increases down Group 2. i.e. Beryllium is significantly less reactive than all the other alkaline earth metals. It only burns if powdered and heated, and it only reacts with halogens on heating. 5
Reactions of Alkaline Earth Metals with Acids The alkaline earth metals react (quite violently!) with acid to produce hydrogen gas and the corresponding salt: Exercise: What volume of hydrogen gas is produced if 2.50 g of magnesium metal is dissolved in excess hydrochloric acid in a fumehood with a temperature of 25 °C and a pressure of 1 bar (100 kPa)? 6
Why M 2+ Cations? If it requires more energy to make M 2+ than M +, why do we always see alkaline earth metal cations as M 2+ ? Look at the Born-Haber cycles for (hypothetical) MgCl vs. MgCl 2 ! Calculate the enthalpy of formation for each. Which is more stable? 8 Mg (s) Δ subl H I1I1 Mg (g) Mg + (g) ½ Cl 2(g) ½ Δ BD H Δ EA H Cl (g) Cl - (g) MgCl (s) Δ LF H + + ΔfHoΔfHo -676 kJ/mol ½(+243 kJ/mol) -349 kJ/mol kJ/mol +148 kJ/mol
The Alkaline Earth Metals (Group 2) 9 Mg (s) Δ subl H I1I1 Mg (g) Mg + (g) Cl 2(g) Δ BD H 2Δ EA H 2 Cl (g) 2 Cl - (g) MgCl 2(s) + + ΔfHoΔfHo Mg 2+ (g) I2I kJ/mol 2(-349 kJ/mol) kJ/mol kJ/mol +148 kJ/mol Δ LF H kJ/mol
Aqueous Reactions with Acids and Bases Consider what happens when we add BeO, MgO or Al 2 O 3 to water: BeO(s) + H 2 0 ? What about adding them to aqueous acid? BeO(s) + H H ? What about adding them to aqueous base? BeO(s) + H OH - ? 10
Applications of Alkaline Earths 11 Limestone (mostly CaCO 3 but also some MgCO 3 ) and lime (CaO) are of particular economic importance: Limestone is spread on fields to neutralize acidic compounds in soil and provide Ca 2+ and Mg 2+. Limestone can also be thermally decomposed to make lime: Historically, lime was mixed with sand and water to make mortar (used to bind stone or bricks):
Portland Cement 12 Portland cement is the basis of all modern mortars and concrete. Lime and shales (mixtures of aluminosilicates) are ground together then heated to 1500 °C., releasing CO 2 to form clinker. The clinker is ground to a fine powder, stabilized by adding calcium sulfate (CaSO 4 ) 26% Ca 2 SiO 4, 51% Ca 3 SiO 5, 11% Ca 3 Al 2 O 6 When adding a complex hydration reactions take place: The main product of this hydration is called tobermorite gel which forms crystals that bind sand and gravel added to give volume and strength.
Cement Plant 13 The main advantage of cement over lime mortar is that is doesn’t need external carbon dioxide to set. The photo below is of the Lafarge Cement Plant at Exshaw (just east of Banff National Park). The limestone is harvested directly behind the plant; the shales come from Seebe, 7km downstream along the Bow river. Photo by Prof. T.W. Swaddle, Dept. of Chemistry, University of Calgary
Solubility of Alkaline Earth Salts Calcium carbonate CaCo 3 – like all other carbonates and sulfates of the alkaline earth metals – is insoluble in neutral water. Why is that? These carbonates are, however, soluble in acid, reacting to produce carbon dioxide: Barium sulfate is can be used as a contrast agent for x-rays, however it must be completely free of barium chloride or barium carbonate. Why? The dissolved barium cation is poisonous. 14
Water Hardness Calcium and magnesium cations are present in “hard water”, as it is is typically acidic due to its dissolved CO 2 : Thus, carbonates that would otherwise be insoluble (primarily MgCO 3 and CaCO 3 ) dissolve in the water: When this water is heated, the solubility of the carbon dioxide decreases? This leaves “hard” rings in your pots, kettles, bathtubs, etc. Soaps precipitate. 15
Water Softeners Water is typically “softened” by ion-exchange- calcium and magnesium cations are exchanged with sodium cations. As sodium carbonate is soluble in water – acidic, basic or neutral – Also at low concentrations does not cause soap to precipitate etc. 16 Image from
Group 13 Metals Boron is a metalloid much harder than the other elements in the group Between corundum (9) and diamond (10) on the Mohs scale; aluminum has a hardness of 2.75 – slightly harder than fingernails) and has much higher melting points. All other elements in the group are metals. 17 Melting Point Boiling Point Density (at 20 °C) Boron2075 °C4000 °C2.34 g/cm 3 Aluminum °C2467 °C2.698 g/cm 3 Gallium29.78 °C2403 °C5.907 g/cm 3 Indium °C2080 °C7.310 g/cm 3 Thallium °C1457 °C11.85 g/cm 3 Images from Ga
Group 13 Valence Three valence electrons Al is [Ne]3s 2 3p 1 Ga is [Ar]4s 2 3d 10 4p 1 All can form +3 cations. Indium and thallium can also form +1 cations. Aluminum is an excellent reducing agent with a standard reduction potential of V Thallium is extremely toxic 18
Group 13 Reactions Reaction with oxygen: Forms passivation layer: a thin layer of oxide that forms on the surface of the metal protects it from rusting. Aluminum, this layer is thickened (up to 0.02 mm) by an electrolytic process called anodic oxidation. Anodized aluminum is also harder than regular aluminum. Reaction with water: Not at room temperature. The thin oxide layer on the metal’s surface protects it. 19
The Metals of Group 13 Reaction with acid: Oxidizing acids (e.g. HNO 3 * ) do not react with group 13 metals because the surface aluminum is already oxidized into a protective coating. Strong non-oxidizing acids (e.g. HCl) dissolve group 13 metals, producing hydrogen gas: Reaction with base: 20
The Metals of Group 13 Aluminum oxide, (“corundum”) is not just a protective layer. One of its forms is extremely hard and therefore useful as an abrasive and polishing material. The main component of rubies and sapphires, when chromium(III) and titanium(IV), resp. “Activated alumina” used in chromatography. This form of aluminum oxide is synthetic, prepared by heating aluminum hydroxide to dehydrate it: 21
Solubility of Aluminum Aluminum hydroxide is amphoteric. i.e. It acts as either an acid or a base. Aluminum hydroxide is insoluble in water. Why is that? It reacts with aqueous base, though, to give a soluble anion: It also reacts with aqueous acid to give a solvated cation: 22
Aluminum is not found in pure form in nature. Produced by electrolysis of Alumina Bauxite ore contains mostly Al(OH) 3 (gibbsite) and/or AlO(OH) (diaspore or boehmite); Iron(III) oxide (Fe 2 O 3 ) and other contaminants are removed before electrolysis. The Bayer Process: i) The ore is dissolved in hot sodium hydroxide solution. ii) The “Red mud” is removed from solution by filtration. iii) Al(OH) 3 is precipitated from saturated solution by seeding crystallization with Al(OH) 3 crystals. iv) Al(OH) 3 is heated to 1200 o C and converted to Al 2 O Production of Alumina by the Bayer Process
Pure aluminum oxide can be melted and electrolyzed. High melting point 2020 °C !! Problem! Aluminum oxide can be dissolved in the molten cryolite (Na 3 AlF 6 ) at 1000 °C and is also a good conductor. It costs twenty times as much to produce aluminum from bauxite Recycling it, due to the energy costs electrolysis. 24 Aluminum Production The Hall-Heroult process