2.2 Group 2, the alkaline earth metals

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  • Chemistry 2.2 Group 2, the alkaline earth metals
    • Intro
      • All metals with a shiny silvery white appearance
      • Compounds all found in Earth's crust and widely distributed in rock structures
      • Ca  and Mg  are most common, then Sr and Ba and finally Be and Ra are rare
      • Alternative name, abalone earth metals, comes from their metal oxides
    • Trends in physical properties
      • 2 e- in outer s sub-level. Metallic bonding where each atom transfers 2 e- to create sea of delocalised e-.
      • Trend in atomic atomic radius (nm)
        • Radii increases moving down Group 2.
        • Each successive lement has extra energy level of e- and larger nuclear charge.
        • Additional energy level of e- shields outer 2s e- from nuclear charge.
        • Outer e- attracted less strongly and move further from nucleus, increasing radii.
        • Atomic radius is distance from centre of an atom to its outermost e-.
      • Trend in 1st ionisation energy (kJmol?¹)
        • 1st IE is energy needed to remove 1 mole of e- from 1 mole of atoms (g).
        • 1st IE decreases down Group 2.
        • Extra energy levels of e- added. Shield outer 2s e- from nuclear charge.
        • Easier to remove e-.
      • Trend in melting points (?)
        • Melting point of metals depends on strength of bonds formed when atoms join to make giant structures with metallic bonding.
        • When metals melt, ions gain sufficient energy to break free from fixed positions.
        • Moving down Group 2, successive energy levels shield metal ion nucleus from delocalised e-, reducing attraction.
        • Becomes easier to break metallic bonds and melting power.
        • Mg lower than expected because strength of bond also depends on pattern in which metal ions pack together.
        • B and Mg have hexagonal close packed structures but Ca, Sr and Ba have cubic structures that aren't as closely packed.
    • Extracting  titanium
      • Ti has high tensile strength, similar to steel but is 1/2 as dense and does not corrode. However it is difficult and expensive to extract.
      • Extracted from TiO? and FeTiO? but reduction with C forms titanium carbide which is useless.
      • Reduction of oxide with more reactive metals causes impurity problems and is difficult and expensive to eliminate reducing agent from Ti.
      • TiCl? is more easily reduced. 2 step process
      • 1st TiO? is converted to TiCl? by heating C in a stream of Cl?(g).
        • Reaction: TiO?(s) + 2Cl?(g) + 2C(s) ? TiCl?(g) + 2CO(g)
      • 2nd TiCl? is reduced using Mg as reducing agent (Kroll process). TiCl? and Mg(l) heated to 1200? in an inert atm of He/Ar(g) in a furnace.
        • Reaction: TiCl?(l) + 2Mg(l) ? Ti(l) + 2MgCl?(l) ?H????= -504kJmol?¹
        • Takes 35-50 hours + 4 days to cool so batch process.
    • Trends in chemical properties
      • Group 2  Elements with water
        • The 2e- in outer energy level are readily transferred to other elements and therefore they are good reducing agents.
        • The lower the element is in Group 2, the easier it is to transfer 2e-.
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