The Steel Story

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Titrations

Acid-base titrations

  • Use pipette to measure out alkali and put in flask with indicator.
  • Do rough titration - find whereabouts end point is.  Regularly swirl.
  • Do accurate titration.  Run acid in within 2cm^3 of end point, then dropwise.  Indicator changes colour = neutral

Redox titrations

  • Let you find out how much oxidising agent needed to exactly react with a quantity of reducing agent.
  • Fill burette with MnO4- solution.
  • Use 25cm^3 pipette for sodium ethanedioate.
  • Place in flask and acidify with H2SO4.
  • Add MnO4- solution slowly near end point.
  • Until permanent pink colour.
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Redox

Rules

1. Uncombined elements and diatomic molecules have oxidation state of 0.

2. State of monatomic ion is same as its charge.

3. Combined oxygen always -2, except in peroxides -1.

4. Combined hydrogen +1, except in metal hydrides -1.

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Electrode potentials

  • In electrochemical cells, e-s flow through wire from most reactive metal.
  • Electrochemical cells usually makde from 2 diff metals dipped in salt solutions of their own ions and connected by a wire.
  • Also a salt bridge, preventing the build up of charge in solution.
  • Can have half cells with 2 aqueous ions of same element.
  • Conversion occurs on surface of electrode, which needs to be solid, conduct electricity and be inert, so won't react with anything - PLATINUM OR GRAPHITE.
  • Standard electrode potential of half cell is voltage measured under standard conditions when half cell is connected to a standard hydrogen electrode.
  • SHE always on LEFT and is 0.00V.
  • STANDARD CONDITIONS: 1.00moldm-3, 25C, pressure must be 100kPa.
  • Ecell = ERHS -  ELHS
  • FEASIBILITY 
    • If Ecell value is GREATER for what is being REDUCED, then is feasible.
    • May be feasible, but not see anything.
    • Doesn't say anything about rate of reaction.
    • Feasible = possible - may NOT happen.
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Iron - RUSTING

  • If Fe exposed to O2 + H2O, turn into RUST.
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Preventing rusting

1. PAINT WITH POLYMER

2. OIL/GREASE

The above act as BARRIERS - prevent O2 + H2O getting in.

3. SACRIFICIAL METHOD - place MORE REACTIVE metal with Fe, so O2 + H2O react with it instead of with Fe.

Zn/Zn2+ more -ve than Fe/Fe2+, so OXIDISED to Zn2+.

GALVANISING - spray coat of Zn.

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Recycling iron

Extracting new ore - lots of E, POLLUTION + LANDFILL SPACE.

Aerosol cans emptied by special facilities before recycled.

1. SEPARATE IRON - magnets.

2. CLEAN IRON - melt in FURNACE and BLOW O2 through to BURN OFF IMPURITIES.

3. ADD CARBON AND ELEMENTS in carefully controlled amounts to get DESIRED properties.

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Extracting metals from ores

  • Metal needs to be REDUCED to be extracted.

1. HEAT WITH CARBON - C is oxidised, so reduces metal.  Cheap - lots of sources of C.  Doens't work with reactive metals - C not strong enough oxidising agent.

2. REDUCTION BY MORE REACTIVE METAL - Na powerful reducing agent.  Expensive as Na costs lots.  E.g Na heated with TiCl4 to get pure Ti.

3. ELECTROLYSIS - pass electric current though MOLTEN ore.  Expensive as high temps and electricity.  Used to PURIFY metal.

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Transition metals

DEFINITION - metal that can form at least one stable ion with an INCOMPLETE D-SUBSHELL.

SCANDIUM + ZINC aren't TMs.  Both only form 1 ion each.

Sc3+ has an EMPTY d-subshell.        Zn2+ has a FULL d-subshell.

In general, the 3d orbitals fill up singly at first, then fill up 4s, then go back to double their 3d.

When ions formed, 4s electrons are REMOVED FIRST.

Cr + Cu 4s subshells have 1 electron.

Chemical properties

  • Form COMPLEX IONS.
  • Exist in variable oxidation states.
  • Form coloured ions.
  • Make good catalysts as can change oxidation states, so transfer e-s to speed up reactions.
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Complex ions

  • DEFINITION - complex ion is a metal ion surrounded by COORDINATELY BONDED LIGANDS.
  • A ligand is any ion or molecule that forms a coordinate bond with a metal ion.
  • Coordination number is number of coordinate bonds formed with central ion.
  • Usually 6 if ligands are small (H2O, CN-, NH3) and 4 if big (Cl-)
  • 6 coordinate bonds = OCTAHEDRAL SHAPE
  • 4 coordinate bonds = TETRAHEDRAL OR SQUARE PLANAR
  • Ligands form bonds using LP of e-s.
  • Ligands with 1LP = monodentate (H2O, NH3, Cl-) 2LPs = bidentate.
  • LIGAND SUBSTITUTION - one ligand can be swapped for another ligand.  Causes a COLOUR CHANGE.
  • If ligands are SIMILAR SIZE (H2O and NH3) coordination number and shape don't change
  • If ligands DIFFERENT SIZES (H2O and Cl-) coordination number and shape change.
  • Sometimes substitution is only PARTIAL.
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More on transition metals (NaOH)

  • NaOH + TRANSITION METAL ION ----> COLOURED PRECIPITATE + H2O
  • TM ions in aq solutions form COMPLEX IONS and adding OH- removes H+ from water ligands, taking +ve charge away.
  • When enough H+ have been removed, complex ion becomes neutral and forms a PRECIPITATE.

LEARN THESE:

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More on transition metals (NH3)

  • Ammonia reacts with TM ion solutions in 2 ways.
  • If LITTLE ammonia solution added to aq solution of Cu2+ ions causes same reaction as OH- do... Removes H+ and forms blue neutral complex precipitate.
  • If LOTS ammonia solution added, precipitate will DISSOLVE and form an INTENSE BLUE COLOUR - because NH3 molecules can also act as LIGANDS.  NH3 REPLACE H2O.

LEARN THESE:

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More on transition metals (catalysts)

  • HOMOGENEOUS CATALYST - in same phase as reactant chemicals.
  • TM compounds good catalysts as change oxidation states easily.
  • HETEROGENEOUS CATALYST - in different phase from reactants.
  • TMs good heterogeneous catalysts as can use their s and d orbitals for bonding to reactant molecules.
  • 1. Reactant molecules attracted to catalyst surface and stick - ADSORPTION.
  • 2. Catalyst surface activates molecules, so react more easily - interaction with catalyst WEAKENS bonds making them easier to break and reform products.
  • 3. Product molecules leave surface - DESORPTION.
  • To be a good heterogeneous catalyst must:
    • ATTRACT REACTANT MOLECULES STRONGLY ENOUGH so are held to surface long enough to REACT.
    • NOT ATTRACT PRODUCT MOLECULES SO STRONGLY they won't be able to DESORB and block catalyst from fresh reactants.
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Colorimetry - Transition metal ion colour

  • When white light hits TM ion, one frequency of light is ABSORBED.
  • Rest of frequencies are TRANSMITTED - the colours you see.
  • Mix 2 complementary colours of light together = white light.
  • So, if copper ions appear blue, the must absorb red part of spectrum.
  • A colorimeter measures absorbance of a colution and can be used to work out concentration of TM ions in a solution because as conc increases, absorbance does.
  • Colorimeter works by filtering source of white light into monochromatic light which only has one frequency - the frequency of light which is ABSORBED.  This is why you pick the filter which is complementary to the colour transmitted.
  • First, need to set colorimeter to zero by measuring absorbance of a BLANK SAMPLE - sample of solvent metal ions are dissolved in - WATER.  This means the colorimeter will only be measuring absorbance of metal ions and not solvent too.
  • Samples placed in cuvettes - doesn't absorb any light.  
  • Light goes through filter, through sample, where some is absorbed, and remaining light travels to detector, which compares absorbance of sample to blank.
  • High absorbance reading = lots light absorbed = very concentrated.
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Using colorimeters to find concentrations

  • Need calibration graph - measure absorbance of samples with known conentration.
  • Samples made by dissolving different amounts of metals in fixed volume of water or make consecutive dilutions.
  • Must contain same metal ion and be in same solution as unknown sample.
  • Plot absorbance against concentration to get calibration graph.
  • Measure absorbance of unknown sample and use standard curve to read off its concentration.
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