The Steel Story
- Created by: Lalala
- Created on: 04-04-13 17:53
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.
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.
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.
Iron - RUSTING
- If Fe exposed to O2 + H2O, turn into RUST.
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.
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.
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.
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.
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.
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:
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:
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.
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.
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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