- 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
- 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.
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.
- 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
- 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.
1. PAINT WITH POLYMER
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.
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.
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.
- 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.
- 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.
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.
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.