chem / electrochemistry.mdon commit [chem] electrolysis intro (b0d3855)
   1# Electrochemistry
   2
   3## Oxidation states
   4
   5Indicates charge (ionisation) of an element
   6
   7**Oxidation** - loss of e- (at anode)  
   8**Reduction** - gain of e- (at cathode)
   9
  10Main group elements (i.e. group 2) - generally one oxidation state:
  11
  12| elements      | valence config | oxidation state |
  13| ------------- | -------------- | --------------- |
  14| alkali metals | $s^1$          | +1              |
  15| alkali earths | $s^2$          | +2              |
  16| aluminium     | $s^2 p^1$      | +2              |
  17| nitrogen      | $s^2 p^3$      | -3              |
  18| oxygen        | $s^2 p^4$      | -2              |
  19| halogens      | $s^2 p^5$      | -1              |
  20| noble gases   | $s^2,\>s^2p^5$ | n/a             |
  21
  22Transition metals (d shell) may have several oxidation states.
  23
  24Common oxidation numbers:
  25
  26| elements          | common ox. state | exceptions          |
  27| ----------------- | ---------------- | ------------------- |
  28| main group metals | valency          |                     |
  29| hydrogen          | +1               | metal hydrides (-1) |
  30| oxygen            | -2               | ce{H2O2} (-1)       |
  31| halogens          | -1               |                     |
  32
  33
  34### Rules for oxidation states
  35
  36- oxidation states >3 may only exist in compounds
  37- oxidation number of free element is 0
  38- oxidation number of simple ion is the charge of the ion
  39- sum of oxidation numbers in polyatomic ion is the charge of the ion
  40- sum of oxidation numbers of a neutral compound is zero
  41
  42## Electrochemical series
  43
  44- Top is most likely to be reduced
  45- Strongest reductants are bottom right
  46- Strongest oxidants are top left
  47- Strong oxidants have weak conjugate reductants
  48- $E^0$ values are measured relative to ce{H2} / ce{H^+} = 0V
  49
  50## Conjugate redox pairs
  51
  52Oxidant and conjugate *reduced form*  
  53e.g. ce{Cu^2+} / ce{Cu}, $\quad$ ce{Zn^2+} / ce{Zn}
  54
  55Usually one member of pair is used as electrode (except for *inert electrodes*, e.g. platinum)
  56
  57## Electrochemical/galvanic cells
  58
  591. Find two half reactions involved (between electrode and solution)
  602. Higher equation will proceed left to right
  613. Lower equation will proceed right to left
  62
  63emf for each cell is calculated as $E^0(\text{red}) - E^0(\text{ox})$
  64
  65For a *spontaneous* (primary/fuel cell) reaction to occur, species on left must be in electrical contact with species on lower right
  66
  67### Primary cells
  68
  69Used for low-current electronic devices. Fixed quantity of reactants.
  70
  71- **Zinc-carbon dry cell** - carbon rod cathode and zinc anode (case) in ammonium chloride/zinc chloride electrolyte
  72- **Alkaline cell** - steel cathode (case) and steel/brass rod anode in potassium hydroxide electrolyte
  73- **Silver-zinc cell** - zinc anode, graphite/silver-oxide electrolyte, potassium hydroxide electrolyte
  74- **Lithium cell** - magnesium oxide anode, nickel/steel cathode (case), lithium, electrolyte. Lithium is low on electrochemical series enables higher voltage
  75
  76### Fuel cells
  77
  78Used for vehicles/long-lasting applications. ce{H2} may be replenished. 
  79
  80- **Alkaline fuel cell** - ce{KOH(aq)} electrolyte, ce{H2(g)} from tank, ce{O2(g)} from atmosphere, water out of hydrogen side
  81- **Acid fuel cell** - ce{H3PO4} electrolyte, water out of oxygen side
  82
  83## Electrolysis reactions
  84
  85- Opposite of reactions in electrochemical (galvanic) cells
  86- Non-spontaneous
  87- Electrical energy $\rightarrow$ chemical energy
  88
  89### Electroplating
  90
  91- For each ion going into plated object, an ion is replaced from the anode (+ve)
  92- Electrolyte balances charges
  93
  94### Coulomb's law
  95
  96$$Q=It$$
  97
  98### Faraday's first law
  99
 100$$m \propto Q$$
 101
 102i.e. mass produced at cathode is proportional to charge supplied
 103
 104*Faraday* - charge on 1 mol of electrons = 96500 C.
 105
 106### Faraday's second law
 107
 108> To produce one mole of substance by electrolysis, a whole number of mole of electrons is needed