chem / electrochemistry.mdon commit [spec] start vector calculus (42fe40b)
   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
  59Spontaneous reaction
  60
  611. Find two half reactions involved (between electrode and solution)
  622. Higher equation will proceed left to right
  633. Lower equation will proceed right to left
  64
  65emf for each cell is calculated as $E^0(\text{red}) - E^0(\text{ox})$
  66
  67For a *spontaneous* (primary/fuel cell) reaction to occur, species on left must be in electrical contact with species on lower right
  68
  69### Primary cells
  70
  71Used for low-current electronic devices. Fixed quantity of reactants.
  72
  73- **Zinc-carbon dry cell** - carbon rod cathode and zinc anode (case) in ammonium chloride/zinc chloride electrolyte
  74- **Alkaline cell** - steel cathode (case) and steel/brass rod anode in potassium hydroxide electrolyte
  75- **Silver-zinc cell** - zinc anode, graphite/silver-oxide electrolyte, potassium hydroxide electrolyte
  76- **Lithium cell** - magnesium oxide anode, nickel/steel cathode (case), lithium, electrolyte. Lithium is low on electrochemical series enables higher voltage
  77
  78### Fuel cells
  79
  80Used for vehicles/long-lasting applications. ce{H2} may be replenished. 
  81
  82- **Alkaline fuel cell** - ce{KOH(aq)} electrolyte, ce{H2(g)} from tank, ce{O2(g)} from atmosphere, water out of hydrogen side
  83- **Acid fuel cell** - ce{H3PO4} electrolyte, water out of oxygen side
  84
  85## Electrolysis reactions
  86
  87- Occurs when electricity passes through ionic compound or electrolyte solution
  88- Opposite of reactions in electrochemical (galvanic) cells
  89- Non-spontaneous
  90- Electrical energy $\rightarrow$ chemical energy
  91- Anode +ve; cathode -ve
  92- Cathode & anode swap relative to galvanic cell
  93- Molten e.g. ce{Na+(l)} vs electrolyte e.g. ce{Na+(aq)}
  94- Molten - use echem series to determine probability of electrolyssis of solution or water
  95- Min voltage = e_oxidising - e_reducing
  96
  97### Factors affecting electrolysis
  98
  99- concentration of electrolyte
 100- - e- values must be "close" for electrolysis rxn to prevail over \ce{H2O}
 101- nature of electrodes
 102
 103### Electroplating
 104
 105- For each ion going into plated object, an ion is replaced from the anode (+ve)
 106- Electrolyte balances charges
 107
 108### Coulomb's law
 109
 110$$Q=It$$
 111
 112### Faraday's first law
 113
 114$$m \propto Q$$
 115
 116i.e. mass produced at cathode is proportional to charge supplied
 117
 118*Faraday* - charge on 1 mol of electrons = 96500 C.
 119
 120### Faraday's second law
 121
 122> To produce one mole of substance by electrolysis, a whole number of mole of electrons is needed