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# Electrochemistry
## Oxidation states
Indicates charge (ionisation) of an element
-**Oxidation** - loss of e-
-**Reduction** - gain of e-
+**Oxidation** - loss of e- (at anode)
+**Reduction** - gain of e- (at cathode)
Main group elements (i.e. group 2) - generally one oxidation state:
Transition metals (d shell) may have several oxidation states.
-$$\ce{H_2}$$
-
Common oxidation numbers:
-| elements | common ox. no. in compounds | exceptions |
-| ----------------- | --------------------------- | ------------------- |
-| main group metals | valency | no |
-| hydrogen | +1 | metal hydrides (-1) |
-| oxygen | -2 |
+| elements | common ox. state | exceptions |
+| ----------------- | ---------------- | ------------------- |
+| main group metals | valency | |
+| hydrogen | +1 | metal hydrides (-1) |
+| oxygen | -2 | ce{H2O2} (-1) |
+| halogens | -1 | |
### Rules for oxidation states
- oxidation number of simple ion is the charge of the ion
- sum of oxidation numbers in polyatomic ion is the charge of the ion
- sum of oxidation numbers of a neutral compound is zero
+
+## Electrochemical series
+
+- Top is most likely to be reduced
+- Strongest reductants are bottom right
+- Strongest oxidants are top left
+- Strong oxidants have weak conjugate reductants
+- $E^0$ values are measured relative to ce{H2} / ce{H^+} = 0V
+
+## Conjugate redox pairs
+
+Oxidant and conjugate *reduced form*
+e.g. ce{Cu^2+} / ce{Cu}, $\quad$ ce{Zn^2+} / ce{Zn}
+
+Usually one member of pair is used as electrode (except for *inert electrodes*, e.g. platinum)
+
+## Electrochemical/galvanic cells
+
+1. Find two half reactions involved (between electrode and solution)
+2. Higher equation will proceed left to right
+3. Lower equation will proceed right to left
+
+emf for each cell is calculated as $E^0(\text{red}) - E^0(\text{ox})$
+
+For a *spontaneous* (primary/fuel cell) reaction to occur, species on left must be in electrical contact with species on lower right
+
+### Primary cells
+
+Used for low-current electronic devices. Fixed quantity of reactants.
+
+- **Zinc-carbon dry cell** - carbon rod cathode and zinc anode (case) in ammonium chloride/zinc chloride electrolyte
+- **Alkaline cell** - steel cathode (case) and steel/brass rod anode in potassium hydroxide electrolyte
+- **Silver-zinc cell** - zinc anode, graphite/silver-oxide electrolyte, potassium hydroxide electrolyte
+- **Lithium cell** - magnesium oxide anode, nickel/steel cathode (case), lithium, electrolyte. Lithium is low on electrochemical series enables higher voltage
+
+### Fuel cells
+
+Used for vehicles/long-lasting applications. ce{H2} may be replenished.
+
+- **Alkaline fuel cell** - ce{KOH(aq)} electrolyte, ce{H2(g)} from tank, ce{O2(g)} from atmosphere, water out of hydrogen side
+- **Acid fuel cell** - ce{H3PO4} electrolyte, water out of oxygen side
+