2. Higher equation will proceed left to right
3. Lower equation will proceed right to left
-emf for each cell is calculated as $E^0(\text{red}) - E^0(\text{ox})$
-Then total emf is $\sum_{i=1}^2 \Sigma E^0({i})$
+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
-For a spontaneous reaction to occur, species on left must be in electrical contact with species on lower right
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