Question

What are some common mistakes students make in thermochemistry?

Answer 1

Off the top of my head, here are some spots for confusion:

• Recalling that `"E"^@""_ "cell" = "E"^@""_ "red"+"E"^@""_ "ox"`, but that the potential values are typically given only as reduction potentials, so the oxidation potentials are the version with the opposite sign.
• The oxidation reactions are also the reverse (flipped reactants or products) of the reduction reactions typically provided.
• Cell potentials for a galvanic cell should always be positive -- set up reduction and oxidation potentials to achieve the greatest cell potential possible.
• In a galvanic cell, oxidation occurs at the anode and reduction at the anode.
• Electrons flow from the anode to the cathode.
• Anions from the salt bridge flow to the anode, and cations flow towards the cathode.
• When balancing half-reactions, pay attention to acidic or basic surroundings and add `"H"^+`, `"OH"^-`, and/or `"H"_2"O""` accordingly.
• Balance electrons in the half reactions -- the number of electrons when balanced is the number of electrons used in equations like `Delta"G"^@=-n"FE"^@`.
• To move from `"C"` to `"mol e"^-`, divide by Faraday's constant. To go the other way, from moles to coulombs, multiply by `96485`.
• When calculating Gibbs free energy, not realizing that enthalpy is often given in `"kJ mol"^-1` whereas entropy is commonly given in `"J mol"^-1` `"K"^-1`, so the enthalpy must often be multiplied by `1000` when using `Delta"G"^@=Delta"H"^@-"T"Delta"S"^@`.