# What are some common mistakes students make in thermochemistry?

Mar 2, 2016

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

• Recalling that $\text{E"^@""_ "cell" = "E"^@""_ "red"+"E"^@""_ "ox}$, 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 ${\text{H}}^{+}$, ${\text{OH}}^{-}$, and/or $\text{H"_2"O}$ accordingly.
• Balance electrons in the half reactions -- the number of electrons when balanced is the number of electrons used in equations like $\Delta {\text{G"^@=-n"FE}}^{\circ}$.
• To move from $\text{C}$ to ${\text{mol e}}^{-}$, 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 ${\text{kJ mol}}^{-} 1$ whereas entropy is commonly given in ${\text{J mol}}^{-} 1$ ${\text{K}}^{-} 1$, so the enthalpy must often be multiplied by $1000$ when using $\Delta {\text{G"^@=Delta"H"^@-"T"Delta"S}}^{\circ}$.