Bonus - Electrochemistry Flashcards
Describe differences between galvanic (voltaic) cells and electrolytic cells.
Uses a battery or power source – E
Typically has two compartments – G
Involve a thermodynamically favorable reaction – G
Has a negative cell potential (Ecell) – E
Has a negative free energy change (ΔG) – G
Has an equilibrium constant less than 1 – E
Describe a similarity between galvanic (voltaic) cells and electrolytic cells.
For all electrochemical cells, oxidation occurs at the anode (lose electrons) and reduction occurs at the cathode (gain electrons).
So electrons flow from the anode to the cathode
How do you relate Ecell, ΔG, and K?
Positive Ecell = negative ΔG = K > 1 = thermodynamically favored reaction
Negative Ecell = positive ΔG = K < 1 = not thermodynamically favored reaction
*need an applied voltage greater than or equal to the magnitude of Ecell in order to power the cell
How do you determine which half reaction is reduction?
The half reaction with the more positive standard reduction potential (E°red) is reduction.
The half reaction with the less positive or negative standard reduction potential (E°red) is oxidation.
This will produce a standard cell potential (E°cell) that is positive and therefore thermodynamically favorable.
How do you rank cell potentials given the concentrations of the ions in NONSTANDARD cells?
Calculate Q for each cell by dividing [product] by [reactant]. Only include ions, not solids.
Smaller Q = larger cell potential (Ecell)
Standard conditions?
Shown by the ° symbol
concentration – 1.0 M
pressure – 1 atm
temperature – 298 K
Standard galvanic cells vs nonstandard galvanic cells
Under standard conditions (shown by the ° symbol), all concentrations of the ions are 1.0 M so Q = 1. In a galvanic cell, K > 1 (always thermodynamically favorable). Therefore Q < K, so the reaction moves in a forward direction.
In a nonstandard cell, decreasing Q relative to its value in the standard cell will cause it to move further from K (because Q < K in standard conditions). Therefore, the value of the cell potential will decrease.
If in a nonstandard cell you change the initial mass of the electrode, there will be no effect on cell potential. This is because solids do not appear in the Q expression
How does the cell potential of galvanic (voltaic) cells change as they operate?
As they operate, the [product] increases while the [reactant] decreases. As a result, Q increases.
Increasing Q relative to its value in the standard cell causes it to move closer to K and the reaction to shift left (because Q < K in standard conditions).
Therefore, the value of the cell potential will decrease during operation as the reaction progresses
Explain why for an electrolytic cell to operate, the ionic compound must be in a liquid state rather than a solid state.
In solid state, ionic compounds will not conduct electricity because the ions are held in a crystal lattice structure.
In order for the cell to operate, the compound must be in the liquid state so that the ions are mobile and are able to move to the electrodes to react.
What will happen in the salt bridge as the reaction progresses?
Ions are produced from neutral atoms, so their concentration increases as the cell operates.
Anions flow through the salt bridge into the anode (to compensate for the negative charge lost when the electrons leave through the wire).
Cations flow through into the cathode (to neutralize the negative charge added when electrons enter from the wire, keeping the charge balanced).
