Ch. 12: Electrochemistry (Complete) Flashcards
defn: electrochemical cells + 3 fundamental types
contained systems in which oxidation-reduction reactions occur
galvanic cells (voltaic cells)
electrolytic cells
concentration cells
what are 5 commonalities between the 3 types of electrochemical cells?
- all contain electrodes where oxidation and reduction take place
- all have a reduction reaction at cathode
- an oxidation reaction at anode
- current flowing from cathode to anode
- electron flow from anode to cathode
what are two differences between the 3 types of electrochemical cells?
galvanic and concentration: house spontaneous reaction, connected by a conductive material/housed in different compartments
electrolytic cells: contain nonspontaneous reactions, housed in the same compartment
defn + mnemonic: anode + cathode
ANODE = electron where oxidation occurs
CATHODE = electron where reduction occurs
AN OX, RED CAT
defn: electromotive force (emf)
what does it indicate when emf is positive? negative?
corresponds to the voltage or electrical potential difference of the cell
emf positive = cell can release energy (delta G < 0) = spontaneous
emf negative = cell must absorb energy (delta G > 0) = nonspontaneous
what is the direction of electrons and of current for all electrochemical cells?
- movement of electrons: anode to cathode
- current (I) runs cathode to anode
what is the relationship between the flow of electrons and the current?
they are always of equal magnitude but in opposite directions
are batteries influenced by temperature? how?
yes!
most galvanic cells fail in cold weather
what is the relationship between the free energy change and the emf?
they always have opposite signs
char (4): galvanic/voltaic cells
- all nonrechargeable household batteries you own
- spontaneous reactions
- delta G < 0 (the reactions free energy is decreasing) as the cell releases energy to the environment
- Ecell (emf) = positive
what is the basic setup of a galvanic/voltaic cell? (5)
- two electrodes of distinct chemical identity are placed in separate compartments (called half-cells)
- the two electrodes are connected to each other by a conductive material (like copper wire)
- along the wire, there may be other various circuit components (i.e. resistors, capacitors, etc.)
- surrounding each electrode is an aqueous electrolyte solution composed of cations and anions
- connecting the two solutions is a salt bridge (consists of an inert salt)
can the cations in the two half-cell solutions be of the same element as the respective metal electrode in a galvanic/voltaic cell?
yes!
what is the process that occurs when the electrodes in a galvanic/voltaic cell are connected to each other by a conductive material? (4)
- charge will begin to flow as the result of an redox reaction that is taking place between the two-half cells
- the redox reaction is spontaneous, so the change in Gibbs free energy is negative
- as the spontaneous reaction proceeds towards equilibrium, the movement of electrons results in a conversion of electrical potential energy into kinetic energy
- by separating the reduction and oxidation half-reactions into two compartments, we are able to harness this energy and use it to do work by connecting various electrical devices into the circuit between the two electrodes
Daniell cell: defn
set up
what are the anode and cathode?
a specific galvanic/voltaic cell
a zinc electrode is placed in an aqueous ZnSO4 solution
a copper electrode is placed in an aqueous CuSO4 solution
anode: zinc bar (Zn (s) is oxidized to Zn2+ (aq))
cathode: copper bar (Cu 2+ (aq) is reduced to Cu (s))
what are the half-cell reactions, net reaction, and Ered/Ecell for each reaction?
Zinc: Ered = -0.762 V (anode)
Copper: Ered = +0.340 V (cathode)
Net: Ecell = +1.102 V
what direction do electrons flow in the Daniell cell?
from the zinc anode through the wire to the copper cathode
what flows external from the salt bridge into the Daniell cell? (2)
anions (Cl-) flow externally from the salt bridge into the ZnSO4
cations (K+) flow externally from the salt bridge into the CuSO4
what would happen if the two half-cells were not seperated?
the Cu2+ ions would react directly with the zinc bar, and no useful electrical work would be done
what would happen if only a wire were provided for this electron flow?
the reaction would soon stop because an excess positive charge would build up on the anode and an excess negative charge would build up on the cathode
the excessive charge accumulation would provide a countervoltage large enough to prevent the redox reaction from taking place and the current would cease
func: salt bridge
to exchange of anions and cations to balance/dissipate newly generated charges
what does the salt bridge contain?
an inert electrolyte (KCl or NH4NO3 usually) which contains ions that will not react with the electrodes or with the ions in solution
explain how the salt bridge works in the context of the Daniell cell
the anions from the salt bridge (Cl-) diffuse into the solution on the anode side (ZnSO4) to balance out the charge of the newly created Zn2+ ions
the cations of the salt bridge (K+) flow into the solution on the cathode side (CuSO4) to balance out the charge of the sulfate ions left in solution when the Cu2+ ions are reduced to Cu and precipitate onto the electrode
defn: plating or galvanization
the precipitation process of the reduced Cu (in the case of the Daniell cell) onto the cathode
what accounts for the relatively short lifespan of the Daniell cell?
the flow of anions and cations from the salt bridge into the half-cells and the finite quantity of Cu2+ in the solution
mnemonic: flow of electrons in all electrochemical cells
A –> C
alphabetical
electrons flow from Anode to Cathode
defn + example for Daniell cell: cell diagram
shorthand notation representing the reactions in an electrochemical cell
Zn (s) | Zn2+ (1 M) || Cu2+ (1 M) | Cu (s)
what are the 3 rules for constructing a cell diagram?
- reactants and products are always listed from L to R in this form:
anode | anode solution (concentration) || cathode solution (concentration) | cathode - a single vertical line indicates a phase boundary
- a double vertical line indicates the presence of a salt bridge or some other type of barrier
char: electrolytic cell (3)
- house nonspontaneous reactions that require the input of energy to proceed
- delta G > 0
- Emf negative
defn: electrolysis
oxidation-reduction reaction driven by an external voltage source, and in which chemical compounds are decomposed
what materials are electrodes made of in electrolytic cells?
any material so long as it can resist the high temperatures and the process’ corrosion
explain the process of what happens in an electrolytic cell (2)
in this example: molten NaCl is decomposed into Cl2 (g) and Na (l)
- the external voltage source (a battery) supplies energy sufficient to drive the redox reaction in the direction that is thermodynamically unfavorable (nonspontaneous)
- Na+ ions migrate toward the cathode, where they are reduced to Na (l). Cl- ions migrate toward the anode, where they are oxidized to Cl2
why do the half reactions in an electrolytic cell not need to be separated?
because the desired reaction is nonspontaneous