W3 Introductory Electrochemistry Flashcards
what are electrolytes
substances that dissolve in water or other solvents to produce ions
properties electrolyte concentration
position of most solution equilibria depends on electrolyte concentration of medium, even when added electrolyte contains no common ion with those involved in equilibrium
magnitude of electrolyte effect highly dependent on charges of the participants in equilibrium
when only neutral species involved > equilibrium position independent of electrolyte concentration
what is effect of added electrolyte on equilibria
effect of added electrolyte independent of chemical nature of electrolyte, but depends on ionic strength
properties of ionic strength
ionic strength of a solution of strong electrolyte consisting solely of singly charged ions is identical to its total molar salt concentration
ionic strength is greater than molar concentration if solution contains ions with multiple charges
what is the electrolyte effect
arises from electrostatic repulsive and attractive forces that exist between ions of electrolyte and ions in equilibrium > cause each ion from dissociated reactant to be surrounded by sheath of solution that contains slight excess of electrolyte ions of opposite charge
hence effective concentration of ions becomes less as ionic strength of solution increases
what is activity,a
used to account for the effects of electrolytes on chemical equilibria
what is activity coefficient
a factor that adjusts the concentration of ions in a solution to account for the interactions between those ions
properties of activity coefficients
in extremely idly solutions where ionic strength is minimal > activity coefficient is unity (close to 1)
activity coefficient of an unchanged molecule is unity
for a given ionic strength, activity coefficient of an ion departs farther from unity as the charge carried by the species increases
at any given ionic strength, the activity coefficient of ions of the same charge are approximately equal
the activity coefficient of an ion describes it effective behaviour in all equilibria in which it participates
what happens at cathode and anode
cathode: where reduction occurs
anode: where oxidation occurs
difference between galvanic and electrolytic cell
galvanic cell operates under equilibrium conditions; cathode is positive, anode is negative
electrolytic cell operates under non equilibrium conditions; cathode is negative, anode is positive
what happens in galvanic cell
electrochemical reaction occurs spontaneously and involves flow of electrons from anode to cathode via conductor
have low resistance circuit
what happens in electrolytic cell
requires external source of energy
potential must be greater to drive reaction in reverse direction to galvanic cell
what is potential difference
measure of electrical potential difference between 2 electrodes
free energy = -nFEcell
potential difference must be positive for spontaneous reaction
if reactants and products in standard state > resulting cell potential is standard cell potential > =-RTlnKeq
what are the three rules to determine the cell potential and spontaneous direction for two half cell reactions
- write reactions at the two electrodes as reductions involving one electron only. if data is supplied as oxidation > sign of potentials must be swapper
- write down cell in shorthand notation so that the couple with more negative reduction potential is on the left hand side of reaction (and write as species that undergoes oxidation)
- subtract reaction at left hand electrode from reaction at right hand electrode to find formal cell potential
what are the different definitions of “potentials”
electrode potential: potential of a half-cell reaction relative to a carefully defined reference electrode
standard potential: electrode potential when the activities of the reactants and products are all unity and T=25 degrees
formal potential: measured potential of the half cell when the species ox and red are present at concentrations such that the ratio [ox]/[red] is unity AND when specified substances are present at stated concentrations
