Electrochemistry Flashcards
what’s an oxidation -reduction (redox) reaction ?
a reaction in which the oxidation numbers of the reactants change
in a redox reaction atoms will gain or loose electrons as new bonds are formed. the total number of electrons are not changed, they are just re distributed among the atoms.
oxidation
when an atom looses electrons and its oxidation number increases
reduction
when an atom gains electrons , the oxidation number decreases
LEO says GER
loose electrons : oxidation
gain electrons : reduced
reducing agent
oxidized atom , bc it gives up its own electrons to reduce another atom
oxidation agent
atom that is reduced, or gained electron . bc it oxidized another atom
in order to better see the exchange of electrons, a redox reaction can be broken down into a pair of half reactions that ….
show the oxidation and reduction separately.
galvanic cells
redox reaction involves the transfer of electrons and the flow of electrons constitutes an electric current that can do work. therefore we can use a spontaneous redox reaction to generate an electric current .
a device for doing this is called a galvanic cell (voltaic) cell .
components of the galvanic cell
one electrode composed of a metal (anode) get oxidized and the electrons it looses travels along a wire to the second metal electrode (cathode) which gets reduced. anode is the electron source and the cathode is the electron sink. electrons always go from the anode to the cathode. this electron flow is the electric current that is produced by the spontaneous redox reaction between the electrodes.
anions from the salt bridge go to the ?
anode (site of oxidation)
cations from the salt bridge go to the ?
cathode( site of reduction)
short hand notation called a cell diagram , is used to identify species in a galvanic cell
anode, anodic solution concentration,,cathodic solution concentration, cathode.
if the concentrations are not specified assume they are 1 M.
Standard reduction potentials
to determine weather the redox reaction of a cell is spontaneous and can produce an electric current, we need to figure out the cell voltage. each half reaction has a potential (E) which is the voltage it would have if the other electrode were the standard reference electrode.
cells at standard conditions :
25 deg celcius
1 atm
I M concentrations
the standard reference electrode
has a potential of 0 volts.
tables of half reaction potentials are given for reductions only . so to get the oxidation half reaction:
we get the potential of the oxidation by reversing the sign of the corresponding reduction potential.
the free energy change , delta G standard for a redox reaction in which cell voltage E is given by the equation
delta G standard = -nFE
n= number of moles of electrons transferred
F= faraday (96500 coulombs).
since a reaction in a cell is spontaneous if delta G is negative , this equation tells us that the redox reaction in a cell will be spontaneous
if the cell voltage is positive.
if the cell voltage is positive :
then the reaction is spontaneous
you have to make sure that the overall reaction is electron balanced but…
do not multiply the potentials by those coefficients.
this is because the potentials are intrinsic to the identities of the species involved and do not depend on the number of moles of the species.
if a reduction half reaction has a large negative potential
means its reduction is not spontaneous and it would rather get oxidized - or loose electrons therefore it is a
a good reducing agent —means it gets oxidized itself.
if the reduction half reaction has a large positive potential then it is a good..
oxidizing agent.. bc it gets reduced!!!
when the conditions are not standard we cannot use the tabulated values for reduction potentials for the half reactions
then in non standard conditions we must use the Nernst equation!!!
E = E std - (RT/nF) lnQ
NERNST equation
describes how deviations in temp and concentration of reactants can alter the voltage of a reaction under non standard conditions. the concentrations of the products and the reactants will change until the Q= Keq , and E=0
concentration cell
is a galvanic cell that has identical electrodes but has had cells with different ion concentrations. since the electrodes and the relevant ions have the same identities , the standard cell voltage E would be zero.
this cell is not standard bc the concentrations are not 1M.
concentration cell
even tho electrodes are the same, the concentrations are different and there will be a potential difference between them!!! and electric current will be made.
how do electrons flow in the concentration cell
the electrons will flow through the conducting wire to the Half cell with the higher concentration of positive ions.
when the concentration of the solutions become equal , the reaction will stop
redox titrations
at the half equivalence point the value of E is equal to the E std for the redox couple being titrated.
electrolytic cells
uses external voltage source (like a battery) to create an electric current that forces a non spontaneous redox reaction to occur. this is known as electrolysis.
-eletrolytic cells are used for plating a thin layer of metal on top of another material- known as electroplating.
galvanic vs electrolytic cells.
in both anode is site of oxidation and cathode is site of reduction.
- electrons always move from anode to cathode.
- also in both: anions migrate to anode
- cations move to the cathode.
however, in galvanic cell : anode is negative and cathode is positive (electrons move from anode to cathode)
in electrolytic cell : anode is positive and the cathode is negative since electrons are being forced to move to where they don’t want to go.
Faradays law of electrolysis
the amount of chemical change is porportional to the amount of electricity that flows through the cell.
amount of electricity
charge (q)= It (current x time)
1 mol of electrons =
9650 coulombs
