Electrochemistry

Question 1

what’s an oxidation -reduction (redox) reaction ?

Answer 1

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.

Question 2

oxidation

Answer 2

when an atom looses electrons and its oxidation number increases

Question 3

reduction

Answer 3

when an atom gains electrons , the oxidation number decreases

Question 4

LEO says GER

Answer 4

loose electrons : oxidation

gain electrons : reduced

Question 5

reducing agent

Answer 5

oxidized atom , bc it gives up its own electrons to reduce another atom

Question 6

oxidation agent

Answer 6

atom that is reduced, or gained electron . bc it oxidized another atom

Question 7

in order to better see the exchange of electrons, a redox reaction can be broken down into a pair of half reactions that ….

Answer 7

show the oxidation and reduction separately.

Question 8

galvanic cells

Answer 8

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 .

Question 9

components of the galvanic cell

Answer 9

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.

Question 10

anions from the salt bridge go to the ?

Answer 10

anode (site of oxidation)

Question 11

cations from the salt bridge go to the ?

Answer 11

cathode( site of reduction)

Question 12

short hand notation called a cell diagram , is used to identify species in a galvanic cell

Answer 12

anode, anodic solution concentration,,cathodic solution concentration, cathode.
if the concentrations are not specified assume they are 1 M.

Question 13

Standard reduction potentials

Answer 13

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.

Question 14

cells at standard conditions :

Answer 14

25 deg celcius
1 atm
I M concentrations

Question 15

the standard reference electrode

Answer 15

has a potential of 0 volts.

Question 16

tables of half reaction potentials are given for reductions only . so to get the oxidation half reaction:

Answer 16

we get the potential of the oxidation by reversing the sign of the corresponding reduction potential.

Question 17

the free energy change , delta G standard for a redox reaction in which cell voltage E is given by the equation

Answer 17

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.

Question 18

if the cell voltage is positive :

Answer 18

then the reaction is spontaneous

Question 19

you have to make sure that the overall reaction is electron balanced but…

Answer 19

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.

Question 20

if a reduction half reaction has a large negative potential

Answer 20

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.

Question 21

if the reduction half reaction has a large positive potential then it is a good..

Answer 21

oxidizing agent.. bc it gets reduced!!!

Question 22

when the conditions are not standard we cannot use the tabulated values for reduction potentials for the half reactions

Answer 22

then in non standard conditions we must use the Nernst equation!!!
E = E std - (RT/nF) lnQ

Question 23

NERNST equation

Answer 23

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

Question 24

concentration cell

Answer 24

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.

Question 25

concentration cell

Answer 25

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.

Question 26

how do electrons flow in the concentration cell

Answer 26

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

Question 27

redox titrations

Answer 27

at the half equivalence point the value of E is equal to the E std for the redox couple being titrated.

Question 28

electrolytic cells

Answer 28

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.

Question 29

galvanic vs electrolytic cells.

Answer 29

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.

Question 30

Faradays law of electrolysis

Answer 30

the amount of chemical change is porportional to the amount of electricity that flows through the cell.

Question 31

amount of electricity

Answer 31

charge (q)= It (current x time)

Question 32

1 mol of electrons =

Answer 32

9650 coulombs