Chapter 17: Electrochemistry Flashcards
what is electrochemistry
a branch of chemistry that relates electricity and chemical reactions
what does the law of conservation of mass say
amount of each element must be the same on both sides
what can be used to balance the gains and losses of electrons
half reaction metod
what is oxidation
lose electrons (more positive)
what is reduction
gain electrons ( more negative)
what is the half reaction method
- write the oxidation and reduction as two separate processes
- balance the 1/2 reactions then combine them to attain the balanced equation for the overall reaction
if there is oxidation in a reaction where are the electrons
on the right (products)
if there is reduction where are the electrons in the rxn
electrons are on the left (reactants)
if a reaction occurs in a basic solution how do you balance it
balance it as if it occurred in an acid
what happens to the electrons in a spontaneous redox rsn
electrons are transferred and energy is released
why are redox reactions in energy
have the potential to generate electrical current
- use the energy to make the electrons go through an external device
what is another name for the voltaic cell
galvanic
in a voltaic cell, setup do the metals touch
the metals are not direct
why do we separate the reduction and oxidation half reactions
so that we can create a flow of electrons
what is the purpose of an electrode
electron flow though a wire with ion flow through a solution
what does electrode require
conductive solid electrode
what is an electrode
two solid metals connected by external circut
where does oxidation occur in a voltaic cell
anode
where does reduction occur in a voltaic cell
cathode
in the cell how do the electrons flow
leave the anode and flow through the wire to the cathode
what happens when the electrons leave the anode
cations formed dissolve into the solution in the anode compartment
- anode loses mass and the soln becomes more concentrated
what happens when electrons reach the cathode
cations in the solution the cathode is in are attracted to the now negative cathode
- cathode gains mass and the soln it is in becomes less concentrated
- electrons form neutral metals
what is a salt bridge
u shaped tube that contains a salt solution, to keep the charges balanced
what is the salt bridge composed of
ions that will not react with other ions in the cell or with the electrode materials
why do we use an electrolyte in thecell
to keep it from pouring out into the two half cells
what sign is the anode labled with
negative
what sign is the cathode labeled with
+
what is the formula for cell notation
electrode | electrolyte || electrolyte | electrode
in cell notation what is on the left
oxidation half cell
in cell notation what is on the right
reduction half cell on the right
what does || mean in cell notation
salt bridge
what does | mean in cell notation
what do you use when mulltiple electrolytes are in the same phase
, (comma)
what is potential difference
the difference in potential energy per electrical charge between two electrodes
what is a potential difference measured in
Volts
what is the coulomb the derived SI unit for
electrical charge
what does 1 volt equal
1 v = 1 J/C
- C = 1A x 1S
what is the charge of one electron
1.60 x 10^-19 C
what is the electromotive force
potential diff. between the anode and cathode
what does electromotive mean
causing electron motion
what is another name for emf (electromotive force)
cell potential
how does a voltmeter work
by drawing current through a known resistance
what happens in the voltmeter
- current flows through wire generating frictional heat and wastes some energy
what happens in a digital voltmeter
draw only a negligible current and are physically and financially convenient
what will happen to the cell potential it it is +
cell rxn proceeds spontaneously
Ecell is referred to and measured in what
cell voltage and measured in volts
what does the emf depend on
the particular cathode and anode
what is the name for cell potential under standard conditions
standard emf
what is cell potential
difference between the two electrode potentials
electrode potentials at standard conditons are for what
reduction reactions
why is it impossible to measure the standard reduction potential of a half reaction directly
the voltaic cell involves two half cells
what is SHE
standard hydrogen electrode
what is the SHE
the reduction of H+ to H2 under standard conditions
what is the reduction potential for hydrogen
0 V
what does a SHE consist of
electrode with finelt divided pt (platinum) in contact with H2(g)
what happens when SHE is the cathode
two H+ ions each accept an electron from the pt electrode and are reduced to H atoms
when SHE is the anode what happens
H2 molecule at the electrode surface loses two e-s and is oxidized to H+
what is cell potential based on
the potential energy per unit of charge (intensive property)
what happens if we increase the amount of substances in a redoz reaction
increaese the energy and charges but the ratio of energy to charge would remain constant
what does the Ered measures
the driving force for red to occur
what happens if the Ered is more positive
the greater the driving force
in a voltaic cell what has a more positive value for Ered
the cathode
what is the greater force of the cathode used for
used to force the anode rxn to occur in reverse
what is the equation for the Ecell
Ecell = Ered (cathode) + (−Ered (anode)
what is another name for the standard cell potential
Ecell
what are the strongest oxidizers and what does it mean if they are the strongest
the one with the most Ered, they have a greater tendency for the reactant of the half rxn to be reduced and oxidize another species
the strongest reducers have the most
negative Ered
what is the half rxn with the smallest ered
reversed as an oxidation
how do you find E standard
E = Ered (reduction) − Ered (oxidation
what does a positive E indicate
a spontaneous process
when is 1 Joule of work produced
when 1 coulomb of charge is transferred between points
what is the equation for cell potential and electrical work
emf = potential difference = E = 1V = work(J)/ charge (c)
what is the equation for energy involving work
-w/q
what is the equation for q that involves F
q = nF
n = moles of electrons
-F = faraday’s constant (96,485 C/mol e-)
what is the equation for max work in a cell
wmax = −qEmax
how do you find change in free energy with max work
∆G = wmax = −qEmax = −nFE
for a spontaneous rxn at standard conditions
what is the sign of G, K, and E
G = -
K = +
E = +
what happens when a voltaic cell is discharged
the reactants are consumed and the products are produced
- concentrations of reactans and products are not constant they change over time
what will E reach when the emf decreases
0
what can we use to calulate the emf under nonstandard conditions
nernst equation
who came up with the nernst equation
walther nernst
what is the nernst equation
= E − RT nF ln Q = E − 2.303 RT nF log Q
what is the nernst equation for room temperature
E = E − 0.0592 V/ n log Q
what is the nernst equation when Q = K
E = 0.0592 V n log K
what is a conc. cell
a cell based on the emf generated because of a difference in concentration
where do electrons flow when the cell concentrations are different
electrons flow from the side with less conc soln (anode) to the side with more conc( cathose)
what will happen if there is oxidation of the electrode in the less conc.
soln will increase the ion conc in the soln
what does the reduction of the soln in the more concentration do to the ion concentration of the soln
it reduces the ion conc
what is cell potential sensitive to
concentrations of reactans and products
what can you use measured potentials for
used to determine the concentration of an ion
what does a pH meter do
measures the conc using an observed potential and consists of three main componenets
what are the 3 main components of a pH meter
- ) standard electrode of known potential (typical Ag/AgCl)
- ) special glass electrode that changes potential
- ) potentiometer that measures the potential btwn electrodes
what solution is in the glass electrode of a pH meter
HCL
what does the electrical potential depend on in a pH meter
difference in protons btwn the reference soln and the soln the electrode is in
- potential varies with the pH of the soln tested
what is the potentiometer between electrodes converted into
a direct reading of pH
what are ion-selective electrodes
electrodes that are sensitive to the concentration of a particular ion
example: glass electrode for pH meter
how can you make an electrode sensitive to other ions
by changing the composition of the glass
what ions can be detected by replacing the glass membrane with an appropriate crystal
F-, Ag+, S-2
what is a battery
portable self contained electrochemical power source that has 1 or more voltaic cells
how can you achieve greater voltages from a battery
- using multiple voltaic cells
- using multiple batteries in a series
how are the cells connected in a battery?
how are the cathode and anode connected?
they are connected in a series
- cathode of one cell is connected to the anode of another
what is the total voltage
what sign is used to represent a cathode
+
what sign is used to represent an anode
-
what does the lifetime of a battery depend on?
the quantities of the substances that are oxidized and rediced
what are the two main types of batteries
primary and secondary cell batteries
what are primary cell batteries
cannot be recharged; discarded when emf drops to zero
what is a secondary battery
can be recharged from an external power source after its emf drops
what is the composition of a lead batter
automotive battery that consists of 6 cells in a series
-
what is the anode in a lead battery
Pb
what is the cathode in a lead battery
PbO2
what is the cell voltage in a lead battery
12 V
what is recharging and what does it generate
an external source of energy is used to reverse the overall cell reaction
- generates Pb and PbO2
what does the alternator do
provides the energy needed to recharge
what is an alkaline battery
most common primary (non-rechargeable) battery
what is the anode in an alkaline battery
Zn or Mg
what is the cathode in an alkaline battery
MnO2 and graphite
what is the eleectrolyte in an alkaline battery
KOH
what is an alkaline battery separated by
porous fabric
what is the cell voltage of an alkaline battery
1.55 V @ 25 degrees
what is the nickel-cadium battery and what is it used in
most rechargeable battery
- used in cell phones, laptops, video cameras
what is the anode and cathode for nickel cadium batter
- anode = cadmium metal
- cathode = nickel oxyhyrdoxide
what is the electrolyte and cell voltage of nickel-cadium battery
- electrolyte: KOH
- cell voltage = 1.30 V
what are the pros and cons of a nickel cadium batter
- pros: rechargeable, long life, and lightweight
- cons: Cd is toxic
what is the difference between metal hydride battery compared to NiCad
- an improvement to NiCad batteries
- cathode is the same anode is different
what is anode for NiMH battery
metal alloy with dissolved hydrogen
what is the electrolyte and cell voltage for a NIMH battery
electrolyte = KOH
cell voltage = 1.30 v
what is the lithium ion battery
newest rechargeable battery
what is the Li-ion battery used in
cell phones and laptops
what is the anode and cathode of the Li-ion battery
anode = graphite
cathode = lithium cobalt oxide
what is Li-ion electrolyte and cell voltage
- electrolyte = KOH
cell voltage = 3.7 V
what are the pros of the li-ion battery
rechargeable, long life, very light more environmentally friendly
what can the thermal energy released by combustion turn into
electrical energy
what are fuel cells
the direct production of electricity from fuels by a voltaic cell yields a higher conversion rate
what does PEM stand for
proton exchange membrane
what is corrosion
the spontaneous oxidation of metals by chemicals in the environment
what does the rusting of iron require
oxygen and water
what is an electrolysis reaction
- forces a current through a cell to produce a chemical change for which the cell potential is negative
what are the practical purposes
the charging a battery producing aluminum, purifying metals, and electroplating
where does electrolysis take place
electrolytic cells
- requires an external power source to supply greater energy than EMF
what is the 3rd most abundant element on earth and where is found
aluminum, found in nature as an oxide
impure metals can be purified by what
electrolysis
what is electroplating
uses electrolysis to deposit a thin layer of one metal on another metal
