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Level 128

Electrochemistry


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Reduction
Cathode is the (oxidation or reduction) part of the reaction?
Galvanic cell
Aka voltaic cell, a spontaneous redox reaction is used to generate a flow of current.
Anode
A positive electrode
Cathode
A negative electrode
cell potential
describes how vigorous a reaction will be
volt
unit of electrical potential
OIL RIG
Oxidation Is Loss (of electrons) Reduction Is Gain (of electrons)
Reducing agent
A reagent that reduces (adds electrons to) another species
Oxidizing agent
An element/compound that's reduced.
Voltaic/galvanic cell
An electrochemical cell that PRODUCES electrical current from a spontaneous chemical reaction
Electrolytic cell
An electrochemical cell that USES electrical current to drive a non-spontaneous reaction
salt bridge
_______ allows ions to flow between the cells to balanced the charge in the half-cells.
+E°cell
Voltaic cell that works. Set up correctly
-E°cell
Voltaic cell that does NOT work. Set up backwards
Electrode
a metal strip that conducts electricity
corrosion
Example of a slow chemical reaction
electroplating
deposition of a thin layer of a metal on an object in an electrolytic cell; did to protect the surface of the base metal from corrosion or to make it more attractive
anions
Elements which gain electrons become _______. (anions or cations)
Cations
Ions with a positive charge.
Warm salt water
Conditions that speed up corrosion.
Lithium
Light, reactive metal used in cellphone and pacemaker batteries
electrochemical cell
system in which electric current flows while a chemical reaction is occurring
electrolysis
an electric current forces a nonspontaneous redox reaction to occur
half-reaction
shows either the oxidation or reduction portion of a redox reaction
oxidation number (state)
number assigned to keep track of electron gain or loss in redox reactions
redox
an oxidation-reduction reaction
voltaic cell
electrochemical cell in which a spontaneous chemical reaction causes a flow of electrons
Must add up to zero
Oxidation #s in a neutral molecule
Oxidation #s in an ionic compound
Must add up to its charge
Is always zero
Oxidation # of pure elements
Is always +1
Oxidation # of Hydrogen
Is always -2
Oxidation # of Oxygen
It is almost always -1
Oxidation # of Fluorine when combined with other elements
-1
Oxidation # of Chlorine and Bromine (unless combined with fluorine or oxygen)
Activity Series
A list of metals arranged in order of decreasing ease of oxidation
Half Reactions
Equations that show either oxidation or reduction alone
Balancing Equations in Basic Solution
Neutralize H+ ions with OH- to form H2O. Add same number of OH- as there is of H+ to both sides of equation.
Electrodes
Anode (Positive), Cathode (Negative) this only applies for electrolysis
Ion migration
Anions always migrate toward the anode and cations toward the cathode
Electron migration
Electrons flow from the anode through the external circuit to the cathode
Electromotive Force (emf)
Another term for Ecell because the potential difference provides the driving force that pushes electrons through the external circuit
Standard Reduction Potentials
Standard half-cell potentials for reduction reactions
The more positive the value of Eºred
The greater the tendency for reduction under standard conditions.
Strengths of Oxidizing and Reducing Agents
The lower the tendency for a half reaction to occur in one direction, the greater the tendency for it to occur in the opposite direction. The half reaction with the most negative reduction potentia…
Strogenst oxidizing agent
Most positive and easiest to reduce
Weakest oxidizing agent
Most negative and most difficult to reduce
Strongest reducing agent
Most negative and easiest to oxidize
Weakest reducing agent
Most positive and difficult to oxidize
Free Energy and Redox Reactions
A positive value of E [Eºred(reduction) - Eºred(oxidation)] indicates a spontaneous process; a negative value of E indicates a nonspontaneous process
Nernst Equation
Increasing concentration of reactant and decreasing concentration of product increases the emf of the cell
Concentration Cells
A cell based solely on the emf generated because of a difference in concentration because the same species are in both half cells
Oxidation in concentration cell
Occurs in the half cell containing the more dilute solution
Reduction in concentration cell
Occurs in the half cell containing the more concentrated solution
Battery
A portable, self contained electrochemical power source that consists of one or more voltaic cells
Lead-Acid Battery
Cathode = Lead dioxide (PbO2)
Alkaline Battery
Most common non-rechargeable (primary) battery
Nickel Cadmium Battery
Rechargeable (eventually unable to)
Nickel Metal Hydride Battery
Used in hybrid gas electric automobiles; recharged by the electric motor while breaking
Lithium Ion battery
found in cell phones and laptops
Hydrogen Fuel Cells
voltaic cells that perform the conversion of chemical energy to electrical energy using conventional fuels
Chemical potential
The increase in the total energy of the thermodynamic system produced by the reversible addition of one mole of the component to the system under constant conditions
Metals consist of..
Fixed cations in a lattice with free moving electrons through the lattice
Solutions contain..
Ions which move through the solution, positive and negative charges are carried through the solution by these ions
When 2 charged phases come into contact
The metal dissolves / deposition of metal ions onto the metal
Interfacial potential difference
Occurs at equilibrium across an interface
Electric potential difference
The work done per unit change, by an external force in moving a unit positive charge from a point A to a point B
Liquid junction potential
The steady state potential difference between 2 solutions at different concentrations separated by a porous barrier
EMF
Electromotive force, the force available to drive electrons through an external circuit connected between 2 metals
Catholic and anodic charge transfer coefficients
Reflect the amount of the applied potential which actually goes into reducing the activation barrier for electron transfer in the forward and reverse directions
High positive/negative over potentials, anodic/cathodic Tafel equations
What are the limits for the Bulter-Volmer equation
Tafel equations
Predict an exponential increase in the current with over potential
Redox reactions within batteries..
Must take place a different sites within the battery, which causes electrons to flow through an external circuit between the anode and cathode
Voltage of the battery depends on..
The choice of electrode reactions taking place at the anode and cathode, the kinetics and the internal resistance of the battery
Current
A
Capacity
The amount of charge that can be obtained from the battery which depends on the size of battery
The discharge rate
The current required to discharge the capacity C in n hours
Electrical storage density
The charge stored in the battery per uni weight
How can a high storage density be achieved
Good design, use of anodes consisting of Li, Al or Mg.
Common battery systems
Lead-acid, Nickel-cadmium, Zinc-Carbon
Lithium-ion battery design
Li+ ions are intercalated into the positive electrode in the discharged state and into the negative electrode in the charged state
The anode of a lithium-ion battery
Lithium dissolved as ions into a carbon
Lithium liberating compounds (LiCoO2)
The cathode of a lithium-ion battery
The cell voltage of the lithium-ion battery
The difference in free energy between the Li+ ions in the crystal structures of the two electrode materials
Unlike a battery, a fuel cell...
Never runs down or requires recharging, it will produce energy in the form of electricity and heat directly from the fuel
Two electrodes separated by an electrolyte
What does a fuel cell consist of
What happens inside a fuel cell
Oxygen passes over one electrode and hydrogen over the other. The hydrogen fuel is oxidised in the fuel cell generating electricity, water and heat.
What happens when hydrogen fuel is fed into a fuel cell
On the anode surface the hydrogen molecule is dissociated and the hydrogen atom is oxidised into a proton and an electron.
What can a fuel reformer do
Utilise the hydrogen from any hydrocarbon fuel.
What is different about a DMFC cell
The liquid methanol is itself oxidised and so eliminating the need for a fuel reformer
Thermodynamic conditions for corrosion
Ecell > 0 G < 0
Pourbaix diagram
E vs pH - defines areas of stability for the metal in the electrolyte in terms of corrosion
Horizontal lines in pourbaix diagrams
Only electrons - independent of pH
Vertical lines in pourbaix diagrams
Equilibria which involve only hydrolysis - no electron transfer
Sloping lines in pourbaix diagrams
Equilibria involving electron transfer and hydrolysis
Dashed lines in pourbaix diagrams
Potentials of the O2/H2O and H+/H2 reactions
How can corrosion be prevented
Slowing down the rate of anodic dissolution of the metal, removing the electrolyte or increasing its resistance, decreasing the rate of cathodic reaction, changing the electrode potential.
potentiometry
analytical methods based on potential measurments
oxidation number
charge that atom in a molecule is going to have if electrons are completely transferred
electrochemical cells
a set of chemical species, cathodes, anodes and a salt bridge which uses redox reactions to generate energy.
electrode potential
the potential difference that develops between the electrodes of the cell.
standard electrode potential
the potential of a half reaction written as a reduction when the activities of all reactants and products are in unity.
reference electrodes
a half cell of known potential which remains the same and is independent of the analyte solution
indicator electrodes
electrodes whose potential changes in a known way with the variations of the concentration of the analyte
junction potential
Ej; the potential that develops across the salt bridge.
electrode of the first kind
Metal electrode in own ion Ag/Ag
electrode of the second kind
Metal in concentration of own ion and another ag/agcl
calomel
Hg2Cl2; a type of half cell in which mercury is coated with calomel and the electrolyte is a solution of KCl with saturated calomel.
amplitude
(A) length of the electric field at wave maximum
Period
(p) time required for successive maxima or minima to pass through a point in space
frequency
1/period
wavelangth
linear distance from successive maxima and minima
wavenumber
number of waves per centimeter
power
how fast work is being done
intensity
power per unit solid angle in 3D space
molar absorptivity
constant of proportionality between A and bc, a measure of the probability of an electronic transitiion
monochromatic
a light source with only one frequency
polychromatic
a light source which has multiple frequencies of light
monochromator
a light source with only a single wavelength type
transducer
a type of detector that converts chemical and phsical quantities into electrical signals sach as current, charge, or voltage.
LOG(T)a
Relationship between absorbance and transmittance
SHE
Standard hydrogen Electrode. 2H+2e- = H2
SCE
Stanard Calomel Electrode
silver silver chloride
Ag/AgCl AgCl + e- = Ag + Cl
Nernstian Behaviour
If the electrode follows the Nernstian equation and potential follow 0.059 /nV per ten fold concentration.
beer's Law
A is proportional to concentration
John Lambert
absorbance is portional to pathlength.
5 componenets in all optical
source, wavelength selector, sample, detecter, signal processor and readout
for electricity to be generated
there must be a transfer of electron which implies an oxidation reduction reaction
physically separate the 2 half reactions
to get useful work from a battery
positive voltage
reaction is spontaneous
negative voltage
reaction is NOT spontaneous
reversing a reaction
changes the sign of voltage
voltage
intensive property
electrolytes
all aqueous acids are
4.07v
battery must be >_______ to occur
stronger oxidizing agent
bottom of oxidizing agent column
cathodic protection
process of protecting metal by sacrificing another more active metal
standard state conditions for voltaic cells (same as ?G)
1 ATM, 25.0°C (room temp), solutions are 1.0 M
nuclear chemistry
concerned with the nucleus, not electrons
Transmutation
A change in the identity of a nucleus as a result of a change in the number of its protons
alpha particle
42He or a
beta particle
°?1e or ß?
positron
°1e or ß?
proton
¹1P
neutron
¹0n
gamma radiation
°0 ?
left side
captured, absorbed, bombarded, taken in
right side
emitted, given off, released
radioactive and undergo spontaneous transmutation
all elements with atomic # >83
abbaaaaabbabba
pattern for beta and alpha
²³892U?42He + ²³490Th,
first 3 transmutations of U-238
relative abundance of elements in the universe
obtained by spectral analysis of star light
lighter elements
more abundant than heavier elements
even #s of protons
generally more abundant that odd #s of protons
elements with atomic # >83
radioactive and undergo spontaneous transmutation
heavier elements
produced in supernovi
depends on the element
how long does radioactive decay take?
half life
amount of time required for half the original decay
5730 years
half life of ¹46C
after 10 half lives
there is no longer enough material to detect (60,000 years for ¹46C)
60
cannnot carbon date anything older than about _______ years
fission
the splitting apart of nuclei (nuclear power)
Fusion
Melting
nuclear energy
all nuclear energy today is accomplished by fusion
fission and fusion
tremendous release of energy associated with _______ and _______
products and reactants
small change in mass in _______ and _______
energy, E=MC²
the small change in products and reactants translates to a HUGE change in _______ according to _______
ionizing radiation
contains enough energy to overcome ionization energies or break bonds
Geiger Counter, Scintillation Counter
how to detect ionizing radiation
nuclear power
all nuclear power utilizes nuclear fission
energy, fission
tremendous amount of _______ is released during _______
fission, steam, turbine, generator
we capture the heat from _______ and use it to make _______ that turns a _______ that turns a _______
nuclear bomb
divide critical mass into 2 sub critical masses. detonate dynamite to drive 2 halves together to start chain reaction.
nuclear reactor, fission, neutrons
in a _______ we must control how quietly the _______ occurs which means we have to control _______
control rods
absorb neutrons and can be lowered into the reactor to slow the chain reaction
moderator
a substance that slows down neutrons
oxidation-reduction reactions
In which one or more electrons are transferred are called
F
weak nucleophile
0
the oxidation state/ number of an atom in an uncombined element is
T
Temperature
-2
except with peroxides
O2²?
peroxides: compounds containing the .... group
-1
in the case of peroxides (O2²?) in which each oxygen is assigned an oxidation state of
1
hydrogen is assigned an oxidation state of
Zero
A plot of reactant concentration versus time gives a straight line. What is the order of the rxn for this reactant?
Charge
C
half-reactions
are equations that have electrons as reactants or products
equal
If the moles of two reactants are equal, then the rates of reactant concentration for each reactant (-?[A1]/?t & -?[A2]/?t), is presumably...?
Electrochemistry
Any chemistry that produces electricity
electrochemical battery/ galvanic cell
is a device powered by an oxidation-reduction reaction where the oxidizing agent is separated from the reducing agent so that the electrons must travel through a wire from the reducing agent to the oxidizing agent
strong
if a substance gives up electrons readily it is said to be a //// reducing agent
oxidizing
if a substance gains electrons readily it is said to be a ...... agent
1
all metals of group 1 (alkali metals) oxidation number is
2
all metals of group 2 (alkaline-earth metals) oxidation number is
-1
all halogens in group 17 oxidation number is
1
the oxidation number of H is /// in all its compounds except the metal hydrides
-1
in metal hydrides (LiH, CaH2) the hydrogen oxidation number is
Hydrogen
Acids always contain this element
decreasing
reference table = substances are arranged in order of ...... oxidizing strength
Li+
lithium ion
F2
strongest oxidizing agent is
number
the _______ of standard atomic orbitals added together always equals the _______ of hybrid orbitals formed
+
a net voltage (E°) = ...... in a spontaneous redox reaction
electrolytic cells
involves electrolysis or electroplating.
Electrical conductivity
A property of a substance that enables it to conduct an electric current through the flows of an electron
species
when its an aqueous electrolytic cell problem you have competing ....
SO
RA
Sr
ion symbol
metal
_______ + nonmetal is an example of an ionic bond
internal circuit
the flow of electrons through a salt bridge or porous cup
external circuit
the flow of electrons through the wire and metal electrodes
salt bridge/porous cup
function to maintain electrical neutrality in each half cell by allowing ions to migrate
Decreases
nucleophilicity _______ across a row of periodic table
increases
nucleophilicity _______ down a column of periodic table in polar protic solvents
coating
methods of combating corrosion thru paint enamel grease etc
alloying
methods of combating corrosion thru stuff like "stainless steel"
-1
in H2O2 the oxidation number of oxygen is
F (most electro negative element)
oxygen will have a postive oxidation number when combined with
both
all redox reactions involve ...... the gain and the loss of electrons
simultaneously
the oxidation half-reaction and the reduction half-reaction occur .......... in redox reactions
replacement
redox reactions = single
mass and charge
in a redox reaction there is a conservation of both .....
ger
- =
leo
+ =
reducing
when a substance is oxidized it acts as a ..... agent
electric
a voltaic cell differs from an electrolytic cell because in a voltaic cell there is a redox reaction that produces an ..... current
gain
ions near cathode ..... electrons
oxidized
in becoming ions, metals are
true
(ex mg)
0
voltage of a chemical cell that has reached equilibrium
flip (negate/change sign too)
oxidation you ..... the voltage shown on standard electrode potentials table
negative (cathode)
in an electolytic cell, cation migrates to the ...... electrode
Solid
Fixed shape and size, atoms are close together, ordered structure, not compressible
give
strongest reducing agent will most readily .... up electrons
H
Hydrogen