CHEM 160 Exam 4

Voltage

the difference in potential energy between the reactants and products, also called the potential different

units = volt (V)

the amount of force pushing the electrons through the wire = electromotive force

the driving force on electrons that pushing it from anode to cathode

Current

the number of electrons that flow through the system per second

the amount of electric charge that passes a point in a given period of time

units = ampere (A)

Electrochemistry

the study of redox reactions that produce or require an electric current

• conversion between chemical and electrical energy which is carried out in an electrochemical cell

Voltaic/galvanic cell

spontaneous redox reactions take place in these

does not require external energy to proceed

Electrolytic cell

nonspontaneous redox reactions can be made to occur in an electrolytic cell by the addition of electrical energy

Diagram/drawing Set Up

electrons travel through wire to get to other beaker and react, as it travels it powers some electrical device

theres an anode and cathode, with the anode normally on the left

electrons leave the anode and go to the cathode

electrodes are there too, they are conductive solids and allow the transfer of electrons

salt bridge has ions in it and makes sure ion charged are balanced, positive ions go to cathode and negative ions go to anode, ions don’t participate with the reaction

half reactions kept separate in half cells

electrons only flow on surfaced of metal

(anode electrode is dissolving, cathode is getting bigger)

Inert electrode

piece of metal that doesn’t oxidize or reduce metal itself, no participation in reaction

it only moves electrons

used when the redox reaction involves the oxidation or reduction of an ion to a different oxidation state or the oxidation or reduction of a gas

Cell Notation

electrode|electrolyte||electrolyte|electrode

normally anode half cell is on the left and cathode half cell is on the right

flow of electrons is from left to right (→)

|| = salt bridge

| = phase barrier, if they;re in the same phase then use commas to separate them

Anode (-)

where there is oxidation

where electrons are produced

what anions migrate towards

Cathode (+)

where there is reduction

where electrons are consumed

what cations migrate towards

Cell Potential

electromotive force: the force or electrical potential that pushes the negatively charged electrons away from the anode and pulls them toward the cathode aka cell potential (E) or cell voltage

Standard Reduction Potential

a half cell that’s a reference for everything in the universe

standard hydrogen electrode (SHE)

• H₂ (g, 1 atm) → 2H⁺ (aq, 1 M) + 2e^-

• ask what’s the force between anode/cathode and H⁺ & H₂ beaker then combine numbers to find out V for the anode and cathode

voltmeter: measures voltage

half-reactions with a stronger tendency toward reduction than the SHE have a positive value E°_red

half-reactions with a stronger tendency toward oxidation than the SHE have a negative value for E°_red

E°_oxidation = -E°_reduction

Calculating cell potentials under standard conditions

E°_cell = E°_{reduction, cathode (reduction)} - E°_{reduction, anode (oxidation)}

• never multiply the values of E°_red/E°_ox

(+) cell potential = spontaneous

(-) cell potential = non-spontaneous

Free Energy Changes

∆G = -nFE

∆G° = -nFE°_cell

Faraday constant = 96,485 C/mol e^-

n = the number of moles of electrons transferred in the balanced redox reaction

E°_cell, ∆G° and K

E°_cell = 0.0592 V/n (log K)

for a spontaneous reaction that proceeds in the forward direction: ∆G° < 1, E° > 1, K > 1

Cell Potential when Ion Concentrations are not 1 M

E_cell = E°_cell - 0.0592 V/n logQ

Radioactivity

release of high-energy particles and/or high-energy electromagnetic radiation from the nucleus of an atom

radioactive decay = when radioactive nuclei spontaneously decompose into smaller nuclei

parent nuclide (reactant) = nucleus undergoing radioactive decay

daughter (product) = new nucleus that’s formed

Writing down elements

mass number on top

atomic number on the botttom

Alpha Decay

alpha particle is a 4/2 He nucleus

238/92 U → 234/90 Th + 4/2 He

most ionizing, but least penetrating

protection = paper or light cloth

Beta decay

neutron becomes a proton

beta particle = 0/-1 e, an electron-like particle

14/6 C → 14/7 N + 0/-1 e

10 times more penetrating than an alpha particle but only half the ionizing ability

protection = heavy cloth

Gamma emission

gamma rays are high energy photons

no change in the composition of the nucleus

gamma ray = 0/0 y

238/92 U → 234/90 Th + 4/2 He + 0/0 y

*first part is alpha decay and 0/0 y is the gamma emission

least ionizing but most penetration so it’s the most dangerous

protection = lead plates and thick cement walls

Positron emission

proton goes to a neutron

0/+1 e (anti electron)

30/15 P → 30/14 Si + 0/+1 e

Electron capture

electron and proton combine to form neutron

92/44 Ru + 0/-1 e → 92/43 Tc

Nuclear stability

particles in the nucleus are held together by the strong force

the neutrons help stabilizing since they add to force but don’t repel

for small elements having the same number of protons and neutrons allows it to be stable but as they get heavier they quire more neutrons to be stable (sometimes there is no stable form for an element)

Mass and energy

putting mass together releases energy with some mass lost as energy

breaking bonds can lead to energy turning into mass

Fission

the large nucleus splits into two smaller nuclei (breaks apart)

release enormous amounts of energy

accelerating a neutron into it breaking an atom apart and then you get 3 neutrons out of it leading to a chain reaction that can happen

chain reaction = when a reactant in the process is also a product of the process

critical mass = the minimum amount of fissionable isotope needed to sustain the chain reaction

fissionable isotopes = U-235, Pu-239 and Pu-240

have to make/enrich U-235 since natural u-235 is < 1%

nuclear reactors use fission energy to generate electricity, it produces heat which boils water and then the steam turns a turbine that generates electricity

Concerns: water loss leading to heat melting core, waste that’s radioactive

Fusion

small nuclei can be accelerated to smash together to make a larger nucleus (come together)

ex: sun uses fusion of hydrogen isotopes to make helium as a power source

produces 10x more energy than fission (so also releases enormous amounts)

no radioactive by-products

*high energy particles can be smashed into target nuclei and result in the production of new nuclei

*PET scan injects positron/F-18 isotope to then trakc that activity