Quiz 2 Flashcards
(25 cards)
The energy difference between bands is called the ___
band gap
What does the band diagram of an insulator look like?
very large energy difference between valence band and conduction band
What does the band gap look like for metals? 2types
- metal with partially filled band , medium band gap, but half of valence band is unoccupied
- metal with overlapping bands - no band gap, part of the conduction band is filled
What does the band gap of a semiconductor look like? 3 types
intrinsic semiconductor - small band gap - Silicon
p type extrinsic conductor - impurity has fewer electrons (Boron) - has unfilled acceptor band close to valence band
n type extrinsic conductor - impurity has more electrons (Arsenic) - has filled donor band closer to conduction band
Explain the electrical conductivity of insulators, metals, and semiconductors
insulator - low electrical conductivity
metals - good conductivity that decreases with temperature
semiconductors - good conductivity that increases with temperature
Define electrical conduction
how easy it is for electrons to flow from one place to another
What is the difference between the amount of energy needed to create electrical conduction?
metals - electrons easily move from filled states
semiconductors and insulators - significant energy may be needed to promote -insulators need more - substituted semiconductors need less than normal semiconductors
Define thermal conductivity in the context of bands
promotion of electrons across the bandgap when thermal energy and bandgap energy are similar
Define photoconductivity
When light with greater energy than the bandgap strikes the semiconductor, electrons are promoted
Explain why some materials are colored?
Metals - band gap energy is 0 so all light is absorbed (black or shiny)
Semiconductors - as band gap energy increases - higher energy light only is absorbed so materials have color
Insulators - band gap energy is too large so no light is absorbed - white
Explain the properties of an alkaline fuel cells including reactions
Input and output at the anode are both H2(g)
Input and output at the cathode are O2(g)
Electrolyte is KOH(aq)
Anode reaction -
2H2(g) + 4OH-(aq) — 4H2O(l) + 4e-
Cathode reaction
O2(g) + 2H2O(l) + 4e- — 4OH-(aq)
Net -
O2(g) + 2H2(g) — 2H2O(l)
Explain the properties of a polymer electrolyte membrane(PEM) fuel cell (also called a proton exchange fuel cell)
Input and output at the anode are both H2(g)
Input and output at the cathode are air and water
Pt catalysts on either side of membrane
- polymer membrane that only allows H+ to cross- Nafion
efficiency can reach 50% but has a low operating temperature
Anode -
H2 — 2H+ + 2e-
Cathode-
4H+ + 4e- + O2 — 2H2O
Explain the reactivity of H2(g)
- not reactive under ambient conditions
- kinetically but not thermodynamically stable - needs activation to break H-H bond
- on metal surfaces such as Pt - H2 will split into 2H+
Explain the reactivity of O2(g), what is required to drive the reaction?
- requires large amounts of energy to break double bonds
- overpotential is required to drive reaction
- means that operating voltages of fuel cells are much less than desired
Explain the function of a solid oxide fuel cell including half reactions
Input and output at the anode are H2 and both H2 and H2O are outputs
Input and output at the cathode are air in and unused air out (o2)
Uses yttria stabilized zirconia (YSZ) as ion conductor
- substitutes Y3+ in ZrO2 creating oxygen vacancies that conduct ions
- need very high temperatures but less need for catalysts
Anode
H2 + O^2- —- H2O + 2e-
Cathode
1/2O2 + +2e- —- O^2-
Why cant we get H2 from chemical reactions involving metals as many metals react with water to form hydrogen
hahaha metal go boom on big scale
What are the main industrial reactions that produce H2?
Water gas reactions
C + H2O —- CO + H2
CO + H2O —- CO2 + H2
Steam reforming
CH4 + H2O —- CO + 3H2
Explain the function of a methanol or ethanol fuel cell?
Input and output at anode is CH3OH and CO2
INput and output at cathode is O2 and H2O
Pt used as a catalyst
Ethanol enters the cell and reacts at the anode providing electrons that can exit the cell and do work along with protons which get pushed across a semipermeable polymer/electrolyte in the center of the cell where they will react with air (O2) to form water at the cathode
Anode -
CH3OH + H2O — CO2 + 6H+ + 6e-
Cathode-
3/2 O2 + 6H + 6e- — 3H2O
Three industrial applicaitons of H2?
Haber bosch process for fixation of nitrogen to form ammonia
hydrogenation of unsaturated fats - margerine formation
synthesis of organic compounds IE MeOH
H2 is often produced in _____
in situ meaning in the reaction vessel due to difficulty storage (low density and reactive)
Explain the properties of metals to resist oxidation, include the Nernst equation
E1/2 for O2
O2 + 4H+ + 4e- — 2H2O = 1.229V
- this means that any metal with a half potential less than 1.229 V will the oxidized by oxygen
Nernst equation is 1.229 + 0.059(pH) - changes the volts required to be oxidized / resist oxidation
What is the formula for Ecell?
Ecell = Ered - Eox
if Ecell is positive then deltaG is negative meaning the reaction is spontaneous
What happens at high doping concentrations in semiconductors?
the bands overlap and the sample becomes metallic
What is the relationship between bond length and band gap energy?
shorter bonds hold electrons more tightly to the atom, meaning that shorter bond length will require a higher band gap energy to promote
