Quiz 2

Question 1

The energy difference between bands is called the ___

Answer 1

band gap

Question 2

What does the band diagram of an insulator look like?

Answer 2

very large energy difference between valence band and conduction band

Question 3

What does the band gap look like for metals? 2types

Answer 3

• 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

Question 4

What does the band gap of a semiconductor look like? 3 types

Answer 4

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

Question 5

Explain the electrical conductivity of insulators, metals, and semiconductors

Answer 5

insulator - low electrical conductivity
metals - good conductivity that decreases with temperature
semiconductors - good conductivity that increases with temperature

Question 6

Define electrical conduction

Answer 6

how easy it is for electrons to flow from one place to another

Question 7

What is the difference between the amount of energy needed to create electrical conduction?

Answer 7

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

Question 8

Define thermal conductivity in the context of bands

Answer 8

promotion of electrons across the bandgap when thermal energy and bandgap energy are similar

Question 9

Define photoconductivity

Answer 9

When light with greater energy than the bandgap strikes the semiconductor, electrons are promoted

Question 10

Explain why some materials are colored?

Answer 10

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

Question 11

Explain the properties of an alkaline fuel cells including reactions

Answer 11

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)

Question 12

Explain the properties of a polymer electrolyte membrane(PEM) fuel cell (also called a proton exchange fuel cell)

Answer 12

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

Question 13

Explain the reactivity of H2(g)

Answer 13

• 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+

Question 14

Explain the reactivity of O2(g), what is required to drive the reaction?

Answer 14

• 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

Question 15

Explain the function of a solid oxide fuel cell including half reactions

Answer 15

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-

Question 16

Why cant we get H2 from chemical reactions involving metals as many metals react with water to form hydrogen

Answer 16

hahaha metal go boom on big scale

Question 17

What are the main industrial reactions that produce H2?

Answer 17

Water gas reactions
C + H2O —- CO + H2
CO + H2O —- CO2 + H2

Steam reforming
CH4 + H2O —- CO + 3H2

Question 18

Explain the function of a methanol or ethanol fuel cell?

Answer 18

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

Question 19

Three industrial applicaitons of H2?

Answer 19

Haber bosch process for fixation of nitrogen to form ammonia

hydrogenation of unsaturated fats - margerine formation

synthesis of organic compounds IE MeOH

Question 20

H2 is often produced in _____

Answer 20

in situ meaning in the reaction vessel due to difficulty storage (low density and reactive)

Question 21

Explain the properties of metals to resist oxidation, include the Nernst equation

Answer 21

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

Question 22

What is the formula for Ecell?

Answer 22

Ecell = Ered - Eox
if Ecell is positive then deltaG is negative meaning the reaction is spontaneous

Question 23

What happens at high doping concentrations in semiconductors?

Answer 23

the bands overlap and the sample becomes metallic

Question 24

What is the relationship between bond length and band gap energy?

Answer 24

shorter bonds hold electrons more tightly to the atom, meaning that shorter bond length will require a higher band gap energy to promote

Question 25

Why is there a relationship between band gap energy and electronegativity differences?

Answer 25

As the electronegativity difference increases, the energy of the atomic orbitals of the more electronegative element decreases, and the energy of the atomic orbitals of the less electronegative element increases producing a larger band gap energy.