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CHEM2017 > Solid state chemistry > Flashcards

Solid state chemistry Flashcards

1
Q

2 types of solids but 1 has 2 subtypes.

A

1.Crystalline: Single crystals, Polycrystalline (many small crystals pack together)
2. non crystalline/amorphous (egglass)

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2
Q

Electrostatic bonding of ions equation. What is z and r?

A

F = -z1z2/4πEr^2 where z=charges of ions and r = distance between the 2 point charges

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3
Q

What is the internal energy (U) based on work done on ions (work = force x distance)

A

U = -z1z2/4πEr

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4
Q

Properties of ionic solids

A

Brittle, electronically insulating, high MP

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5
Q

Props of covalent solids

A

Rigid, electrically insulating, high MP

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6
Q

Props of metallic solids

A

Ductile, electrically conducting, high MP

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7
Q

Props of molecular solids

A

Soft, electrically insulating, low MP

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8
Q

Why are ionic solids brittle

A

If ions are displaces, all ions line up with similar charges which repel. It reduces the bonding so they separate

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9
Q

Why are ionic solids insulators

A

Extra electrons on anions are firmly attached so can’t carry electric current. Ions locked in position by electrostatic forces. Will conduct once melted tho

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10
Q

2 diff ways and names of enthalpies used when dissolving ionic solid

A

ΔHlattice = boiling into gaseous ions (very +ve) then ΔHsolvation (hydration) = water to gaseous ions (very -ve). OR ΔHsolution where solid dissolved in water. Sometimes +ve or -ve

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11
Q

Equation for gibbs E

A

ΔG = ΔH - TΔS

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12
Q

Why are cations smaller than anions

A

+ve charge pulls e- inwards

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13
Q

2 evidences for ionic solids

A
  1. Electron density maps: shows where e- density is. Metal + covalent have e- between atoms but ionic doesnt.
  2. Thermodynamic properties predicted using ionic model agree with experimentally measured props
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14
Q

First ionisation energy, Ie, definition

A

Change in internal energy for gas atom -> gas atom+. AT 0K! (0 kelvin)

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15
Q

What sign is ionisation energy always

A

Postive as E has to be put into the system to remove an e-

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16
Q

What does it mean to have a positive value for electron affinity Ea

A

Energy is released, exothermic. Different to normal

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17
Q

What sign is always 2nd electron affinities

A

Always endothermic so -ve. Energy is required

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18
Q

What does the Born-Lande potential equation represent

A

total coulombic interaction between 2 ions based on repulsion (electrons) + attraction (+ve and -ve charge). It is just the one you draw on the graph that goes below then above and plateaus

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19
Q

What is the unit for the Born-Lande potential equation

A

J per ion pair

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20
Q

What does the Born-Mayer equation calculate + in what unit

A

Lattice energies in J per mol

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21
Q

What is the constant A in Born-Mayer equation

A

Madelung’s constant

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22
Q

How is Madelungs constant calculated in a certain structure

A

Taking every distance between one ion to all other ions in a lattice and adding together. Repulsive = +ve, attractive = -ve

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23
Q

When is Kapustinskii’s equation used and what it works out

A

If structure and therefore Madelungs const is unknown, equation using number of ions etc in formula of compound works out lattice energy

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24
Q

What does rho in Born-Mayer equation come from

A

Compressibility data

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25
Q

What is Kapustinski’s equation based on

A

The fact that the Madelung constant/V (number of ions in the formula) is nearly constant for all structures

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26
Q

What is r0 in Born-Mayer equation + other eqa

A

The minimum distance between opposite ions = their equilibrium separation. Energy of system is a minimum

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27
Q

How is experimental value of UL found (lattice)

A

Bron-Haber/Thermodynamic cycle

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28
Q

3 approximations used in solid state thermodynamic cycles

A
  1. Enthalpy used rather than internal energy because H is at constant pressure but U is constant volume. ΔH = U + PΔV where ΔV is very small for solids so ΔH ≈ ΔU
  2. Ie & Ea are defined at 0K but thermodyn quants defined at 298 K. Only introduces a small error of 2-5kJ mol-1.
  3. ΔH are often so big that they determine if reaction is spontaneous. Don’t bother with ΔG and ΔS much. May need to for solubility when terms are closely balanced
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29
Q

Enthalpy equation based on internal energy

A

ΔH = ΔU+ PΔV

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30
Q

What is important to remember in Born Haber cycle about electron affinity. Thing used instead of Ea

A

If it is +ve then make it negative in calculation as it actually is exothermic. ΔHa (enth of electron gain) used in stead which is -Ea -5/2 RT but last bit not important bc very small

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31
Q

5 characterisation methods

A

Thermogravimetric analysis (TGA), Differential scanning calorimetry (DSC), Crystallography and X-ray diffraction, electrical+magnetic+optical properties (not covered), solid state NMR (not covered)

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32
Q

How does thermogravimetric analysis (TGA) work

A

Determines amount of volatile components in materials. Weight measured. Heated up and plot weight against temperature. Then can find x in eg Al2Si2O7 . xH2O as an integer

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33
Q

How does Differential Scanning Calorimetry (DSC) work

A

Empty crucible (ref) + sample crucible heated linearly. Power applied adjusted so both are at same temperature. Thermocouples measure temp. Graph of Difference in power (Pref - Psample) against temperature and time shows where more power needed (dip in graph) = endothermic = eg when H2O driven off. Peak = exothermic phase change eg from kaolinite -> mullite (exothermic phase change). Area in peak gives energy (power * time) -> shows enthalpy change

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34
Q

Lattice points definition

A

Positions within a crystal which all have identical environments

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35
Q

Motif (or Basis) definition

A

Atoms/molecules associated with each lattice point

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36
Q

Crystal structure def

A

Periodic arrangement of MOTIFS - has translational symmetry

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37
Q

Crystal lattice def

A

Periodic arrangement of LATTICE POINTS

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38
Q

What does a primitive unit cell have

A

Lattice point at each of the 8 corners. Vectors of any length + any angle. Only 1 lattice point per unit cell so 1 motif per unit cell.

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39
Q

Bcc structure

A

1/8 lattice point in each corner + 1 in middle. Total 2

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40
Q

Fcc (ccp) structure

A

1/8 in each corner, 1/2 on each face. Total 4 lattice points

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41
Q

Why are some non-primitive unit cells chosen over primitive

A

All crystals have primitive ones but can be harder to draw + visualise

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42
Q

What are 3 types of symmetry for crystals

A

Translational, point group and combined translation and point group symmetry

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43
Q

What is point group symmetry

A

Rotations and reflections about certain points in the crystals that leave crystal looking the same

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44
Q

What is combined translation + point group symmetry

A

Glide planes: translate crystal by 1/2 a unit cell in direction parallel to glide plane, then reflect crystal across glide plane. Screw axes also exist but not covered in this course.

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45
Q

What are the “only” rotation axis that exist

A

1 fold (no symmetry at all), 2 fold (180), 3, 4, 6. 5 is only quasi crystals. This is bc unit cells must tesselate and other rotational symmetry will create gaps. You can’t have gaps if you also have unit cells

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46
Q

What is tessellation

A

Arranging shapes closely together with no gaps - happens in crystal so only 1,2,3,4,6 fold rotations can happen

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47
Q

How many fundamental types of lattices are there and what is their name

A

14 and Bravais lattices

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48
Q

What are the seven crystal systems

A

1) Triclinic: non-equiv or perpendicular sides. No axes of symmetry. 2) Monoclinic: 2 faces at right angles to third. 3 edges different lengths. 3) Trigonal (rhombohedral): all edges are the same length. 4) Hexagonal: 2 edges same length (120° between them). 3rd at right angles to them. 5) Orthorhombic: all faces at right angles + all different lengths. 6) Tetragonal: All faces right angles. 1 square face. 7) Cubic: All faces right angles + same length edges

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49
Q

How many ways are there of stacking the 7 possible crystal systems (block shapes) for crystals

A

14 - Bravais lattices

50
Q

Energy of X-ray equation

A

E=hv=hc/λ. = very small number. Divide by e- charge = around 6200 eV

51
Q

When is diffraction the most pronounced

A

When wavelength is comparible to spacing

52
Q

What is a thermal neutron

A

The speed/energy they would have if neutrons were a gas at room temperature

53
Q

What is the Bragg equation + meaning + use

A

nλ=2dsinθ. d is distance between atom layers. n is order of diffraction. If equation is true for a θ, then it is constructive interference as beam bouncing off lower layer will continue in phase with beam off upper layer of crystal. All other θ are destructive. Use: Find length between atom layers in crystals (d) since we know λ and can find θ

54
Q

Which values are known/measured in Bragg equation

A

nλ=2dsinθ. We know λ from what we aim at crystal. θ is measured and d is worked out.

55
Q

How much of X-ray is diffracted by crystal

A

About 0.01%, but each layer of crystal reflects that much which results in a strong beam

56
Q

Separation d(hkl) between planes given by what eqn

A

d(hkl) = a/sqrt(h^2+k^2+l^2)

57
Q

Use of miller indices

A

We use the three miller indices (h k l) to refer to sets of planes with a particular spacing d(hkl). They are used in reciprocal space.

58
Q

How to produce X-ray using electrons (3 ways but 2 are similar)

A
  1. e- excited then releases photon as it drops back into hole.
  2. e- deceleration gives Bremsstrahlung radiation = continuum of white (all freqs) radiation.
  3. OR e- accelerated in magnetic field gives curve with increasing radius. Diff radii gives diff rate of change of velocity = white radiation emitted
59
Q

4 devices used for generating X-rays + explain first 2

A
  1. Standard X-ray generator: e- from filament go through field and hit copper target. Sudden breaking + the acceleration releases X-rays + peaks (2 or more) from e- in copper being excited then releasing photons
  2. Synchrotron bending magnet: accelerates e- around circle. Gives Bremsstrahlung x-rays (all)
  3. Synchrotron Wiggler
  4. Synchrotron Undulator
60
Q

What devices can be used for synchrotons

A
  1. Synchrotron Wiggler. Makes e- wiggle down then back up in normal path to increase x-ray intensity since it gets accelerated (change in direction)
  2. Synchrotron undulator: Wiggles it loads and loads. Since e- is almost at x-rays releaseds speed but longer path length means interference - causes intermittent peaks of higher intensity than wiggler.
61
Q

Hi

What process causes electrons to emit the x-rays used in a diffraction experiment

A

(Although Bremsstrahlung takes place in a sealed x-ray tube it produces polychromatic radiation.) In an x-ray diffraction we require monochromatic x-rays (a single wavelength) so we tune the instrument to use a characteristic peak (transition of electrons between energy levels) of the anode emission spectrum. Bending and wiggling electrons in a magnetic field to generate x-rays are methods only used in large synchrotron facilities.

62
Q

Conditions for diffracted beams to show up in Simple cubic, bcc and fcc

A

Simple: All allowed
bcc: sum of no. must be even so 110 yes but 100 no.
fcc: all must be even or all odd so 200 yes but 300 no.

63
Q

Care must be taken when using X-ray diffraction experiments to determine the location of hydrogen atoms in organic molecules. Why is this the case?

A

The electron density associated with hydrogen atoms will be pulled towards the adjacent carbon atom. The peak in the electron density map will not be centred on hydrogen atom itself.

64
Q

2 types of powder x-ray diffraction and their difference.

A

Single crystal and crystalline powder diffraction. Hard to grow single crystal so powder has many random orientations. Produces Debye-Scherrer rings instead of dots.

65
Q

What are Debye-Scherrer rings

A

Result of crystalline powder diffractions

66
Q

How are Debye-Scherrer rings analysed

A

Plotted on graph of intensity vs 2theta to find crystal structure using missing diffraction beams

67
Q

Why are beam stops used

A

99% of x-rays go through so used to stop them so machine doesn’t catch fire

68
Q

What causes the intensities of diffraction beams

A

The motif

69
Q

What causes the shape of diffraction pattern

A

The lattice shape

70
Q

Rules for close packing

A
  1. All atoms are hard spheres - no distorion.
  2. Spheres packed as tightly as possible.
  3. Big spheres leave spaces where little spheres can fit into
71
Q

What structure of layers is hcp. Coord number?

A

ABAB, coord: 12

72
Q

What structure of layers is ccp/fcc. Coord number?

A

ABCABC.., 12

73
Q

How much of available volume does fcc, hcp, bcc, simple cubic and fcc diamon structure occupy

A

fcc, hcp: 74%
bcc: 68%
Simple cubic: 52%
Fcc diamond: 34%

74
Q

What is polymorphism and example of what material does that

A

This is where a material has different crystal structures under different experimental
conditions (e.g. temperature or pressure). Iron.

75
Q

Structure of tetrahedral hole. Coord number?

A

Between 3 A-layer atoms and 1 B-layer atom. Coord of 4

76
Q

Structure of octahedral holes. Coord number?

A

Between 3 A-layer atoms and 3 B-layer atoms. Coord of 6

77
Q

Amount of octahedral and tetrahedral holes for FCC (ccp)

A

Octa: 1 on each edge (12) ans 1 in middle = total 4. Tetra: 8

78
Q

Sodium chloride structure NaCl

A

Cl- in FCC (ccp) structure. Na+ in octahedral holes.

79
Q

Calcium fluoride structure CaF2

A

Ca2+ in FCC lattice (coord 8). F- ions occupy all tetrahedral holes (coord 4)

80
Q

Zinc Blende (ZnS) stucture

A

Zn2+ in FCC lattice (coord 4). S2- occupy half of tetrahedral holes diagonal to each other. (coord 4)

81
Q

Diamond structure

A

Not ionic, covalent. Carbons in lattice points of FCC unit cell. Carbons in half of tetrahedral holes (same as ZnS blende)

82
Q

In diagram to show structure of fcc, why does it just say 0 in corners, not (1, 0)

A

Becasue we know there are atoms at the top corners

83
Q

Rules for ionic radius ratios in crystal structures

A
  1. Cations + anions in contact to minimise energy.
  2. As many anions as possible surround cations (normally anions are bigger so makes sense).
  3. Geometry must minimise anion-anion contact
84
Q

CsCl structure

A

Bcc structure and every other atom is Cs or Cl

85
Q

List of r+/r- rations compared to coordination number (structure)

A

1-0.732: 8 (CsCl)
0.732-0.414: 6 (NaCl)
0.414-0.225: 4 (ZnS)
0.225-0.155: 3
0.155-0: 2

86
Q

Difference between conductivity and conductance

A

Conductance depends on dimensions but conductivity is a property of the material

87
Q

How does conductivity depend on temp for conductors

A

Decreases as temp increases. Cations vibrate more -> obstructs e-, slows them down - stops free path of e-

88
Q

How does conductivity depend on temp for semi-conductoros

A

Increases with temp increase

89
Q

Numbers of conductivity for conductors, semiconductors and insulators

A

Conducts: 10^6-10^8 Ω-1m-1
Semi: 10^-4 - 10^5
Insulators: <10^-4

90
Q

2 theories of behaviours of conductors, semi and insulators

A

Free e- theory, Linear Combination of Atomic Orbitals theory

91
Q

What does density of states mean

A

DOS: number of orbitals (states) there are per eV, per atom in band structure

92
Q

What is the Aufbau principle

A

Electrons fill lower-energy atomic orbitals before filling higher-energy ones

93
Q

Why can metals conduct electricityin terms of e- and orbitals

A

Small increase in KE -> e- jumps up from filled to empty orbitals. Can conduct elec because of proximity of empty to filled orbitals.

94
Q

Why are metals opaque

A

All wavelengths of radiation can be absorbed because a filled level always has an empty level above it so e- can jump up

95
Q

Why are metals colourless

A

No wavelength is absorbed more strongly than another

96
Q

2 types of semiconductors

A

Intrinsic and extrinsic

97
Q

What is an intrinsic semiconductor

A

Ultra-pure materials

98
Q

What is an extrinsic semiconductor

A

Deliberately made impute by embedding foreign atoms in the crystal to change electrical properties

99
Q

What is valence band, band gap and conduction band

A

Valence: Full lower level, Band gap where no orbitals exist at all, Conduction band is empty band of orbitals

100
Q

Where is fermi level in band gap

A

Half way up the gap

101
Q

What are p type charge carrier

A

An apparent positive hole left in valence band as e- moves to conduction band. Can also carry current. Positive hole = p type charge carriers

102
Q

What are n type charge carriers

A

e- that moves up into conduction band. Lots of closely spaced empty levels available to carry current- Negative e- = n type charge carriers

103
Q

How does migration of a p type work

A

e- hops into hole left by movement of previous e-.

104
Q

Why are semiconductors opaque

A

Light absorbed if photon energy is higher than band gap (eV). e- from valence band and excite to conduction. For a lower photon energy eg IR, material is transparent.

105
Q

Reason for optical transparency

A

Band gap higher than 3ev so optical photons can’t be absorbed

106
Q

Explain the reason for photoconductivity of semiconductors

A

Absorption of a photon promotes an electron from the valence band to the conduction band, leaving a hole in the valence band. There is an n-type in conduction band (e-) and p-type charge carrier in valence band (+ve hole). Both e- and hole can carry current so conductivity is proportional to amount of light absorbed

107
Q

Why conductivity increases with temperature

A

No. of e-excited from valence to conduction band goes up exponentially. Conoductivitydepends on charge carriers so more e- and holes makes conductivity increas exponentially too

108
Q

Why do insulators not conduct electricity and why are they transparent

A

Band gap too large to excite e- across so no charge carriers. Transparent bc no light absorbed

109
Q

What is an n type semiconductor (Si)

A

Si doped with P. P has one more e- than Si. P energy level lies just below bottom of conduction band. P donates e- to become P+. Ion is fixed but e- can move so is an n type semiconductor.

110
Q

What is a p type semiconductor (Si)

A

Si doped with Al. Al one less e- than Si. Al energy level just above valence bond. So Al = acceptor -> Al- ion immobile in silicon lattice but +ve hole left = charge carrier = p type semiconductor

111
Q

What can be done by linking together p and n type semiconds.

A

Circuits and computer chips

112
Q

Why are H hard to see in electron density maps

A

Often lost in noise of map if quality isn’t high.
Also, electron density is pulled towards carbon so more difficult to measure correctly.

113
Q

Why does electron affinity of F->I have descending eV. Outlier?

A

Electrostat attraction between e- and nucleus gets smaller as atom size increases. Less E given out. F is unusually small because of high e- density which gives higher repulsion of e-

114
Q

Enthalpy of solution vs enthalpy of hydration (solvation)

A

Solution = NaCl (s) -> Na+ (aq) + Cl-(aq) but hydration is them in gas phase into aq

115
Q

What is lattice constant

A

a = the length of side of the unit cell cube, called the cubic lattice constant

116
Q

What is Kapustinskis equation based on? What makes it work?

A

The fact that madelungs constant divided by V (number of ions in formula) is very nearly constant

117
Q

What is de Broglies equation (used when calc electron wavelength and kinetic energy)

A

p = h/𝜆
where p = momentum = mv

118
Q

Use of thermogravimetric analysis (TGA)

A

To determine composition - amounts of volatile components

119
Q

Use of Differential Scanning Calorimetry (DSC)

A

Determine phase changes in solid

120
Q

Use of Crystallography and X-ray Diffraction

A

Most important technique for structure determination

121
Q

Photons of visible light have energies spanning range…?

A

1.5-3 eV

122
Q

What does Kapustinski’s equation assume?

A

Purely ionic bonding comprising only
electrostatic (or Coulombic) interactions as a result of complete transfer of electrons from the cations to the anions.

CHEM2017 (2 decks)

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