Solid State Structures

Question 1

which combination of cation/anion gives highest degree of polarisation/high covalent character?

Answer 1

small highly charged cations and large polarisable anions

Question 2

large covalent character + Born-Landé equation

Answer 2

underestimates lattice energy (actual value obtained from a Born-Haber cycle = higher)

Question 3

drawback of Madelung constant + interionic distance (r)

Answer 3

requires detailed knowledge of structure which may not be available

Question 4

value of ‘k’ in Kapustinskii equation

Answer 4

107 900 pm kJ mol-1

Question 5

predicting bond types - greater electronegativity

Answer 5

greater ionic character

Question 6

predicting bond types - high electronegativity values

Answer 6

tends to be covalent

Question 7

predicting bond types - low electronegativities

Answer 7

substance with be an alloy with metallic bonding

Question 8

iodine

Answer 8

strong covalent bonds between atoms

weak intermolecular interactions

low mp/bp

sublimes with mild heating at atmospheric pressure

Question 9

silicon

Answer 9

covalent network

each atom forms 4 covalent bonds

Question 10

covalent network structures

Answer 10

infinite

atoms linked by covalent bonds

examples = boron, phosphorus + boron nitride

Question 11

allotropy

Answer 11

elements that can form more than 1 structure

e.g. carbon (diamond, graphite, Buckminsterfullerene)

Question 12

allotropes of tin

Answer 12

β tin (= white) - stable at rtp, metallic

α tin (= grey) - stable below 13°C, non-metallic

Question 13

polymorphism

Answer 13

compounds that can also form more than 1 structure

e.g. quartz and β-cristobalite = polymorphs of SiO2

Question 14

unit cell

Answer 14

smallest possible repeating unit for a structure

Question 15

unit cell - vertex

Answer 15

1/8

Question 16

unit cell - edge

Answer 16

1/4

Question 17

unit cell - face

Answer 17

1/2

Question 18

unit cell - body

Answer 18

1

Question 19

cell projection diagrams - 0 and 1 meaning

Answer 19

0 = bottom of cell
1 = top of cell

Question 20

interstitial sites

Answer 20

free space between spheres

Question 21

how many octahedral sites are there in cubic-close packing?

Answer 21

4

Question 22

how many tetrahedral sites are there in cubic-close packing?

Answer 22

9

Question 23

what is an alloy?

Answer 23

contains 2 or more metals or metal + non-metal

Question 24

why are alloys more commonly used than pure metals?

Answer 24

stronger - different sized atoms ∴ slide less easily

less susceptible to corrosion

Question 25

substitutional alloys

Answer 25

atoms of 1 metal exchanged for those of another metal

usually occurs when atoms are similar in size (alloy formation doesn’t cause major distortions to lattice)

Question 26

interstitial alloys

Answer 26

atoms of one metal are present in interstitial sites of another metal lattice

Question 27

binary compounds

Answer 27

contains 2 elements

close-packed structure (one element in interstitial sites)

most ionic + some covalent described in this way

Question 28

limiting radius ratio

Answer 28

smallest value of radius ratio - if radius is any smaller, cation-anion contact is lost

Question 29

enthalpy change of formation

Answer 29

change in enthalpy when 1 mole of a compound is formed in a reaction between its elements in their standard states

Question 30

enthalpy change of atomisation

Answer 30

change in enthalpy when 1 mole of an element is converted from solid to one mole of gas = 1/2 bond dissociation enthalpy

Question 31

first ionisation energy

Answer 31

change in enthalpy when one mole of gas is converted to one mole of gaseous cations

Question 32

electron gain enthalpy

Answer 32

change in enthalpy when 1 mole of gas is converted to 1 mole of gaseous anions = minus the e- affinity

Question 33

lattice enthalpy

Answer 33

enthalpy change for conversion of 1 mole of ionic solid to gaseous ions
= +ve

Question 34

value for charge on electron

Answer 34

1.6022 x10^-19 C

Question 35

value for permittivity of vacuum

Answer 35

8.8542 x 10-12 C2 J-1 m-1

Question 36

polarisable

Answer 36

large anions - e- density is further from nucleus + less controlled

small cations with higher charges - high charge density

Question 37

kapustinskii equation

Answer 37

allows ionic radii for polyatomic ions to be estimated

Madelung constant requires detailed knowledge of structure - may not be available

Question 38

madelung constant

Answer 38

scale factor

accounts for interactions in crystal
-> cations + anions = attractive
-> same charge = repulsive