13a - lattice energy

Question 1

what is the definition of lattice enthalpy?

Answer 1

standard enthalpy change when 1 mole of ionic crystal lattice forms from its constituent ions in gaseous form

Question 2

what is the definition of enthalpy change of atomisation?

Answer 2

enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state

Question 3

what is the definition of electron affinity?

Answer 3

enthalpy change when 1 mole of gaseous atoms gain 1 mole of e- to form 1 mole of gaseous ions with 1- charge

Question 4

which electron affinity is exothermic?

Answer 4

1st

Question 5

is the 2nd electron affinity exothermic or endothermic? why?

Answer 5

endothermic
- energy is needed to overcome repulsive forces between e- and negative ion

Question 6

what is the definition of ionisation enthalpy?

Answer 6

enthalpy change required to remove 1 mole of e- from 1 mole of gaseous atoms to form 1 mole of gaseous ions in 1+ charge

Question 7

what is the trend in 1st electron affinity down group 7? why?

Answer 7

less exothermic down group 7
- atoms get bigger
- more shielding
-> less easy to attract a e-

Question 8

what 2 factors determine the trend in lattice enthalpies?

Answer 8

1- size of ions
2- charge of ions

Question 9

how does the size of ions affect lattice enthalpies?

Answer 9

larger size = less negative value of LE
- charges are further apart -> weaker attraction

Question 10

how does the charge of ions affect lattice enthalpies?

Answer 10

larger charge = higher strength of LE (more negative)
- greater attraction between ions

Question 11

what does lattice enthalpy provide a measure for?

Answer 11

ionic bond strength

Question 12

what does a theoretical lattice energy assume?

Answer 12

a perfect ionic model where ions are 100% ionic & spherical, attraction are purely electrostatic

Question 13

what is different from the experimental value to the theroretical value?

Answer 13

anion is polarised when it is distorted by the cation -> addition covalent character which leads to a stronger lattice

Question 14

describe the 2 factors that increases polarising power in cations.

Answer 14

• decreasing ionic radii
• increasing charge

Question 15

describe the 2 factors that increases the polarisibility in anions.

Answer 15

• increasing charge
• increasing ionic radii

Question 16

what is the definition of enthalpy change of solution?

Answer 16

standard enthalpy change when 1 mole of ionic solid dissolves in large amount of H2O to ensure ions are well seperated and do not interact

Question 17

whatis the definition of enthalpy change of hydration?

Answer 17

enthalpy change when 1 mole of gaseous ions become hydrated such that further dilution causes no further heat change

Question 18

how does the sizes of ions affect lattice enthalpy?

Answer 18

• larger ionic radii means charge is further apart -> weaker attractive force -> weaker lattice
  = less negative LE

Question 19

how does charges of ion affect lattice enthalpy?

Answer 19

• higher charge has a stronger attraction
  -> stronger LE (more negative)

Question 20

why is the enthalpy change of hydration always exothermic?

Answer 20

energy is given out as H2O bind to metal ions

Question 21

how does the solubility of a substance relate to the values of the enthalpy change of solution?

Answer 21

• substance more likely to be soluble if exothermic
• if endothermic: LE is much larger than ^Hhyd and therefore not energetically favourable to break up lattice

Question 22

by which 2 factors & how are enthalpy change of hydration affected?

Answer 22

1- charge of ion
- higher charge attracts water molecules more strongly
-> more energy released when bond is made & more exothermic

2- size of ion
- higher charge density when ionic radii decreases
- attracts water molecules more strongly