Lattice energy, ΔlatticeH, and Born-Haber cycles

Question 1

The standard enthalpy change of atomisation of an element is

Answer 1

the enthalpy change measured at a stated temperature, usually 298 K, and 100 kPa when one mole of gaseous ATOMS (not molecules!!) is formed from an element in its standard state
ΔatH

Question 2

bond enthalpies can be used as a measure of the strength of……………………in molecules
the equivalent energy change for ionic bonding in ionic compounds is

Answer 2

the covalent bonding in molecules

lattice energy, ΔlatticeH (or lattice enthalpy)

Question 3

The lattice energy (or lattice enthalpy) of an ionic compound is

Answer 3

the energy change when one mole of the ionic solid is formed from its gaseous ions
is standard conditions are applied (298 K and 100 kPa) then the energy change is called the standard lattice energy

Question 4

lattice energy of formation is another way of describing…………………….

Answer 4

lattice energy when an ionic compound is made from its ions

the energy change is negative

Question 5

lattice energy of dissociation is the

Answer 5

other definition of lattice energy where one mole of a compound is broken down (dissociated) into its ions
the energy change is positive

Question 6

the equation that represents the standard lattice energy of sodium chloride is

Answer 6

Na+(g) + Cl-(g) → NaCl(s)

ΔlatticeH = -780 kJ mol-1

Question 7

factors affecting lattice energy are: (4 factors)

Answer 7

→the charge of the ions involved (e.g. Mg2+ has twice the charge of a sodium ion, so the lattice energy is much larger, more negative, for magnesium chloride than sodium chloride)
→the number of cation-to-anion interactions (e.g. magnesium chloride has more interactions because there are two Cl- ions for each Mg2+ ion, while for NaCl the ration is 1:1)
→the distance between the centres of the cations and their neighbouring anions. this is determined by calculating the sum of their ionic radii. this is effected by the relative sizes of the ions involved (e.g. Mg2+ ions are smaller than Na+ ions. therefore reducing the sum of the ionic radii)and by the type of lattice structure the compound has (though the relative ion sizes themselves determine the lattice structure
→covalent interactions between the ions also affect the magnitude of the lattice energy and can be taken into consideration through a Born-Haber cycle

Question 8

a decrease in the distance between the centres of two ions (the inter-ionic distance) results in a ………..value for the lattice energy

Answer 8

smaller inter-ionic distance = more negative value

Question 9

an increase in charge in even just one of the ions involved leads to a ……………value for the lattice energy

Answer 9

more negative

Question 10

the equation representing the standard enthalpy change of atomisation of hydrogen at 298 K and 100kPa is:

Answer 10

1/2H2(g) → H(g)

Question 11

the first electron affinity, Eea(1), of an element is

Answer 11

the energy change when each atom in one mole of atoms int eh gaseous state gains an electron to form a -1 ion

Question 12

the first electron affinity has a ……………..value for many elements except for …………

Answer 12

negative value usually, but not for noble gases

Question 13

explain why noble gases do not have a negative value as their first electron affinity value.

Answer 13

→repulsion caused by the electrons already present in the valence shells results in a positive value for the first electron affinity
→this is because the additional electron would have to occupy a new valence shell

Question 14

the second electron affinity tends to have a …………. value

Answer 14

positive

Question 15

show the equations representing the first and second, then combined, electron affinities of oxygen

Answer 15

O(g) + e- → O-(g) Eea(1) = -141 kJ mol-1
O-(g) + e- → O2-(g) Eea(2) = +798 kJ mol-1
O(g) + 2e- → O2-(g) Eea(1) + Eea(2) = +657 kJ mol-1

Question 16

a positive enthalpy change value means the process is exo-/endo-thermic?

Answer 16

endothermic, as energy is ‘added’ to the system

Question 17

the order of energy changes for sodium chloride in a Born-Haber cycle is:

Answer 17

ΔatH [Na(s)]
IE(1) [Na(g)] (first ionisation energy of Na)
ΔatH[Cl2(g)]
Eea[Cl(g)]
ΔfH [NaCl(s)]

Question 18

calculating ΔlatticeH of sodium chloride Born-Haber cycle =

Answer 18

ΔlatticeH = ΔatH [Na(s)] + IE(1) [Na(g)] + ΔatH[Cl2(g)] + Eea[Cl(g)] + ΔfH [NaCl(s)]

Question 19

exothermic = less/more energy required?

Answer 19

less

Question 20

endothermic = less/more energy required?

Answer 20

more