19
Lattice Energy
----- LATTICE ENERGY & ENTHALPY CHANGE OF ATOMISATION -----
Enthalpy change (ΔH) refers to the amount of heat energy transferred during a chemical reaction at a constant pressure
Enthalpy change of Atomisation
ΔHatꝋ is the enthalpy change when 1 mol of gaseous atoms is formed from its element under standard conditions. Standard
conditions in the syllabus are 298K and 101 kPa.
ΔHatꝋ is always endothermic as energy is always required to break any bonds between the atoms in the element, to break the
element into its gaseous atoms → will always have a + value.
A(s) → A(g)
Lattice energy
The lattice energy ΔHlattꝋ is the enthalpy change when 1 mol of an ionic compound is formed from its gaseous ions under
standard conditions.
It is always exothermic as there is a large release of energy → will always have a very large -
value.
The large negative value suggests ionic compounds are much more stable than gaseous ions.
- Due to electrostatic forces of attraction between ions
- There are no electrostatic forces between gaseous ions so they are less stable
- The more exothermic / negative, the stronger the ionic bonds
A+(g) + B-(g) → AB(s)
----- ELECTRON AFFINITY -----
The first electron affinity (EA1) is the enthalpy change when 1 mol of e- is added to 1 mol of gaseous atoms to form 1
mol of gaseous ions each with a single negative charge under standard conditions.
X(g) + e- → X-(g)
EA1 is usually exothermic as energy is released. EA2 and EA3 are endothermic as energy is released. This is because the
incoming e- is added to an already negative ion. Energy is needed to overcome the repulsive forces between the incoming
e- and negative ion.
Factors affecting electron affinity
Nuclear charge- the greater the nuclear charge, the stronger the attractive forces between and incoming e- and nucleus
Distance- the greater the distance between the nucleus and the outermost shell/orbital where the e- is added, the weaker
the force of attraction.
Shielding- the greater the nº of shells, the greater the shielding effects & the weaker the force of attraction
Trends in electron affinity of group 16 & 17
Ea of non-metals become more exothermic across periods with a max at G17.
Going down both groups, it generally becomes less exothermic apart from the first elements of each group (O2 & F).
Going down G16 & G17
- Outermost e- are held less tightly to the nucleus as they are farther
- The n of e- shells increases causing and increased shielding
- Gets more difficult to add an e- to the outer shell
- Less energy released upon adding an e- to the outer shell
- So generally, the EA1 becomes less exothermic
Fluorine is an exception and has a lower EA1 than chlorine
- Fluorine has a very small atomic radius
- This means that the e- density of fluorine is high
Lattice Energy
----- LATTICE ENERGY & ENTHALPY CHANGE OF ATOMISATION -----
Enthalpy change (ΔH) refers to the amount of heat energy transferred during a chemical reaction at a constant pressure
Enthalpy change of Atomisation
ΔHatꝋ is the enthalpy change when 1 mol of gaseous atoms is formed from its element under standard conditions. Standard
conditions in the syllabus are 298K and 101 kPa.
ΔHatꝋ is always endothermic as energy is always required to break any bonds between the atoms in the element, to break the
element into its gaseous atoms → will always have a + value.
A(s) → A(g)
Lattice energy
The lattice energy ΔHlattꝋ is the enthalpy change when 1 mol of an ionic compound is formed from its gaseous ions under
standard conditions.
It is always exothermic as there is a large release of energy → will always have a very large -
value.
The large negative value suggests ionic compounds are much more stable than gaseous ions.
- Due to electrostatic forces of attraction between ions
- There are no electrostatic forces between gaseous ions so they are less stable
- The more exothermic / negative, the stronger the ionic bonds
A+(g) + B-(g) → AB(s)
----- ELECTRON AFFINITY -----
The first electron affinity (EA1) is the enthalpy change when 1 mol of e- is added to 1 mol of gaseous atoms to form 1
mol of gaseous ions each with a single negative charge under standard conditions.
X(g) + e- → X-(g)
EA1 is usually exothermic as energy is released. EA2 and EA3 are endothermic as energy is released. This is because the
incoming e- is added to an already negative ion. Energy is needed to overcome the repulsive forces between the incoming
e- and negative ion.
Factors affecting electron affinity
Nuclear charge- the greater the nuclear charge, the stronger the attractive forces between and incoming e- and nucleus
Distance- the greater the distance between the nucleus and the outermost shell/orbital where the e- is added, the weaker
the force of attraction.
Shielding- the greater the nº of shells, the greater the shielding effects & the weaker the force of attraction
Trends in electron affinity of group 16 & 17
Ea of non-metals become more exothermic across periods with a max at G17.
Going down both groups, it generally becomes less exothermic apart from the first elements of each group (O2 & F).
Going down G16 & G17
- Outermost e- are held less tightly to the nucleus as they are farther
- The n of e- shells increases causing and increased shielding
- Gets more difficult to add an e- to the outer shell
- Less energy released upon adding an e- to the outer shell
- So generally, the EA1 becomes less exothermic
Fluorine is an exception and has a lower EA1 than chlorine
- Fluorine has a very small atomic radius
- This means that the e- density of fluorine is high
