Topic 13.1 Flashcards
Lattice energy
-Energy change for ionic bonding in ionic compounds is lattice energy ΔlatticeH (or lattice enthalpy).
–> It is the equivalent to bond enthalpies for covalent bonding.
-The lattice energy of a compound is the energy change when one mole of the ionic solid is formed from its gaseous ions.
Factors affecting the magnitude of lattice energy
Lattice energy of magnesium chloride is much larger than that of sodium chloride.
This is because:
1) Magnesium ion carries twice the charge of a sodium ion.
2) There are more cation-to anoint interactions in magnesium chloride because there are twice as many chloride ions per cation.
3) The distance between the centres of the cations and their neighbouring anions.
Standard enthalpy change of atomisation, ΔatH°
The enthalpy change measured at a stated temperature (298K) and 100kPa when one mole of gaseous atoms is formed from an element in its standard state called the standard enthalpy change of atomisation of the element.
Electron affinity (1st)
The first electron affinity of an element, Eea(1), is the energy change when each atom in one mole of atoms in the gaseous state gains an electron to form a -1 ion.
-The first electron affinity has a negative value for many elements, including alkali metals.
-There is a notable exception with the noble gases.
-For these, repulsion caused by the electrons already present in the valence shell results in a positive value for the first electron affinity as the additional electron would have to copy a new valence shell.
2nd electron affinity
In contrast, second electron affinities Eea(2) tend to be positive, eg: O-(g) + e- –> O^2-(g) Eea(2) = +798kJ/mol.
-Therefore, the formation of the oxide ion O^2-, in the gaseous state from its atom in the gaseous state is an endothermic process overall: O(g) + 2e- –> O^21(g) Eea(1) + Eea(2) = +657kJ/mol
Born-Haber cycles
Experimental lattice energy
The Born-Haber cycle allows us to calculate a value for the lattice energy of an ionic compound from knowledge of other energy changes, all of which can be determined experimentally.
Theoretical lattice energy
First make the following assumptions:
-The ions are in contact with one another.
-The ions are perfectly spherical.
-The charge on each ion is evenly distributed around the centre so that each ion can be considered as point changes.
–> The bonding in the lattice has a considerable covalent character, which makes the experimental value for the lattice energy more negative than the theoretical value.
Polarisation of the anion
In an ionic lattice, the electrons are pulled towards the cation, and the anion is said to be polarised.
-The extent to which an anion is polarised depends on several factors known as Fajan’s rules:
-a high charge and small size of the cation.
-a high charge and large size of anion.
High charge and small size of cation
-The ability of a cation to attract electrons from the anion towards itself is called its ‘polarising power’.
-A cation with a high charge and a small nucleus has a large polarising power.
-A value for polarising power can be obtained by calculating its charge density.
-The charge density of a cation is the charge divided by the surface area of the ion.
Charge density ~ charge / r^2
High charge and polarisation
-The ease with which an anion is polarised depends on its charge and its size.
-Anions with a large charge and large size are polarised most easily.
-Eg. The larger iodide ion is more easily polarised, which leads to a greater degree of covalent bonding in silver iodide.
Enthalpy change of solution, ΔsolH
-One of the factors that determines solubility value is enthalpy change of solution.
-Eg. The enthalpy change of solution for sodium chloride is the energy change associated with the following process: NaCl(s) –> Na+(aq) + Cl-(aq).
-Upon dilution, the ions in the solution move further apart and become more hydrated.
Enthalpy change of hydration, ΔhydH
-The enthalpy change when one mole of an ion in its gaseous state is completely hydrated by water.
-Complete hydration is said to have occurred when the solution formed is at infinite dilution.
-For sodium and chloride ions:
Na+(g) –> Na+(aq)
Cl-(g) –> Cl-(aq)
-When an ion is placed in water it immediately interacts with the water molecules.
-The interaction is the result of the attraction between δ- oxygen atom of the water molecule and the cation.
Factors affecting the magnitude of the hydration enthalpy
-As we go down a group the magnitude of ΔhydH becomes less negative.
-This correlates with an increase in ionic radius.
-As the ions become larger, the electrodtatic force of attraction between them and the water molecules decreases, and ∴ the energy released upon hydration decreases.
Relationship between ΔsolH, ΔhydH and ΔlatticeH
