15.1 Energy cycles (HL)
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Lattice enthalpy
Enthalpy change when one mole of an ionic compound is broken apart into its constituent gaseous ions under standard conditions
Is lattice enthalpy positive or negative?
Always positive as energy is absorbed to break the bonds
Opposite of lattice enthalpy
Lattice formation enthalpy
What does an increase in lattice enthalpy mean?
The higher it is, the stronger the electrostatic bonds
What does lattice enthalpy depend on?
Ionic radii and ionic charge
What values do you need to calculate lattice enthalpy?
• Enthalpy of formation • Enthalpy of atomization • Bond dissociation energy • First ionization energy • First electron affinity
What is enthalpy of atomization?
When one mole of gaseous atoms is formed from an element
If bond dissociation energy of O2 is +498kJ/mol, what is enthalpy of atomization?
It will be half as only 1 oxygen is made i.e. 249kJ/mol
What is the general formula for Born-Haber cycle? eg. NaCl
Lattice enthalpy= Enthalpy of atomization(Na) + Enthalpy of atomization(Cl) + First ionization energy (Na) + First electron affinity (Cl) - Enthalpy of formation (NaCl)
What needs to be taken into account for Born-Haber cycle of MgF2?
Second ionization of energy of Mg is required and first electron affinity needs to be doubled for F
What needs to be taken into account for Born-Haber cycle of MgO?
Second ionization of energy of Mg is required and second electron affinity of O is also required
When gaseous ions are hydrated by surrounding water molecules, what bonds are formed?
Ion-dipole forces are formed
Enthalpy of hydration
Enthalpy change when one mole of gaseous ions dissolve in water to form a solution of infinite dilution under standard conditions
Infinite dilution
A solution that has excess of water and addition of more water will not cause more heat to be absorbed or released
Is enthalpy of hydration positive or negative?
Negative
Enthalpy of solution
Enthalpy change when one mole of solute dissolves to form a solution of infinite dilution
Equation for enthalpy of solution
Enthalpy of hydration of all ions + Lattice enthalpy
What does solubility of an ionic compound depend (to some extent) on?
Sign and magnitude of enthalpy of solution
What type of ionic compounds have higher positive enthalpy of solution values and why?
Insoluble ionic compounds as enthalpy of hydration doesn’t compensate for higher lattice enthalpy as dissolving process will be energetically unfavorable
Can those with slight positive values still be soluble?
Yes
Factors affecting enthalpy of hydration
Ionic radii and charge on ion (charge density)
How does ionic radii affect enthalpy of hydration?
Higher the ionic radii, magnitude of enthalpy of hydration decreases as there are weaker ion-dipole forces
How does charge density affect enthalpy of hydration?
As it increases, so does magnitude and it also becomes more exothermic as forces are stronger and more energy is thus released
What would the equation for enthalpy of solution of MgCl2 look like?
[Lattice enthalpy of MgCl2] + [(Enthalpy of hydration of Mg) + (Enthalpy of hydration of Cl x 2)]