thermodynamics
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Define enthalpy of atomisation
the enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state
Define bond dissociation enthalpy
the standard molar enthalpy change when one mole of a covalent bond is broken into two gaseous atoms (or free radicals)
Define enthalpy of sublimation
the enthalpy change when a solid metal is turned to gaseous atoms
Define lattice enthalpy of formation
the standard enthalpy change when one mole of an ionic crystal lattice is formed from its constituent ions in gaseous form.
Define lattice enthalpy of dissociation
the standard enthalpy change when 1 mole of an ionic crystal lattice form is separated into its constituent ions in gaseous form.
Define first ionisation enthalpy
the enthalpy change required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous ions with a +1 charge
Define second ionisation enthalpy
the enthalpy change to remove 1 mole of electrons from one mole of gaseous 1+ ions to produces one mole of gaseous 2+ ions.
Define first electron affinity
the enthalpy change that occurs when 1 mole of gaseous atoms gain 1 mole of electrons to form 1 mole of gaseous ions with a 1- charge is exothermic for atoms that normally form negative ions. This is because the ion is more stable than the atom,and there is an attraction between the nucleus and the electron.
Define second electron affinity
the enthalpy change when one mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions. is endothermic because it take energy to overcome the repulsive force between the negative ion and the electron.
Define enthalpy of hydration
Enthalpy change when one mole of gaseous ions become aqueous ions. This always gives out energy (exothermic,-ve) because bonds are made between the ions and the water molecules
Define enthalpy of solution
the enthalpy change when 1 mole of an ionic substance dissolves in water to give a solution of infinite dilution.
Why are Born-Haber cycles used
to calculate enthalpies of solution for ionic compounds from lattice enthalpies and enthalpies of hydration
The strength of an enthalpy of lattice formation depends on the following factors:
• The sizes of the ions the larger the ions, the less negative the enthalpies of lattice formation (i.e. a weaker lattice). As the ions are larger the charges become further apart and so have a weaker attractive force between them. • The charges on the ion The bigger the charge of the ion, the greater the attraction between the ions so the stronger the lattice enthalpy (more negative values).
The lattice enthalpies become less ________ down any group.
negative
Perfect ionic model
Theoretical lattice enthalpies assumes a perfect ionic model where the ions are 100% ionic and spherical and the attractions are purely electrostatic.
Differences between theoretical and Born Haber (experimental) lattice enthalpies
The Born Haber lattice enthalpy is the real experimental value. When a compound shows covalent character, the theoretical and the born Haber lattice enthalpies differ. The more the covalent character the bigger the difference between the values.
There is a tendency towards covalent character in ionic substances when
the positive ion is small the positive ion has multiple charges the negative ion is large the negative ion has multiple negative charges.
When a compound has some covalent character, it tends to be
giant covalent so the lattice is stronger than if it was 100% ionic. hence the Born-Haber value would be larger than the theoretical value.