solids - energy Flashcards
(21 cards)
what is hess’s law?
the ideal that energy is conserved, so multiple reaction pathways for the same synthesis have = energies/enthalpies
enthalpy change of formation, ΔfH° definition
enthalpy change when 1 mole of a substance is formed from raw elements in standard states + conditions
enthalpy change of atomisation, ΔaH° definition
enthalpy change when 1 mole of atoms is converted into the gas state under standard conditions
- for diatomic molecules this value is 1/2 the BDE, and you’d need to account for it once per atom, twice per molecule
first ionisation enthalpy, ΔiH° definition
enthalpy change when 1 mole of electrons is removed form each atom in 1 mole of gaseous atoms forming 1 mole of gaseous 1+ ions
electron gain enthalpy, ΔegH° definition
enthalpy change when each atom in 1 mole of gaseous atoms gains an electron forming 1 mole of gaseous 1- ions
how are electron affinity and electron gain enthalpy related?
electron affinity = -electron gain enthalpy
lattice enthalpy, ΔLEH° definition
enthalpy change when 1 mole of an ionic solid separates back into its gaseous ions, always +ve in this direction, -ve for reverse
what is the purpose of born haber cycles?
born haber cycles allow lattice enthalpies to be calculated from experimental data - these calculations are based on internal energy changes from ion interactions rather than enthalpy changes, giving us a lattice energy instead, the difference is usually very small/ignored
ΔH = ΔU +pV
where ΔH = enthalpy changes, ΔU = energy change
lattice energy definition
the difference in potential energy between ions in solid lattice and widely spread as a gas
what is the purpose of the madelung constant A and avogadros number in the born-lande equation?
accounts for the many ion-ion interactions between anions + cations in ionic solids, and makes it a molar quantity
what is the purpose of the born exponent n in the born-lande equation?
lattice enthalpy expression assumes ions are point charges, this is untrue and needs to be accounted for + additional short range forces between ions should be included, which arise from repulsions between overlapping electron clouds which rise steeply with decreasing r
Erep = B/(r^n) where Erep = repulsive energy, B and n = constants, r = distance between ions (m)
born exponent can be experimentally determined from compressability of solid or predicted from electron configuration of ions
where does the madelung constant A come from?
ΔUn = energy given by the cumulative interactions of a single ion with its closest neighbours, and then with its next closest neighbours, then its third closest neighbours, so on so on until nth neighbour
this is the madelung constant - a huge number of terms need to be included to get to the actual observed madelung constant
polarisability definition
the degree to which atoms/ions are distorted
give one reason as to why a perfect ionic compound isn’t possible
+vely charged cations can distort the sphericla electron clouds of the anions leading to some degree of covalent character, this is because of polarisability
what anions are most polarisable?
large anions are more polarisable than small anions as outer electron density is further from the nucleus
what cations are most polarising?
small cations with large charges are more strongly polarising as they have high charge densities and are more likely to induce distortions in anions
will a compound containing small cations with large charges and large anions be more ionic or covalent in character?
more covalent due to the high degree of polarisation
how does covalent character affect the born-lande equation?
compounds with a high covalent character tend to have their lattice energies underestimated from the born-lande equation, born-haber cycles for these compounds match poorly to the calculated value which proves this (+ vice versa)
give 1 limitation of the born-lande equation
although this equation gives quite good predictions, it is very complicated and difficult to use - madelung constant and distance r require a detailed knowledge of the structure which may not be available
give 2 advantages of the kaputstinskii equation
- greatly simplifies the born-lande equation while still giving decently accurate results
- allows the ionic radii for polyatomic ions to be estimated from lattice energies of their compounds
thermochemical radii definition
radii of compounds estimated from experimental data of lattice enthalpies
