3.6 enthalpy changes for solids and solutions Flashcards
in ionic bonding, the ions arrange themselves into a _____?
lattic
- so that the ions of opposite charge are next to one another
enthalpy change of formation definition
- the energy transferred when 1 mole of the compound is formed from its elements under standard conditions (298K and 100KPa) with all the reactants and products being in their standard states
is the enthalpy change of formation a positive or negative process?
negative
enthalpy of atomisation definition
- the enthalpy change when 1 mole of gaseous atoms is formed from its own elements in its standard state
is the enthalpy of atomisation a positive or negative process?
positive
first ionisation enthalpy definition
the enthalpy change when one mole of gaseous atoms forms one mole of gaseous 1+ ions
is the first ionisation enthalpy a positive or negative process?
positive
first electron affinity definition
the enthalpy change that occurs when 1 mole of gaseous 1- ions is formed from 1 mole of gaseous atoms
is the first electron affinity a positive or negative process?
negative
second electron affinity definition
the enthalpy change when one mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions
is the second electron affinity a positive or negative process?
positive
enthalpy of lattice formation definition
the standard enthalpy change when 1 mole of an ionic crystal lattice/solid ionic compound is formed from its constituent ions in gaseous form
is the enthalpy of lattice formation a positive or negative process?
negative
enthalpy of lattice dissociation definition
the standard enthalpy change when 1 mole of an ionic crystal lattice (ionic compound) is separated into its constituent ions in gaseous form
is the enthalpy of lattice dissociation a positive or negative process?
positive
categorise these:
1. Na (s) —> Na (g)
2. Na (g) —> Na+ (g) + e-
3. Na+ (g) + Cl- (g) —> NaCl (s)
4. O2 (g) —> 2O2 (g)
- enthalpy of atomisation
- first ionisation enthalpy
- enthalpy of lattice formation
- 2x enthalpy of atomisation
what are born-haber cycles?
- thermochemical cycles that include all of the enthalpy changes involved in the formation of an ionic compound
- lattice enthalpy cant be calculated directly so a born-haber cycle can be used to calculate the lattice enthalpy by applying hess’s law
drawing born-haber cycles:
- start with the elements in their standard states - these have zero enthalpy by definition
- the zero line will beer to be in the middle of the paper
- draw a line upwards from the zero line to the solid ionic compound made. this represents the enthalpy change of formation
- draw a lone upwards to usually atomise the elements individually
- positive (endothermic) changes are shown by an arrow pointing upwards
- negative (exothermic) changes are shown by an arrow pointing downwards
- need to include state symbols in born-haber cycles *
why is the second electron affinity positive while the first is negative?
because of the repulsion, so energy is required for the 2nd electron affinity a
what are some factors affecting the strength of the lattice formation enthalpy?
- size of ions
- the charges on the ions
how does the size of the ion affect the strength of the lattice formation enthalpy?
- the larger the ions, the less negative the enthalpies of lattice formation (weaker lattice)
- larger ions means the charges become further apart and have a weaker attractive force between them
- also larger ions means a smaller charge density
how does the charges on the ion affect the strength of the lattice formation enthalpy?
- the bigger the charge of the ion, the greater the attraction between the ions so the stronger the lattice enthalpy (more negative values)
- this means more energy will be required to separate the ions
what do born-haber cycles assume?
that an ionic compound is ‘perfectly ionic’:
- ions are treated as point charges, with the charge centred in the middle of the ion and 100% spherical
- attractions are assumed to be purely electrostatic
(the born-haber cycles use theoretical data)