A2 Unit F325 - Equilibria, Energetics and Elements
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Revision Powerpoint on Enthalpy and Entropy OCR Chemistry A level 2015
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A2 Unit F325 - Equilibria, Energetics and Elements
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OCR
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A2 Unit F325 - Equilibria, Energetics and Elements
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Enthalpy
and
Entropy
, Lattice enthalpy
Due to the strength of the ionic bonds, electrostatic Gaseous ions
attractions between oppositely charged ions in the
ionic lattice structure, ionic compounds are stable.
This creates a substantial energy barrier that must
be overcome to break down the lattice.
Lattice enthalpy is a measure of the strength of ΔLEHo
ionic bonding in a giant ionic lattice. It is the enthalpy
change that accompanies the formation of 1 mole of
an ionic compound from its gaseous ions under
standard conditions. E.g.
K+(g) + Cl-(g) KCl(s)
Gaseous ions solid ionic compound
Lattice enthalpy involves ionic bond formation from Ionic lattice
separate gaseous ions. It is an exothermic change &
the value for the enthalpy change will always be
negative.
Lattice enthalpy cannot be measured directly & must be
calculated indirectly using known energy changes in an
energy cycle.
, Born-Haber cycle
• The standard enthalpy change of formation (ΔfHo) Gaseous ion (metal)
is the enthalpy change that takes place when 1 mole of ΔEAHo
a compound is formed from its elements in their Gaseous atoms ΔIEHo Gaseous ions
standard states, under standard conditions. The
compound will always be an ionic compound in its ionic Elements in ΔatHo ΔLEHo
lattice. Na(s) + ½Cl(g) NaCl(s) standard states
• The standard enthalpy change of atomisation ΔfHo
(ΔatHo) is the enthalpy change that takes place for the Ionic lattice
formation of 1 mole of a gaseous atoms from the
element in its standard state under standard conditions. Na(s)Na(g) & ½Cl₂(g)Cl(g)
ΔatHo is always an endothermic process because bonds are broken to form gaseous
atoms. When the element is a gas in its standard state, ΔatHo is related to the bond
enthalpy of the bond being broken.
• The first ionisation energy (ΔIEHo) is the enthalpy change required to remove 1
electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions.
Na(g) Na+(g) + e-
Ionisation energies are endothermic because energy is required to overcome the
attraction between a –ve electron & the +ve nucleus.
• The first electron affinity (ΔEAHo) is the enthalpy change that takes place when one
electron is added to each atom in 1 mole of gaseous 1- atoms. Cl(g) + e- Cl-(g)
First electron affinities are exothermic because the electron being added is attracted
in towards the nucleus. Second electron affinities are endothermic as a 2nd electron is
being gained be a –ve ion, which repels the electron away. So energy must be put in to
force the –vely-charged electron onto the –ve ion.
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