21. Thermodynamics Flashcards
(20 cards)
Definitions:
Enthalpy Change, ΔHf:
Change in heat energy at a constant pressure
Definitions:
Standard Enthalpy of Formation, ΔHf:
ΔH when 1 mole of a compound is formed from its elements under standard conditions with all reactants and products being in their standard states
Definitions:
Standard Enthalpy of Combustion, ΔHc:
ΔH when 1 mole of a compound is burned in oxygen under standard conditions with all reactants and products being in their standard states
Definitions:
Mean Bond Enthalpy, ΔHbe:
ΔH when 1 mole of gaseous molecules each break a covalent bond to form 2 free radicals, averaged over a range of compounds
Definitions:
1st Ionisation Enthalpy, ΔHie:
ΔH when 1 mole of e- is removed from 1 mole of gaseous atoms to give 1 mole of gaseous ions, each with a single +1 charge
Definitions:
2nd Ionisation Enthalpy, ΔHie:
ΔH when 1 mole of e- is removed from 1 mole of gaseous 1+ ions to give 1 mole of gaseous ions each with a 2+ charge
Definitions:
Standard Enthalpy of Atomisation, ΔHat:
ΔH when 1 mole of gaseous atoms are formed from an element in its standard state
Example: Na(s) → Na(g), 1/2 Br2(l) → Br(g)
Definitions:
Relationship between ΔHbe and ΔHat:
(only with diatomic molecules)
ΔHbe = ΔHat x 2
Definitions:
1st Electron Affinity, ΔHea:
ΔH when 1 mole of gaseous atoms is converted to 1 mole of gaseous ions, each with a single -1 charge, under standard conditions
Example: O(g) + e- → O- (g)
Definitions:
2nd Electron Affinity, ΔHea:
ΔH when 1 mole of e- are added to 1 mole of gaseous -1 ions, to form 1 mole of gaseous ions, each with a -2 charge
Example: O-(g) + e- → O -2 (g)
Definitions:
Lattice Formation Enthalpy, ΔHlf (exothermic):
ΔH when 1 mole of a solid ionic compound is formed from its gaseous ions
Example: Na+(g) + Cl- (g) → NaCl (s), Mg2+ (g) + 2Br- (g) → MgBr2 (s)
Definitions:
Lattice Dissociation Enthalpy, ΔHld (endothermic):
ΔH when 1 mole of solid ionic compound dissociates into its ions
Definitions:
Standard Enthalpy of Solution, ΔHsol:
ΔH when 1 mole of a solid ionic substance dissolves * to form its aqueous ions
* (in enough solvent to form a solution in which the ions are far enough apart to not interact with each other)
Definitions:
Standard Enthalpy of Hydration, ΔHhyd:
ΔH when 1 mole of gaseous ions are converted to 1 mole of aqueous ions
In terms of bonding/attractions, describe what processes happen when NaCl dissolves in water, state whether these processes are exothermic or endothermic:
- ionic bonds between Na+ and Cl- are broken - endothermic
- water forms attractions to Na+ and Cl- - exothermic
- the δ+H is attracted to the Cl-
- the δ-O is attracted to the Na+
Which structure do you use when comparing lattice enthalpies for different substances?
CRAM
C- charge (not if -ve ion is of same charge)
R- radius
A- attractions between ions (strong/weak)
M - more endo ΔHld / more exo ΔHlf
What does the +ve ion do to the negative ion when the molecules has covalent character?
+ve ion polarises (distorts) the -ve ion (by attracting some of its e- density, resulting in a partial sharing of e-)
Covalent character occurs in ionic compounds more strongly with which features?
+ve ion = small with high charge
-ve ion = large with high charge
Assumptions that the Perfect Ionic Model makes about ionic substances:
- ions are perfect spheres
- ions can’t be polarised
Compare the Perfect Ionic model’s to the Born Haber:
BH:
- accounts for covalent character and ionic bonding
- more ΔH -ve
PI:
- only accounts for ionic bonding (not covalent character)
- less ΔH -ve
(if a compound has similar lattice enthalpy values with both PI model and BH cycle the the compound has very little covalent character)
