Thermodynamics

Question 1

Standard Enthalpy of Formation, ΔHf

Answer 1

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

Question 2

Standard Enthalpy of Combustion, ΔHc

Answer 2

ΔH when 1 mole of a compound is burned in oxygen under standard conditions with all reactants and products being in their standard states

Question 3

Mean Bond Enthalpy, ΔHbe

Answer 3

ΔH when 1 mole of gaseous molecules each break a covalent bond to form 2 free radicals, averaged over a range of compounds

Question 4

1st Ionisation Enthalpy, ΔHie

Answer 4

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

Question 5

2nd Ionisation Enthalpy, ΔHie

Answer 5

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

Question 6

Standard Enthalpy of Atomisation, ΔHat

Answer 6

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),

Question 7

Relationship between ΔHbe and ΔHat

Answer 7

(only with diatomic molecules)
ΔHbe = ΔHat x 2

Question 8

1st Electron Affinity, ΔHea

Answer 8

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)

Question 9

2nd Electron Affinity, ΔHea

Answer 9

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)

Question 10

Lattice Formation Enthalpy, ΔHlf (exothermic)

Answer 10

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)

Question 11

Lattice Dissociation Enthalpy, ΔHld (endothermic)

Answer 11

ΔH when 1 mole of solid ionic compound dissociates into its ions

Question 12

Standard Enthalpy of Solution, ΔHsol

Answer 12

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)

Question 13

Standard Enthalpy of Hydration, ΔHhyd

Answer 13

ΔH when 1 mole of gaseous ions are converted to 1 mole of aqueous ions

Question 14

In terms of bonding/attractions, describe what processes happen when NaCl dissolves in water, state whether these processes are exothermic or endothermic

Answer 14

• 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+

Question 15

Which structure do you use when comparing lattice enthalpies for different substances?

Answer 15

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

Question 16

What does the +ve ion do to the negative ion when the molecules has covalent character?

Answer 16

+ve ion polarises (distorts) the -ve ion (by attracting some of its e- density, resulting in a partial sharing of e-)

Question 17

Covalent character occurs in ionic compounds more strongly with which features?

Answer 17

+ve ion = small with high charge
-ve ion = large with high charge

Question 18

Assumptions that the Perfect Ionic Model makes about ionic substances

Answer 18

• ions are perfect spheres
• ions can’t be polarised

Question 19

Compare the Perfect Ionic model’s to the Born Haber

Answer 19

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)