Thermodynamics
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What does Hess’ Law state?
The enthalpy change for a reaction is independent of the route taken
Define standard enthalpy of formation
The enthalpy change when one mole of a compound is formed from its constituent elements in standard conditions, with all products and reactants in their standard states
What is the standard enthalpy of an element
0 by definition
Define standard enthalpy of combustion
The enthalpy change when one mole of a substance is completely burnt in (excess) oxygen
Define standard enthalpy of atomisation
Enthalpy change when one mole of gaseous atoms is formed from a compound in its standard state in standard conditions
Define first ionisation energy
Enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to form one mole of gaseous 1+ ions
Define second ionisation energy
Enthalpy change when one mole of electrons is removed from one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions
Define first electron affinity
Enthalpy change when one mole of gaseous atoms gains one mole of electrons to form one mole of gaseous 1- moles
Define second electron affinity
Enthalpy change when one mole of gaseous 1- ions gains one mole of electrons to form one mole of gaseous 2- ions
Define lattice enthalpy of formation
Enthalpy change when one mole of solid ionic lattice is formed from its constituent gaseous ions
Define lattice enthalpy of dissociation
Enthalpy change when one mole of solid ionic lattice is dissociated into its gaseous ions
Define enthalpy of hydration
Enthalpy change when one mole of gaseous ions become hydrated / dissolved in water to infinite dilution [water molecules completely surround the ion]
Define enthalpy of solution
Enthalpy change when one mole of solute dissolves completely in a solvent to infinite dilution
Define mean bond dissociation enthalpy
Enthalpy change when one mole of (a certain type of) covalent bonds is broken, with all species in the gaseous state
Write an example equation for: Standard enthalpy of formation Standard enthalpy of combustion Standard enthalpy of atomisation
• Mg (s) + 1/2 O2 (g) —> MgO (s) • CH4 (g) + 2O2 (g) —> CO2 (g) + 2H2O (g) • 1/2 I2 (g) —> I (g)
Write an example equation for: First ionisation energy Second ionisation energy First electron affinity
• Li (g) —> Li+ (g) + e- • Mg+ (g) —> Mg2+ (g) + e- • Cl (g) + e- —> Cl- (g)
Write an example equation for Second electron affinity Lattice enthalpy of formation Lattice enthalpy of dissociation
• O- (g) + e- —> O*2- (g) • Na+ (g) + Cl- (g) —> NaCl (s) • NaCl (s) —> Na+ (g) + Cl- (g)
Write an example equation for Enthalpy of hydration Enthalpy of solution Mean bond dissociation enthalpy
• Na+ (g) —> Na+ (aq) • NaCl (s) —> Na+ (aq) + Cl- (aq) • Br2 (g) —> 2Br (g)
What is a Born-Haber cycle
Thermochemical cycle showing all the enthalpy changes involved in the formation of ionic compound. Start with elements in their standard states (enthalpy of 0)
What factors affect the lattice enthalpy of an ionic compound
Size of ions Charge of ions
How can you increase the lattice enthalpy of a compound? Why does this increase it?
Smaller ions since the charge centres will be closer together Increased charge since there will be greater electrostatic force of attraction between the oppositely charged ions N.B. Increasing the charge on the Anion has a much smaller effect than increasing the charge on the Cation, since increasing anion size also has the effect of increasing ionic size
How can Born-Haber cycles be used to see if compounds could theoretically exist
Use known data to predict certain values of theoretical compounds, and then see if these compounds would be thermodynamically stable Was used to predict the existence of the first noble gas containing compound
What happens when a solid is dissolved in terms of interactions of the ions with water ions
Break lattice - gaseous ions; dissolve each gaseous ion in water. The aqueous ions are surrounded by water molecules (which have a permanent dipole due to polar O-H bond)
What is the perfect ionic model
Assumes that ions are perfectly symmetrical and that there is an even charge distribution (100% polar bonds). Act as point charges