Enthaplies A2 Flashcards
Def of enthalpy of formation
Enthalpy change when 1 mol of a substance is formed from its elements with all substances in their standard states
Symbol and example of enthalpy of formation
(Use sodium chloride)
ΔfH⦵
E.g Na (s) + 1/2Cl2 (g) —> NaCl (s)
Def of the first ionisation enthalpy
Enthalpy change when 1 mol of electrons is removed from 1 mol of gaseous species
Def of enthaply of atomisation (element)
Enthalpy change when 1 mol of gaseous atoms is formed from its element in its standard state
Def enthalpy of atomisation (compound)
Enthalpy change when 1 mol of a compound in gas state is converted into separate gaseous ions
Def bond dissociation enthalpy
Enthalpy change when 1 mol of given covalent bond, of a compound in the gas states is broken
Def of first electron affinity enthalpy
Enthalpy change when 1 mol of electrons is added to 1 mol of gaseous species
Def of lattice enthalpy (also called lattice formation)
Enthalpy change of formation of 1 mol of an ionic lattice from its gaseous ions
Def of enthalpy of hydration
Enthalpy change when one mole of gaseous ions are diluted to give no further temperature change and one mole of hydrated ions
Def of enthalpy of solution
Enthalpy change when 1 mole of solute is dissolved in sufficient solvent to give no interaction between dissolved species
Symbol and example equation for the first ionisation enthalpy
(Use sodium)
Δ1st i.e H⦵
E.g Na (g) —> Na+ (g) + e-
Symbol and example equation for enthalpy of atomisation of an element
(Use sodium and chlorine)
ΔatH⦵
E.g Na(s) —> Na (g)
1/2 Cl2 (g) —> Cl2 (g)
Symbol and example equation for the enthalpy of atomisation of a compound
(Use methane)
ΔatH⦵
E.g CH4 —> C(g) + 4H(g)
Symbol and example equation for bond dissocation enthalpy
(Use methane)
ΔdissH⦵
E.g CH4 (g) —> CH3 (g) + H (g)
Symbol and example equation for first electron affinity enthalpy
(Use chlorine )
Δ1st eaH⦵
E.g Cl (g) + e- —> Cl- (g)
Symbol and example equation for lattice enthalpy
(Use sodium chloride)
ΔLEH⦵
E.g Na+ (g) + Cl- (g) —> NaCl(s)
Symbol and example equation of enthalpy of solution
( use sodium chloride)
ΔsolH⦵
E.g NaCl (s) —> Na+ (aq) + Cl- (aq)
Symbol and example equation of enthalpy of hydration
( use sodium)
ΔhydH⦵
E.g Na+ (g) + H2O (l) —> Na+ (aq)
Draw a Born-Haber cycle to calculate the ΔleH of LiF
If
Δ1st ea H(f) = -328 KJ mol-1
Δ atH(Li) = +159 KJ mol -1
Δ1st ie H(Li) = +520 KJ mol -1
ΔatH(F) = +79 KJ mol-1
ΔfH(LiF) = -616 KJ mol-1
See notes for cycle
ΔleH (LiF) = - Δ1st ea H(f) - Δ atH(Li) - Δ1st ie H(Li) - ΔatH(F) +ΔfH(LiF)
ΔleH (LiF) = -(-328) - (+79) - (+520) - (+159) + (-616)
ΔleH (LiF) = -1046 KJ mol-1
What is the effect of cation (positive) size (down the group) on
1) attraction
2) lattice energy
3) melting point
Na+ < K+ < Rb+
As ionic size increases
1- ionic radius increases
2- attraction between ions decreases
3- lattice energy less negative
4- melting point decreases
What is the effect of increased cation (positive) charge on
1) attraction
2) lattice energy
3) melting point
Na+ vs Ca2+
As ionic charge increases
Attraction between ions increases
Lattice energy becomes more negative
Melting point increases
What is the effect of cation (positive) size (down the group) on
1) attraction
2) hydration enthalpy
Na+ < K+ < Rb+
Ionic radius increases
Attraction between ion and water molecules decreases
Hydration energy less negative
What is the effect of increased cation (positive) charge on
1) attraction
2)hydration energy
Na+ vs Ca2+
Ionic charge increases
Attraction with water molecules increases
Hydration energy becomes more negative
How do you calculate enthalpy of solution of LiF from lattice enthalpies and enthalpies of hydration using Hess cycle or Born-Haber cycle
If
Lattice enthalpy = -1031
Hydration of Li+ = -520
Hydration of F- = -524
Enthalpy of solution (LiF) = - lattice enthalpy + enthalpy of hydration( Li+) + enthalpy of Hydration (F-)
Enthalpy of solution (LiF) = -(-1031) + ((-520) + (-524))
Enthalpy of solution (LiF) = -13 KJ mol-1