Energetics: Enthalpy Change Essay

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1: THERMODYNAMICS
Definitions of enthalpy changes Enthalpy change of formation The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions

Enthalpy of atomisation The enthalpy of atomisation of an element is the enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state Na (s) Na(g) [∆Hat = +148 kJ mol-1] ½ O2 (g) O (g) [∆Hat = +249 kJ mol-1] Bond dissociation enthalpy The bond dissociation enthalpy is the standard molar enthalpy change when one mole of a covalent bond is broken into two gaseous atoms (or free radicals) Cl2 (g) 2Cl (g) ∆Hdiss = +242 kJ mol-1 Or CH4 (g) CH3 (g) + H(g) ∆Hdiss = +435 kJ mol-1 First Ionisation energy The first ionisation energy is the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous ions with a +1 charge (under standard conditions) Mg+ (g) + e- [∆H Ie] Mg (g) First Electron affinity The first electron affinity is the enthalpy change that occurs when 1 mole of gaseous atoms gain 1 mole of electrons to form 1 mole of gaseous ions with a –1 charge (under standard conditions) O (g) + eO- (g) [∆Hea] = -141.1 kJ mol-1]

Second Ionisation energy The second ionisation energy is the energy required to remove 1 mole of electrons from one mole of gaseous 1+ ions to produces one mole of gaseous 2+ ions. Mg+ (g) Mg 2+ (g) + e- [∆ Hi]

second electron affinity The second electron affinity is the enthalpy change when one mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions. O – (g) + eO2- (g) [∆Hea = +798 kJ mol-1]

The first electron affinity is exothermic for atoms that normally form negative ions because the ion is more stable than the atom Enthalpy of lattice formation The Enthalpy of lattice formation is the standard enthalpy change when 1 mole of an ionic crystal lattice is formed from its constituent ions in gaseous form (under standard conditions).
]1-lom Jk 787- = ttaL H [ )s( lCaN

The second electron affinity for oxygen is endothermic because it take energy to overcome the repulsive force between the negative ion and the electron

Enthalpy of lattice dissociation The Enthalpy of lattice dissociation is the standard enthalpy change when 1 mole of an ionic crystal lattice is separated into its constituent ions in gaseous form (under standard conditions).
+ ]1-lom Jk 787+ = ttaL H [ )g( -lC + )g( +aN

Note the conflicting definitions and the sign that always accompanies the definitions Enthalpy of Hydration ∆Hhyd Enthalpy change when one mole of gaseous ions become hydrated (dissolved in water). ∆
1-

Enthalpy of solution The enthalpy of solution is the standard enthalpy change when one mole of an ionic solid dissolves in an large enough amount of water to ensure that the dissolved ions are well separated and do not interact with one another NaCl (s) + aq Na+ (aq) + Cl-(aq)

This always gives out energy (exothermic, -ve) because bonds are made between the ions and the water molecules

Copyright N Goalby Bancroft's School

lom Jk 605- = dyhH

∆

)s( lCaN

∆

ta )g( lC )g( 2lC ½ 1-lom Jk 121+ = H ssidH )g( lC2 )g( 2lC 1-lom Jk 242+ = tnemele eht fo taH x2 sa emas eht si eluclom eht fo ssidH eht selucelom cimotaid roF

]1-lom Jk 2.114- = fH [ )s( lCaN
The enthalpy change for a solid metal turning to gaseous atoms can also be called the Enthalpy of sublimation and will numerically be the same as the enthalpy of atomisation Na (s) Na(g) [∆Hsub = +148 kJ mol-1] ∆ ∆ ∆ ∆ ∆
1-

∆

lom Jk 915- = dyhH

+

-

iL roF

F roF

) g( 2 l C ½ + ) s( a N
)qa( )qa(

)g( -lC + )g( +aN
+

-X

X

qa + )g( -X ro qa + )g( +X

BORN HABER CYCLES
The lattice enthalpy cannot be determined directly. We calculate it indirectly by making use of changes for which data are available and link them together in an enthalpy cycle the Born Haber cycle

Born Haber cycle: sodium Chloride

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