Thermodynamics Flashcards
Define enthalpy change of formation
The standard enthalpy change of formation of a compound is the energy transferred when 1 mole of the compound is formed from its elements under standard conditions (298K and 100kpa), all
reactants and products being in their standard states
Na (s) + ½Cl2
(g) –> NaCl (s) [∂fH = - 411.2 kJ mol-1]
Define enthalpy change 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) [∂atH = +148 kJ mol-1]
Define bond dissociation energy
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) ∂dissH = +242 kJ mol-1
Note: For diatomic molecules the ∂dissH of the molcule is
the same as 2x ∂atH of the element
Cl2 (g) –> 2Cl (g) ∂dissH= +242 kJ mol-1
Define first ionisation enthalpy
The first ionisation enthalpy is the enthalpy change required to
remove 1 mole of electrons from 1 mole of gaseous atoms to form
1 mole of gaseous ions with a +1 charge
Mg (g) –> Mg+
(g) + e- (endothermic)
Define second ionisation enthalpy
The second ionisation enthalpy is the enthalpy change to
remove 1 mole of electrons from one mole of gaseous 1+ ions
to produces one mole of gaseous 2+ ions.
Also endothermic
Define 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
O (g) + e- –> O- (g) [∂ea 1H] = -141.1 kJ mol-1]
The first electron affinity is exothermic for atoms that normally
form negative ions. This is because the ion is more stable than the
atom, and there is an attraction between the nucleus and the
electron.
Define 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) + e- –> O2- (g) [∂ea 2H = +798 kJ mol-1]
The second electron affinity for oxygen is endothermic
because it take energy to overcome the repulsive force
between the negative ion and the electron.
Define 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. Na+
(g) + Cl- (g) –> NaCl (s) [∂LattH = -787 kJ mol-1]
Define enthalpy of lattice dissociation
The Enthalpy of lattice dissociation is the standard enthalpy
change when 1 mole of an ionic crystal lattice form is separated
into its constituent ions in gaseous form.
NaCl (s) –> Na+
(g) + Cl- (g) [∂LattH = +787 kJ mol-1]
Define enthalpy of hydration
Enthalpy change when one mole of gaseous ions become aqueous ions .
X+(g) + aq –> X+(aq)
For Li+ ∂hydH = -519 kJ mol-1 or
X-(g) + aq –> X-(aq)
For F- ∂hydH= -506 kJ mol-1
This always gives out energy (exothermic, -ve) because bonds are made between the ions and the water molecules.
Define enthalpy of solution
The enthalpy of solution is the standard enthalpy change
when one mole of an ionic solid dissolves in a large enough
amount of water to ensure that the dissolved ions are well
separated and do not interact with one another.
What is a spontaneous process?
A SPONTANEOUS PROCESS (e.g. diffusion) will proceed on its
own without any external influence. This is mathematically defined by ∂G<0
Which 2 factors predict entropy changes
In general, a significant increase in the entropy will occur if:
- there is a change of state from solid or liquid to gas
- there is a significant increase in number of molecules between products and reactants.
Define standard enthalpy of combustion
The standard enthalpy of combustion of a substance is defined as the enthalpy change that occurs when one mole of a substance is combusted completely in oxygen under standard conditions. (298K and 100kPa), all reactants and products being in their
standard
