Topic 13: Energetics II Flashcards
Lattice energies, enthalpies of solution/hydration, entropy, Gibbs energy
What is the lattice energy?
The energy change when one mole of an ionic solid is formed from its gaseous ions.
It always has a negative value (it is exothermic)
What assumptions do theoretical values for lattice energy?
- Ions are spherical,
- Ions are point charges i.e. the charge is evenly distributed,
- Ions are in contact.
This would all be the case for pure ionic bonding.
Why is there a difference between theoretical and calculated lattice energies?
Covalent character of the ionic bond affects the values. If the positive ion is very small, it has a higher charge density and therefore more polarising ability on the large negative ion.
The larger the negative ion, the more polarisable it is.
This discrepancy causes a difference in values.
What is the enthalpy of solution?
enthalpy change when 1
mole of an ionic compound is dissolved in water to
form an infinitely dilute solution
What is Enthalpy of hydration?
enthalpy change when 1
mole of gaseous ions are dissolved in sufficient water
to give an infinitely dilute solution. Always negative
values as a result of the formation of strong ion-dipole
attractions.
What is meant by lattice enthalpy?
enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions.
Always negative due to the formation of strong ion-ion
attractions.
Diagram of an ionic solid dissolving?
Ionic solid —-> Dis. ion in soln
\ /
\ /
Gaseous ions
From gaseous ions -> ionic solid = lattice enthalpy
Gaseous ions –> dissolved in solution = enthalpy of hydration
Ionic solid –> dissolved in solution = Enthalpy of solution
What is entropy?
A description of the number of ways that atoms can share quanta of energy.
If the number of ways is high then entropy is high.
What is the equation for the total entropy change?
ΔStotal = ΔSsystem + ΔSsurroundings
What is the equation for ΔSsystem?
ΔSsystem = ∑S products - ∑S reactants
∑ = sum of
What does an increase in entropy mean?
An increase in entropy means that the system becomes energetically more stable
An example of a system that becomes more disordered is when a solid is melted
What is more favourable? Higher or lower entropy?
Higher entropy will be energetically favourable (as the energy of the system is more spread out when it is in a disordered state)
What is Gibbs free energy?
The feasibility of a reaction is determined by two factors
The enthalpy and entropy change
The two factors come together in a fundamental thermodynamic concept called the Gibbs free energy
Gibbs energy equation?
ΔGꝋ = ΔHreaction – TΔSsystem
(under standard conditions)
What must the value for Gibbs be in order for the reaction to be feasible?
Less than or equal to 0
What are the units for Gibbs?
- The units of ΔGꝋ are in kJ mol–1
- The units of ΔHreactionꝋ are in kJ mol–1
- The units of T are in K
- The units of ΔSsystemꝋ are in J K-1 mol–1(and must therefore be converted to kJ K–1 mol–1by dividing by 1000)
What is the equation for ΔSsurroundings?
ΔSsurroundings = -ΔH/T
What is the enthalpy change of atomisation?
enthalpy change when one mole of gaseous atoms is produced from its element in its standard state
Define electron affinity
The first electron affinity is the energy released when 1 mole of gaseous atoms each acquire an electron to form 1 mole of gaseous 1- ions
always has a negative value
What does lattice energy tell us
It provides a measure of ionic bond strength
What is the meaning of polarisation when applied to ions?
What does the polarising ability of a cation depend on?
Charge - larger charge means more polarising ability
Radius - smaller radius means more polarising ability
What does the polarisability of a anion depend on?
Charge
Radius - the larger the radius, the more easily it will be distorted
What is the effect of ionic charge and ionic radius on lattice energy?
