Thermodynamics Flashcards
What is the Born-Haber cycle? When is it used?
‘Special type of Hess’s law cycle’
- Used to calculate the lattice enthalpy of ionic compounds
Define atomisation
Formation of 1 mole of gaseous atoms
Define Ionisation
Formation of gaseous ions
Define standard enthalpy change of atomisation
The enthalpy change when 1 mole of gaseous atoms is formed under standard conditions
OR
The enthalpy change for this reaction:
Ca(s) ——> Ca(g)
Is atomisation ENDO or EXO. Why?
ENDO
because, bonds are being broken
Define standard enthalpy change of the 1st I.E
(1st I.E)
the enthalpy change of this reaction:
Ca(g) ——> Ca+ (g) + e-
Is the 1st I.E ENDO or EXO
Why?
ENDO
because, energy is needed to overcome the electrostatic attraction between the nucleus and the electron being removed
Define standard enthalpy change of the 2nd I.E
The enthalpy change for this reaction:
Ca+(g) ——> Ca2+(g) + e-
Is the 2nd I.E ENDO or EXO
Why?
MORE ENDO
because, more energy is needed to overcome the electrostatic attraction between the nucleus and the electron being removed
Define the standard enthalpy of the first electron affinity
The enthalpy change for this reaction:
F(g) + e- ——> F- (g)
Is the first electron affinity ENDO or EXO
Why?
EXO
because, there is an electrostatic force of attraction between the nucleus of the gaseous atom and the electron being added
Define the standard enthalpy of the second electron affinity
The enthalpy change for this reaction:
O-(g) + e- ——> O2- (g)
Is the second electron affinity ENDO or EXO
Why?
ENDO
because energy is needed to overcome the repulsion between the negatively charged gaseous ion and the negatively charged electron being added
Define Enthalpy of Lattice Formation
The enthalpy change when 1 mole of an ionic solid is formed from the gaseous ions under standard conditions
OR
the enthalpy change for this reaction:
K+(g) + F-(g) ——> KF(s)
Is the lattice of formation ENDO or EXO
Why?
EXO
because, there is a strong electrostatic force of attraction between the oppositely charged ions
Define Enthalpy of Lattice Dissociation
The enthalpy change when 1 mole of an ionic solid lattice is broken to form the separate gaseous ions under standard conditions.
OR
The enthalpy change for this reaction:
KF(s) ——> K+(g) + F-(g)
Is the lattice of dissociation ENDO or EXO
Why?
ENDO
because, energy is needed to break the electrostatic force of attraction between the oppositely charged ions
What 2 factors affect Lattice enthalpy
- charge of ions
- Size/ radius of ion
What do you call lattice enthalpy values from Born- Harber cycles
Experimental lattice energies
What are theoretical lattice energy values
Values calculated from the ‘perfect ionic model’
Name the 2 main assumptions of the ‘perfect ionic model’
- The ions are perfectly spherical /(they are point charges)
- The compound is 100% ionic. There is no distortion or polarisation of the ions (no covalent character present)
Define the enthalpy change of solution
The enthalpy change when 1 mole of an ionic solid lattice is dissolved completely in water under standard conditions
OR
The enthalpy change for this reaction:
KCl(s) ——> K+(aq) + Cl-(aq)
When an ionic solid is dissolved in water, which 2 thermodynamic processes are involved
-Lattice Dissociation
The ionic solid lattice is broken into the gaseous ions
-Hydration
The gaseous ions are converted into aqueous ions
What is the equation for the 🔺H(sol) ?
🔺H(sol)
= lattice dissociation + 🔺H(hyd)
Define the enthalpy change of Hydration
The enthalpy change when 1 mole of aqueous ions is formed from 1 mole of gaseous ions under standard conditions
OR
The enthalpy change for this reaction:
Na+ (g) ——> Na+(aq)
Is the enthalpy of hydration ENDO or EXO
Why?
EXO
because of the strong electrostatic attraction between the ion and the diapoles in water
What 2 factors affect the enthalpy of hydration?
- Size of ion
- Charge of ion
What does it mean if an ion has a higher ‘charge density’
SMALLER ion
HIGHER charge
Define what it means for a reaction to be spontaneous/ ‘feasible’
Can occur freely/ of it’s own accord
What is Entropy and give it’s units
‘The measure of the degree of disorder in a reaction’
UNITS: Jk-1 mol-1
J= joules
K= kelvin
Why is the entropy of elements not zero?
Entropy is measured at 298K
- the particles still have some energy to distribute (still moving)
Define the state of ‘perfect/ maximum order’
The state of a substance when the particles are stationary (only occurs at 0K)
What does a graph showing entropy against temperature show?
Entropy increases as temperature increases
A change in state leads to a BIG INCREASE in entropy
Boiling results in a bigger increase in entropy than melting
What is the equation for the entropy change of surroundings
🔺S = (-)🔺H / T
What is the 2nd law of thermodynamics
For a spontaneous reaction
🔺S total must be positive
What is the equation for ‘Gibbs free energy change’
🔺G = 🔺H - T🔺S
Why might a reaction with negative GFEC not occur spontaneously and give solutions
Rate is too slow
- Add a catalyst
- Increase the temperature
What is the equation for the entropy change of a system
entropy change of (products) - entropy change of (reactants)
