Topic 13 - Energetics 2 Flashcards
What is lattice enthalpy?
the energy change when one mole of an ionic
solid is formed from its gaseous ions
-always -ve
What is enthalpy change of atomisation?
the energy change to form a gaseous atom from its element in its standard state
What is first electron affinity?
the energy released when one electron is added to one mole of gaseous ions
-forming negative ions
What is a Born-Haber cycle used for?
to calculate an unknown enthalpy change
What do arrows going up in a Born-Haber cycle indicate?
energy required
What do arrows going down in a Born-Haber cycle indicate?
energy released
What factors affect lattice enthalpy?
- charge (higher charge = less -ve lattice enthalpy)
- ionic radius (smaller ionic radius = less -ve lattice enthalpy)
What is the difference between theoretical and experimental lattice enthalpy values?
theoretical value -> based upon mathematical model (electrostatic theory) and assumes point charges
-less -ve value (more exothermic)
experimental value -> based upon experimental data (from a Born-Haber cycle)
-takes into account distortion of anion’s electron density (distortion makes bond more covalent + stronger -would release more energy)
What is enthalpy change of solution?
the energy used when one mole of a solid compound is dissolved in sufficient solvent to give an infinitely dilute solution
(whole dissolving process)
ΔHsol =
-ΔHlatt + 2ΔHhyd
can be used on a Hess cycle
What is enthalpy change of hydration?
the energy used when one mole of gaseous ions is dissolved in sufficient solvent to give an infinitely dilute solution
What happens when an ionic solid dissolves?
negative lattice enthalpy/lattice disassociation
- lattice breaks up into separate gaseous ions
- endothermic
hydration enthalpy
- solid is surrounded by water molecules
- attraction between ions (in solid) and dipoles (in water)
- exothermic
What does the magnitude of lattice enthalpy depend on?
the strength of the forces acting on the ions
- ionic charges (higher charge = greater lattice enthalpy)
- ionic radii (smaller radii = greater lattice enthalpy)
What impact does ionic charge have on lattice enthalpy?
the greater the charge, the greater the lattice enthalpy
What impact does ionic radii have on lattice enthalpy?
the larger the radii, the smaller the lattice enthalpy
What does the magnitude of hydration enthalpy depend on?
the strength of the force between the ion and surrounding water molecules
- ionic charge (higher charge = great force)
- ionic radius (smaller radius = greater force)
What impact does ionic charge have on hydration enthalpy?
the greater the charge, the greater the hydration enthalpy
What impact does ionic radii have on hydration enthalpy?
the larger the radius, the smaller the hydration enthalpy
What is entropy?
a measure of disorder
- symbol = S
- units = JK-¹mol-¹
- naturally increases
What are the three rules of thermodynamics?
1) energy can not be created or destroyed
2) every spontaneous process must result in an increase in disorder
- the entropy of the most perfect crystalline solid at 0K (absolute 0) would have an entropy of 0JK-¹mol-¹
What factors affect entropy?
- state (solid→gas = increasing entropy)
- complexity (the higher the MMR of the compound, the more electrons it has so the greater the disorder)
ΔSsys =
ΣΔSprods - ΣSreacts
Why would an entropy change of system be negative?
there’s a decrease in disorder
eg. 2 mols of ordered solid and 1 mol of highly disordered gas are being converted into 2 mols of ordered solid
Why would an entropy change of system be positive?
there’s an increase in disorder
eg. 1 mol of ordered solid is being converted into 1 mol of ordered solid and 1 mol of highly disordered gas
ΔSsurr =
-ΔH
____
T
NB.
ΔH is measured in kJmol-1 however ΔS is measured in JK-1mol-1 so ΔH must be multiplied by 1000 (to convert into Jmol-1)
temp in Kelvin
ΔStot =
ΔSsys + ΔSsurr
When is a reaction feasable?
when ΔStot is positive
Why does a reaction not happen even though it has a positive total entropy?
- reactants are thermodynamically unstable (likely to turn into products)
- however… reactants are thermodynamically stable (high activation energy doesn’t allow them to react)
ΔG =
ΔH-TΔSsys
NB.
ΔG and ΔH are measured in kJmol-1 but ΔS is measured in JK-1mol-1 so ΔS must be divided by 1000 (to convert into kJK-1mol-1)
What does a negative ΔS say about a reaction’s feasibility?
rxn is not feasible
What does a positive ΔS say about a reaction’s feasibility?
rxn is feasible
What does a negative ΔG say about a reaction’s feasibility?
rxn is feasible
What does a positive ΔG say about a reaction’s feasibility?
rxn is not feasible
How do you calculate the minimum temperature to make a non-feasible reaction feasible USING ΔS?
Use ΔSurr = -ΔH/T rearranged to T=ΔH/ΔSsurr
use the opposite +/- to ΔSsys as ΔSsurr
How do you calculate the minimum temperature to make a non-feasible reaction feasible USING ΔG?
For a feasible rxn ΔG<0 so use ΔG=ΔH-TΔSsys to find minimum temp by replacing ΔG with 0 so that ΔH-TΔSsys>0
At equilibrium, what does a -ve ΔStot/a +ve ΔG mean?
backwards reaction is favoured
equilibrium lies to the left
ΔG =
in terms of equilibrium
RTlnK
rearranged:
K = e ^(-ΔG/RT)
ΔStot =
in terms of equilibrium constant
RlnK
