thermodynamics [P2] PAPER 1 Flashcards
gibbs equation and units?
• ΔG = ΔH – TΔS
• ΔG = Gibbs free energy change, kJmol-1
• ΔH = enthalpy change, kJmol-1
• T = temperature, K
• ΔS = entropy change, JK-1mol-1
⚠️ ΔS is in JOULES per kelvin per mole, while the rest are KILOJOULES, so need to convert to kJ when doing calculations
how is the feasibility of a reaction determined using the value of ΔG?
if ΔG is negative, it is feasible
State the meaning of the term enthalpy change.
heat energy change at constant pressure
Define the term enthalpy of lattice formation.
enthalpy change when one mole of ionic lattice is formed from its constituent gaseous ions under standard conditions
define the term electron affinity
the enthalpy change when one mole of gaseous atoms form one mole of gaseous negative ions under standard conditions
Define the term enthalpy of lattice dissociation
enthalpy change when separating one mole of ionic lattice into its constituent gaseous ions under standard conditions
Lattice enthalpy can be calculated theoretically using a perfect ionic model. Explain the meaning of the term perfect ionic model.
• ions are point charges
• ions are perfect spheres
• only electrostatic attraction between ions
• no covalent character
• only ionic bonding
Suggest two properties of ions that influence the value of a lattice enthalpy calculated using a perfect ionic model.
• ion size
• charge density
lattice enthalpy can be defined as…
either enthalpy of lattice dissociation or enthalpy of lattice formation
define enthalpy of hydration
enthalpy change when one mole of gaseous ions forms one mole of aqueous ions under standard conditions
define enthalpy of solution
enthalpy change when one mole of ionic lattice is dissolved in water to infinite dilution, so that the ions no longer interact under standard conditions
define enthalpy of atomisation
enthalpy change when one mole of gaseous atoms are formed from the element in its standard state
why is enthalpy of hydration almost always negative?
• water is polar, with H δ+ and O δ+
• the negative ion attracts the H in water molecules
• the positive ion attracts the O in water molecules
calculating enthalpy of solution?
ΔH(solution) = ΔH(lattice dissociation) + Σ(ΔHhydration)
OR
ΔH(solution) = – ΔH(lattice formation) + Σ(ΔHhydration)
how to calculate at which temperature a reaction becomes feasible using the Gibbs free energy equation?
• ΔG = ΔH – TΔS
• make ΔG = 0 [as a reaction is feasible when ΔG <0], then solve for T
what is entropy?
measure of disorder
calculating entropy change, ΔS?
ΔS = S products – S reactants
ΔG = ΔH – TΔS as y = mx + c, with y-axis as ΔG in kJ/mol, and x-axis as temperature in K
ΔG = ΔH – TΔS
⇒ ΔG = – TΔS + ΔH
• ΔG = y
• ΔH = c
• T = x
• –ΔS = m
solid, liquid and gas states of a substance in order of increasing entropy, S?
S solid < S liquid < S gas