Thermodynamics Flashcards
Define Enthalpy change of Formation?
⦾ ΔH1
⦾ ΔHf
⦾ ΔH when 1 mole of compound is formed from its elements in their standard states
Define Bond Dissociation Enthalpy?
⦾ ΔHdiss
⦾ ΔH when all the bonds of the same type in 1 mole of gaseous molecules are broken
Define Enthalpy change of Atomisation of an element?
⦾ ΔH2 + ΔH3
⦾ ΔHat
⦾ ΔH when 1 mole of a compound in its standard states is converted to gaseous atoms
Define First Ionisation Energy?
⦾ ΔH4
⦾ ΔHie1
⦾ ΔH when 1 mole of a gaseous 1+ ions is formed from 1 mole gaseous atoms
Define Second Ionisation Energy?
⦾ ΔHie2
⦾ ΔH when 1 mole of gaseous 2+ ions is formed from 1 mole of gaseous 1+ ions.
Define First Electron Affinity?
⦾ ΔH5
⦾ ΔHea1
⦾ ΔH when 1 mole of gaseous 1- ions is formed from 1 mole of gaseous atoms.
Define Second Electron Affinity?
⦾ ΔHea2
⦾ ΔH when 1 mole of gaseous 1- ions gains 1 electron per ion to produce gaseous 2- ions
Define Lattice enthalpy of Formation?
⦾ ΔH6
⦾ ΔHlattice
⦾ ΔH when 1 mole of solid ionic compound is formed from its gaseous ions
Define Lattice enthalpy of Dissociation?
⦾ ΔHlattice
⦾ ΔH when 1 mole of solid ionic compound is completely dissociated into gaseous ions
⦾ Endothermic
Define Enthalpy change of Hydration?
⦾ ΔHhyd
⦾ ΔH when 1 mole of aqueous ions is formed from gaseous ions
⦾ Exothermic
Define Enthalpy change of Solution?
⦾ ΔHsolution
⦾ ΔH when 1 mole of ionic substance dissolves in enough solvent to form an infinitely dilute solution
What is an infinitely dilute solution?
⦾ Contains so much solvent that when more liquid is added, there is no change in concentration.
What is Hess’s Law?
⦾ Total enthalpy change of a reaction is the same no matter the route taken
How do you calculate ΔH6?
⦾ ΔH6 = - (ΔH5) - (ΔH4) - (ΔH3) - (ΔH2) + (ΔH1)
How do you calculate ΔH1?
⦾ ΔH1 = (ΔH2) + (ΔH3) + (ΔH4) + (ΔH5) + (ΔH6)
What must you do if you go the wrong way along an arrow?
⦾ Add in a minus sign
What does the purely ionic model of a lattice assume?
⦾ All ions are spherical
⦾ Have their charge evenly distributed around them
How can water form bonds with ionic lattices?
⦾ Water's polar molecule ⦾ O more electronegative than H ⦾ So draws bonding electrons towards itself creating dipole ⦾ +ve ions form weak bonds with O ⦾ -ve ions form weak bond with H
What is the enthalpy change of solution?
⦾ Overall effect of enthalpy of bond breaking and making.
What do you need to know to work out the ethalpy change of a solution using an enthalpy cycle?
⦾ Lattice dissociation enthalpy of the compound
⦾ Enthalpies of hydration of ions
⦾ ΔH3 = ΔH1 + ΔH2
What does ‘entropy’ mean?
⦾ How much disorder
⦾ Measure of no. of ways particles arranged and no. of ways energy can be shared out between particles
State the factors that affect entropy?
1 ⦾ Physical state
2 ⦾ Dissolution
3 ⦾ No. of particles
How does physical state affect entropy?
⦾ Solids fixed in place so least entropy
⦾ Gases bounce in random movement so most entropy
How does Dissolution affect entropy?
⦾ Dissolving solid increased its entropy
⦾ Free to move - no longer held in place
How does no. of particles affect entropy?
⦾ More particles means more entropy
⦾ More ways energy’s arranged
When water evaporate, entropy…
⦾ …increases
⦾ Change from liquid to gas
When NaHCO3 and HCl react, entropy…
⦾ Increases
⦾ More particles in product
⦾ NaHCO3(s) + H+(aq) ➜ Na+(aq) + CO2(g) + H2O(l)
What is the formula for calculating entropy changes?
⦾ ΔS = Sproducts - Sreactants
What are the units for entropy?
⦾ J/K/mol
What is Free-energy change?
⦾ ΔG is measure to predict whether certian reaction is feasible
What is a feasible reaction?
⦾ Reaction that once started will continue onto completion, without any energy being supplied to it
What does it mean if ΔG is -ve or equal to zero?
⦾ Reaction is feasible
What is the formula for calculating free-energy change (ΔG)?
⦾ ΔG = ΔH - TΔS
⦾ ΔG = Free-energy change (J/mol)
⦾ ΔH = Enthalpy change (J/mol)
⦾ T = Temp (K) ΔS = Entropy change (J/K/mol)
Why might a -ve ΔG not guarantee a reaction will happen or tell you about the rate?
⦾ May have a very high activation energy
⦾ So slow you wouldn’t notice it happening
If a reaction is exothermic (-ΔH) and has a +ve entropy change, then the ΔG is always…
⦾ … -ve
⦾ These reactions are feasible at any temperature
If a reaction is endothermic (+ΔH) and has a -ve entropy change, then the ΔG is always…
⦾ … +ve
⦾ These reactions aren’t feasible at any temperature
If a reaction ΔH is positive (endothermic) and ΔS is positive, then the reaction will only be feasible…
⦾ …above a certain temperature
If a reaction ΔH is negative (exothermic) and ΔS is negative, then the reaction will only be feasible…
⦾ … below a certain temperature
