3.1.8: Thermodynamics Flashcards
What is enthalpy of formation (▲fH°)?
enthalpy change when one mole of substance is formed from its constituent elements, all substances in their standard states.
exothermic
What is enthalpy of combustion (▲cH°)?
enthalpy change when one mole of substance undergoes complete combustion in oxygen, all substances in their standard states.
exothermic
What is enthalpy of neutralisation (▲neutH°)?
enthalpy chaine when one mole of water is formed in a reaction between an acid and an alkali under standard condition.
exothermic
What is first ionisation energy (▲ieH°)?
enthalpy change when each atom in one mole of gaseous atoms loses an electron to form one mole of gaseous 1+ ions
endothermic
What is second ionisation energy (▲ieH°)?
enthalpy change when each ion in one mole of gaseous 1- ions gains an electron to form one mole of gaseous 2+ ions
endothermic
What is first electron affinity (▲eaH°)?
enthalpy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous 1- ions
exothermic
What is second electron affinity (▲eaH°)?
enthalpy change when each ion in one mole of gaseous 1- ions gains one electron to form one mole of gaseous 2- ions
endothermic
Why is second electron affinity endothermic?
as it involves adding an electron to a negatively charged ion, which requires energy to overcome the repulsive forces between the negatively charged particles.
What is enthalpy of atomisation (▲atH°)?
enthalpy change when one mole of gaseous atoms is produced from an element in its standard states (equal to 1/2 bond enthalpy)
endothermic
What is enthalpy of hydration (▲hydH°)?
enthalpy change when one mole of gaseous ions becomes hydrated (dissolves in water)
exothermic
What is enthalpy of solution (▲solH°)?
enthalpy change when one mole of an ionic solid dissolves in an amount of water large enough so that the dissolved ions are well seperated and don’t interact
varies
What is bond dissociation enthalpy (▲disH°)?
enthalpy change when one mole of covalent bonds is broken in the gaseous state
endothermic
What is lattice enthalpy of formation (▲LEFH°)?
enthalpy change when one mole of a solid ionic compound/ lattice is broken up from into its constituent ions in the gas phase
exothermic
What is lattice enthalpy of dissociation (▲LEDH°)?
enthalpy change when one mole of solid ionic compound/ lattice is broken u[ from into its constituent ions in the gas phase
endothermic
What is enthalpy of vapourisation (▲vapH°)?
enthalpy change when one mole of a liquid is turned into a gas
What is enthalpy of fusion (▲fusH°)?
enthalpy change when one mole of a solid is turned into a liquid
What is Born-Haber cycle used for?
- to find “experimental value”
- uses Hess’ law, includes all enthalpy changes in forming an ionic com[ound
- seperate step for each enthalpy change
- calculates (▲LEDH°) and (▲LEFH°)
What is lattice enthalpy in Born-Haber cycles?
enthalpy change when ions in one mole of a solid ionic compound/ lattice are broken apart (▲LEDH°) or brought together (▲LEFH°)
- indication of strength of ionic bodning
- greater magnitude of lattice enthalpy, stronger bonding (compound with smaller ions or ions w/ higher charge)
- BHC to scale, arrows represent the amount of energy
What are factors affecting lattice enthalpy?
- ionic radius- if radius increases, decreases lattice enthalpy
- charge- larger chage on metal, increase in lattice enthalpy
What are perfect ions?
perfect ionic model
ions that are perfectly spherical, have an even distribution of charge and interactions are purely electrostatic (no covalent character)
What is the difference between experimental and theoretical lattice enthalpy?
- experimental- real value (calculated by BHC)
- theoretical value- lattice enthalpy considering size, arrangement and charge in a lattice, assming structure is perfectly ionic
LARGER DIFFERENCE BETWEEN 2 VALUES, MORE COVALENT CHARACTER
How are ions have covalent character?
- distortion occurs due to polarising
- nucleus of ion distorts another ions electron cloud
- ions are spherical
- still ions, just have covalent character
What ions have most covalent charcter?
- cations (+ ions)- small and highly charged are very good at distorting as more charge to distort electron cloud with positive charge
- anions (- ions)- large and highly charges are easier to distort as further from nucelus, so easily polarised
What are properties of ions with covalent character?
- low solubility in water
- melting points/ electrical conductivity not as high as expected
- larger difference between experimental and theoretical values
e.g. AlCl3
- should be ionic, have covalent charcter
- low m.p., sublimes under pressure
- poor conductor in molten state
How do you calculate enthalpy of solution?
▲H°sol= ▲H°latt + ▲H°hyd
BHC:
- triangle (gas ions on top, ionic solid on left, dissolved ions on right)
- arrows going down for ▲H°latt diss and ▲H°hyd (as both exothermic)
What is entropy?
- a measure of disorder
- more disordered, greater entropy
- affected by temperature and no. of moles
- tendency for entropy to increase (2nd law of thermodynamics, entropy increases naturally over time)
- gases have most entropy, solids have least entropy
How does temperature affect entropy?
- 3rd law of thermodynamics- entropy of a substance is zero at absolute 0 and increases w/ temperature
- higher temp, more kindetic energy, faster particles, greater disorder/ entropy
- state changes have big increase in entropy
- more moles, greater entropy
How do you calculate entropy change?
▲S= (sum S products) - (sum S reactants)
in Jmol-1K-1
What is gibbs free energy?
▲G= ▲H- T▲S
- combines the 2 thermodynamic factors ▲H and ▲S
- if ▲G≤0, reaction is feasible
- divide T▲S /1000 to convert to kJ
in kJmol-1
What does a feasible reaction mean?
- a reaction can take place
- feasibility dependent on temperature, switched feasibility when ▲G=0
- feasible reaction may not take place due to high Ea
- spontaneous= feasible, reaction is thermodynamically possible
How does gibbs free affect changes of state?
- controlled in the same way
- below m.p. of substance, melting isn’t feasible if ▲G is positive
- at m.p. ▲G=0 melting point becomes feasible, substance melts
What is the chelate effect?
- bidentate and multidentate ligands will always displace monodentate ligands
- due to more moles of product produced, results in increased entropy
- increase T▲S so reaction is more feasible
