3.1.8 Thermodynamics Flashcards
define enthalpy of formation?
this is the enthalpy change when 1 mole of a compound is formed form its constituent elements (EXOTHERMIC)
what is enthalpy of atomisation?
the enthalpy change when 1 mole of a gaseous atom is formed from its elements
what is meant by ‘1st ionisation energy’?
the energy needed to remove 1 mole of an electron from 1 mole of a gaseous atom to form 1 mole of a gaseous 1+ ion (ENDOTHERMIC)
what is meant by ‘1st electron affinity’? (EA1)
this is the energy released when 1 mole of an electron is added to 1 mole of a gaseous atom to form 1 mole of a gaseous 1- ion (EXOTHERMIC)
what is meant by ‘lattice enthalpy of formation?
this is the enthalpy change when 1 mole of an ionic compound is formed from its constituent gaseous ions (EXOTHERMIC)
what is meant by ‘enthalpy of solution’?
this is the enthalpy change when 1 mole of a compound is dissolved in a large volume of water to form an infinitely dilute solution
what is meant by ‘lattice dissociation enthalpy’? (LDE)
energy needed to break 1 mole of an ionic compound to its constituent gaseous ions.
e.g. NaCl(s) -> Na+(g) + Cl-(g)
what is meant by ‘enthalpy of hydration’?
enthalpy change when 1 mole of a gaseous ion is attracted to water molecules to form 1 mole of an aqueous ion
what is the formula for enthalpy of solution?
ΔHsol = LDE + [(ΔHhyd of cation) + (ΔHof anion)]
what has the lowest entropy and why?
Solids have the lowest entropy because:
- they have the lowest level of disorder
- due to very few ways of rearranging the particles
what has the highest entropy and why?
Gases have the highest entropy because:
- they have the highest level of disorder
- as there are many more ways of rearranging the particles
what is the formula for entropy change?
ΔS = ΣS[products]-ΣS[reactants]
how to calculate ΔH using ΔHf?
ΔH = ΣΔHf[products]-ΣΔHf[reactants]
how to calculate ΔH using ΔHc?
ΔH=ΣΔHc[reactants]-ΣΔHc[products]
what is the formula for Gibb’s free energy?
ΔG=ΔH-TΔS
what are the units in the formula for Gibb’s free energy?
- ΔG - KJ/mol
- ΔH - KJ/mol
- T - K
- ΔS - J/K/mol
what needs to happen in order for a reaction to be feasible, according to Gibb’s free energy?
ΔG≤0
what are the best possible conditions for a reaction to be feasible according to Gibb’s free energy?
ΔH = -ve (EXOTHERMIC)
ΔS = +ve (HIGH ENTROPY)
how does the size of an ion affect lattice enthalpy?
- the smaller the size of the ion, the greater the charge density
- stronger attraction to the oppositely charged ion -> LE INCREASES
what happens to lattice enthalpy as you go down the group?
- charge of the ion is the same but ion size increases -> charge density FALLS
- so weaker attraction to oppositely charged ions -> LE DECREASES.
what happens to lattice enthalpy as you go across the period (groups 1-3)?
- across the period (e.g Na+, Mg2+, Al3+), ion size decreases, charge increases, so charge density is GREATER.
- stronger attraction to oppositely charged ion -> LE INCREASES
what happens to lattice enthalpy as you go across the period (groups 5-7)?
- size of the ion decreases, leading to a greater charge density, but charge falls, so charge density is LOWER.
how do you compare experimental and theoretical lattice enthalpies?
- theoretical LE = assumes perfect ionic spheres; doesn’t account for covalent character within the ionic compound
why do all ionic compounds have covalent character?
- positive ion polarises the negative ion
- this changes/distorts its shape, which induces covalent character
