physical chem; thermodynamics Flashcards
define the term enthalpy change of formation
the enthalpy change when 1 mol of a compound is formed from it’s elements in their standard states under standard conditions
define the term lattice enthalpy of formation
the enthalpy change when 1 mole of a solid ionic compound is formed from it’s gaseous ions under standard conditions
define the term lattice enthalpy of dissociation
the enthalpy change when 1 mole of a solid ionic compound is dissociated into it’s gaseous ions under standard conditions
define the term enthalpy change of dissociation
the enthalpy change when 1 mole of bonds of the same type of molecule in the gaseous state is broken
define the term enthalpy change of 1st ionisation
the enthalpy change when 1 mole of gaseous 1+ ions are made from 1 mole of gaseous atoms
define the term enthalpy change of 2nd ionisation
the enthalpy change when 1 mole of +2 ions are made from 1 mole of gaseous +1 ions
define the term enthalpy change of atomisation
the enthalpy change of when 1 mole of gaseous atoms is made from an element in it’s standard state
define the term enthalpy change of 1st electron affinity
the enthalpy change when 1 mole of gaseous -1 ions are made from 1 mole of gaseous atoms
define the term enthalpy change of 2nd electron affinity
the enthalpy change when 1 mole of gaseous -2 ions are made from 1 mole of gaseous -1 ions
what are born-haber cycles used for?
to calculate lattice enthalpies
what do theoretical & experimental lattice enthalpy values depend on?
how pure an ionic compound is as some ionic compounds have covalent characteristics
what is a perfect ionic model?
- perfectly spherical ions
- the charge is evenly distributed in the sphere (point charges)
why do some ionic compounds have covalent character?
the +ve ion distorts the charge distribution in the -ve ion (+ve ion polarises -ve ion). the more polarisation there is, the more covalent character there will be
what do lattice enthalpy values tell us about ionic substances?
how purely ionic a substance is
how can we know that an ionic substance contains some covalent character from looking at lattice enthalpy values?
if the experimental value shows a higher lattice enthalpy than the purely ionic theoretical value
define the term enthalpy change of solution
the enthalpy change when 1 mole of an ionic substance is dissolved in the minimum mount of solvent to ensure no further enthalpy change is observed upon further dilution
name the requirements needed for a substance to dissolve
- substance bonds must break (endothermic)
- new bonds formed between the solvent & substance (exothermic)
explain what happens when ionic substances dissolve in a polar solvent (e.g. water)?
the δ+ charge is attracted to the -ve ions & the δ- oxygen is attracted to +ve ions - structure starts to break down.
the water molecules surround the ions in a process called hydration.
define the term enthalpy change of hydration
the enthalpy change when 1 mole of aqueous ions is made from 1 mole of gaseous ions
what is entropy?
the measure of disorder in a system - more disorder = higher level of entropy
state the disorder of the 3 states in order from lowest to highest
solids - lowest disorder as particles are tightly packed to one another
liquids
gases - highest as particles are free
what affects entropy change?
the no.of particles
if a reaction is in the same state but more moles are produced then entropy increases
explain how enthalpically unfavourable reactions can react spontaneously
increasing entropy is energetically favourable for endo reactions (enthalpically unfavourable) & can still spontaneously react if changes in entropy overcome changes in enthalpy
how do we know if a reaction is entropically favourable?
more moles with the products
increased disorder (e.g. change in state from solid to liquid/gas)
how do we know if a reaction is enthalpically favourable
if its exothermic
how is entropy change calculated?
∆S = S (products) - S (reactants)
how do we know if a reaction is entropically feasible?
if the entropy value is +ve
what is gibbs free energy?
tells us if a reaction is feasible or not
how is gibbs free energy calculated?
∆G = ∆H - T∆S
∆H - enthalpy change
T - temp
∆S - entropy change
how do we know if a reaction is feasible by using ∆G value?
a reaction is feasible in theory if ∆G is -ve or 0
state whether the reaction is feasible if ∆H is -ve & ∆S is +ve & state whether the ∆G value is +ve or -ve
yes, its feasible at any temp
∆G always -ve
state whether the reaction is feasible if ∆H is +ve & ∆S is -ve & state whether the ∆G value is +ve or -ve
never feasible at any temp
∆G always +ve
state whether the reaction is feasible if ∆H is -ve & ∆S is -ve & state whether the ∆G value is +ve or -ve
yes, feasible at lower temps
∆G -ve at lower temps
state whether the reaction is feasible if ∆H is +ve & ∆S is +ve & state whether the ∆G value is +ve or -ve
yes, feasible at higher temps
∆G is -ve at higher temps
how is the temp of a reaction that becomes just feasible calculated?
a reaction is just feasible when ∆G = 0 so the equation can be rearranged to T = ∆H / ∆S
