Thermodynamics Flashcards
define enthalpy change of formation
enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
define lattice enthalpy of dissociation
enthalpy change when 1 mole of a solid ionic compound is dissociated into its gaseous ions under standard conditions
define lattice enthalpy of association
enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions under standard conditions
define bond dissosciation enthalpy
the enthalpy change when 1 mole of covalent bonds is broken into 2 gaseous atoms (or free radicals)
define enthalpy change of atomisation
the enthalpy change when 1 mole of gaseous atoms is made from an element in its standard state
define the enthalpy change of 1st ionisation
enthalpy change when 1 mole of electrons are removed from 1 mole of atoms in their gaseous states to produce 1 mole of 1+ ions also in their gaseous states
define the enthalpy change of 2nd ionisation
enthalpy change when 1 mole of electrons are removed from 1 mole of 1+ ions in their gaseous states to produce 1 mole of 2+ ions also in their gaseous states
define 1st electron affinity
enthalpy change when 1 mole of gaseous 1- ions are made from 1 mole of gaseous atoms
define 2nd electron affinity
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? why?
- used to calculate lattice enthalpies
- can’t calculate from experiments
what is the formula to calculate lattice enthalpy of dissociation?
-ΔH°f + ΔH°at + In + Ea
describe the perfectly ionic model
- ions that are perfectly spherical
- charge is evenly distributed in this sphere (point charges)
what is the perfectly ionic model used for?
calculating theoretical lattice enthalpies
what does it mean if the experimental lattice enthalpy of a compound is different to the theoretical lattice enthalpy?
- compound doesn’t follow the perfectly ionic model so has some covalent characteristics
why might a compound not follow the perfectly ionic model?
- positive ion polarises the negative ion (distorts the charge distribution in the negative ion)
- more polarisation = more covalent character
what is entropy a measure of?
disorder in a system
what symbol represents entropy?
S
what is entropy?
number of ways energy can be shared between particles
state the link between disorder and entropy
more disorder = higher level of entropy
list 4 factors that increase entropy
- inc number of molecules
- inc temperature
- inc randomness of molecule (s -> l -> g)
- inc complexity of molecule
what does enthalpically unfavourable mean?
endothermic
will a reaction tend to more or less disorder? what effect does this have on entropy?
more disorder + therefore inc entropy
give an equation for measuring entropy change
entropy change = entropy of products - entropy of reactants
ΔS = Sproducts - Sreactants
what is entropy change measured in?
J K-1mol-1
what does it mean if the entropy of a reaction is positive?
the reaction is entropically feasible
what is the symbol for Gibbs Free Energy?
ΔG
what does ΔG tell us?
whether or not a reaction is feasible
give an equation to calculate Gibbs Free Energy
ΔG = ΔH - TΔS
gibbs free energy (J mol-1) = enthalpy change (J mol-1) - temp (K) x entropy change (J K-1 mol-1)
what equation is used to calculate the temperature at which a reaction becomes feasible?
T = ΔH / ΔS
- make sure ΔH is in J mol-1
when is a reaction considered feasible in terms of ΔG?
reaction feasible if ΔG is negative or zero
why may a reaction not be observed to be occuring, despite it being calculated to be feasible?
- activation energy may be too high
- rate of reaction may be very slow
if ΔH value is negative and ΔS value is positive, what is the ΔG value? at what temp is the reaction considered feasible?
- ΔG always negative
- feasible at any temp
if ΔH value is positive and ΔS value is negative, what is the ΔG value? at what temp is the reaction considered feasible?
- ΔG always positive
- never feasible at any temp
if ΔH value is negative and ΔS value is negative, what is the ΔG value? at what temp is the reaction considered feasible?
- ΔG negative at lower temps
- feasible at lower temps
if ΔH value is positive and ΔS value is positive, what is the ΔG value? at what temp is the reaction considered feasible?
- ΔG negative at higher temps
- feasible at higher temps
define enthalpy change of solution
enthalpy change when 1 mole of an ionic substance is dissolved in the minimum amount of solvent to ensure no further enthalpy change is observed upon further dilution
what conditions need to be met for a substance to dissolve?
- substance bonds must break (endothermic)
- new bonds form between solvent and substance (exothermic)
- new bonds formed must have same strength or greater than those broken
describe what happens when an ionic lattice in solid form is dissolved in water
- substance bonds broken to create free moving ions
- partially pos H is attracted to negative ions and partially neg oxygen is attracted to positive ions
- structure starts to break down
- water molecules surround the ions (hydration)
- bonds formed between ions and water. the ions are hydrated
are soluble substances more likely to have exothermic or endothermic enthalpies of solution? why?
- exothermic
- new bonds formed must have same strength or greater than those broken
give a formula to calculate enthalpy change of solution
LEdiss + enthalpy change of hydration
define enthalpy change of hydration
enthalpy change when 1 mole of aqueous ions is made from one mole of gaseous ions
