Thermodynamics Flashcards
enthalpy of formation
enthalpy change when one mole of a substance is formed from its constituent elements with all substances in their standard states under standard conditions - exothermic (usually)
enthalpy of combustion
enthalpy change when one mole of a substance undergoes complete combustion in oxygen with all substances in standard states under standard conditions - exothermic
enthalpy of neutralisation
enthalpy change when 1 mole of water is formed in a reaction between an acid and an alkali under standard conditions - exothermic
ionisation enthalpy
first: enthalpy change when each atom in one mole of gaseous atoms loses one electron to form one mole of gaseous 1+ ions
second: enthalpy change when each ion in one mole of gaseous 1+ ions loses one electron to form one mole of gaseous 2+ ions
-endothermic
electron affinity
First: enthalpy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous 1- ions
- exothermic
Second: enthalpy change when each ion in one mole of gaseous 1- ions gains one electron to form one mole of gaseous 2- ions
-endothermic
enthalpy of atomisation
enthalpy change when one mole of gaseous atoms is produced from an element in its standard state - endothermic
hydration enthalpy
enthalpy when one mole of gaseous ions become hydrated (dissolved in water) - exothermic
enthalpy of solution
enthalpy change when one mole of an ionic solid is dissolved - varies
bond dissociation enthalpy
enthalpy change when one mole of covalent bonds is broken in the gaseous state - endothermic
lattice enthalpy of formation
enthalpy change when one mole of a solid ionic compound is formed from its constituent ions in the gas phase - exothermic
lattice enthalpy of dissociation
enthalpy change when one mole of a solid ionic compound is broken into its constituent ions in the gas phase - endothermic
enthalpy of vaporisation
enthalpy change when one mole of a liquid is turned into a gas - endothermic
enthalpy of fusion
enthalpy change when one mole of a solid is turned into a liquid - endothermic
born-haber cycle
ionise metal –> down 2nd ea –> down
1st ea non-metal —> up
up
atomise non-metal lattice
up of
atomise metal formation
up
start line: elements in standard states
down (enthalpy formation)
base line: solid ionic compound <———————————-
enthalpy change definition
change in heat content at constant pressure
Hess’s law?
The enthalpy change for a reaction is independent of the route taken
A —> B = A—>C—>B
enthalpy of formation hess law
elements enthalpy of formation always 0
arrows pointing up from constituent elements
sum of product enthalpy - sum of reactant enthalpy
enthalpy of combustion hess law
arrows pointing down to oxides
reaction across top often enthalpy of formation
bond enthalpy hess law
arrows pointing down to gas atoms
bond enthalpies are mean not specific for compound so not very accurate
equation for enthalpy change and conversion to kJ/mol
(calorimetry)
q = mcAt
q: heat energy given out (J)
m: mass of substance heated (g)
AT: temperature rise (K)
c: specific heat capacity (J/g/K)
Then
/1000 for kJ
Finally
enthalpy change/mol = q/number of moles reacting
problems with calorimetry calculations + solution
heat loss major problem - to reduce you can measure heat capacity of calorimeter as a whole
enthalpy of solution hess cycle
gas ions —-> ionic solid (lattice enthalpy formation)
gas ions —-> dissolved (aq) ions (hydration enthalpy)
arrows pointing down from gas ions to
ionic solid ——-> dissolved (aq) ions (enthalpy of solution)
trend in lattice enthalpy in regards to ionic bonding
greater magnitude of lattice enthalpy, greater ionic bonding and vice versa
compounds with smaller ion and/or greater charge have stronger attraction so greater lattice enthalpy
how to find experimental lattice enthalpy?
born-haber cycle
what is perfect ionic model?
theoretical calculation for lattice enthalpy that considers size, charge and arrangement of ions in lattice - assumes structure is perfectly ionic
problem with perfect ionic model
often distortion of ions in ionic compound (not perfectly spherical) - they have covalent character
bigger difference between theoretical and real value in general, the greater the covalent character
still ionic bonding but some covalent properties such as low solubility, lower melting point/conductivity than expected
entropy
Entropy (S) is how disordered something is and is measured in J/mol/K
Gases have most entropy
Entropy will naturally increase over time
entropy
Entropy (S) is how disordered something is and is measured in J/mol/K
Gases have most entropy
Entropy will naturally increase over time
how does entropy vary with temperature?
Zero at absolute zero and increases with temperature
Big increases in entropy at state changes - greatest from liquid to gas as entropy substantially larger in gases than solids or liquids
entropy change
change in S = [sum S products] - [sum S reactants]
reactions with increase in entropy are favourable
sometimes very small change so unable to tell if it will be positive or negative
Gibbs free energy change
change in G combines change in H and change in S
AG (kJ/mol) = AH (kJ/mol) - T (K) AS (J/mol/K)
need to convert AS to kJ/mol/K
Feasible reaction if AG less than or equal to 0
Depends on temperature if feasible or not
AG = 0 is the point at which reaction becomes feasible
If feasible still may not occur due to having very high activation energy
