thermodynamics Flashcards
(27 cards)
define hess’ law
enthalpy change us independent of the route taken
enthalpy of formation
enthalpy change when one mole of compound is formed from elements in their standard states and conditions
enthalpy of combustion
enthalpy change when one mole of compound is burnt completely in excess oxygen in standard state and conditions
electron affinity
enthalpy change when one mole of gaseous atoms gain a mole of electrons to become a 1- gaseous ion
lattice formation
enthalpy change when one mole of solid ionic compound formed from its gaseous ions
lattice dissociation
enthalpy change when one mole or solid ionic compound dissociates into its gaseous ions
enthalpy of atomisation
enthalpy change when one mile
or gaseous atoms formed from element in their standard state
mean bond enthalpy
enthalpy change when one mole of gaseous molecules each breaks a covalent bond to form 2 free radicals averaged over a range of compounds (diatomic) 2x atomisation
first enthalpy of ionisation
enthalpy change when one mole or electrons is removed from one mole gaseous atoms
what determines which is more exo (lattice enthalpy)
radius
attraction
more or less
higher lattice enthalpies
smaller ions
larger ions
ions increase down group a group in size more exo = stronger ionic bonding
small +ve ion
large -ve ion
more exothermic than perfect ionic model (no covalent character and perfect sphere)
enthalpy of solution
one mole of solute dissolves to form its aqueous ions
enthalpy of hydration
one mole of gaseous ions are converted to one mole of aqueous ions
k+(g) + aq —> k+(aq)
what is perfect ionic model
ions are perfect point spheres
what is entropy and what affects and equation
measures disorder
more disorder +ve entropy
more moles of gas formed
dissolving
entropy = sum of products - sum of reactants
gibbs free when feasible
^G = ^H - T^S
feasible when ^G<0
^H
——
^S
enthalpy of solution of MgCl2
MgCl2 + aq —> Mg2+(aq) + 2-(aq)
why is the first electron affinity of oxygen exothermic
strong attraction between nucleus of O atom and outer e-
which has the highest enthalpy of lattice and why
MgCl2 -2018
MgO -2443
BaCl2 - 3889
MgCl2 = -2443
MgO= -3889
BaCl2 = -2018
small +ve and large -ve
Ba2+ is large so weakest
O2- larger -Ve than Cl-
so strongest attraction therefore most exothermic
if tell increases solubility of
MgCl2 —> Mg2+ + 2Cl-
as temp increases
equilibrium shifts to the left
less moles
more
MgCl2 undissolved
therefore less soluble
explain why hydration of chloride is exothermic
chloride ion is strongly attracted to s+ H on H2O
when chloride binds to s+ H energy released
why does enthalpy of hydration become
less negative from Li+ to K+
size of ion increases
weaker attraction between +ve ion and if -O in water
units and conversions of gibs free:
T=O degrees = 273K
convert ^s to kjmol-1 by dividing by 1000
^G = Kjmol-1
why might a feasible reaction not be observed
activation energy too high or reaction happening very slowly
explain why the bond enthalpy of a cl-cl bond is greater than Br-Br bond
chlorine is a smaller atom than bromine - stronger attraction between nucleus and outer electron is stronger
