Thermodynamics Flashcards
ΔfH definition
Energy transferred when 1 mol of a compound is formed from its elements under standard conditions and all reactants and products being in their standard states
ΔatH definition
Enthalpy change when 1 mol of gaseous atoms is formed from its elements in their standard states
ΔsublimationH definition
enthalpy change for a solid metal turning to gaseous atoms and equals ΔatH
ΔdissH (bond dissociation) definition
standard molar enthalpy change when one mole of a covalent bond is broken into two gaseous atoms
First IE definition
ΔH required to remove 1 mol of e- from 1 mol gaseous atoms to form 1 mol gaseous ions with a 1+ charge
First electron affinity definition
ΔH when 1 mol of gaseous atoms gain 1 mol of electrons to form 1 mol of gaseous ions with a 1- charge
Why is first electron affinity exothermic for atoms that normally form 1- ions (group 7 for instance)
The ion is more stable than the atom
There is an attraction between the nucleus and the electron
Why is the second electron affinity for oxygen endothermic?
Energy is required to overcome the repulsive force between the negative ion and the electron
ΔlatformH definition
standard ΔH when 1 mol of a solid ionic crystal lattice is fromed from its consituent ions in gaseous form
ΔlatdissH definition
standard ΔH when 1 mole of an ionic crystal lattice forms its consituent ions in gaseous form
ΔhydH definition
(exo or endothermic?)
enthalpy change when 1 mol of gaseous ions become aqueous ions
always exothermic as bonds are formed between ions and water
ΔsolH definition
standard enthalpy change when 1 mol of an ionic solid dissolves in a large enough amount of water to ensure that the ions are well separated and do not interact with each other
Opposite of enthalpy of lattice formation?
enthalpy of lattice dissociation
Why use born haber cycles to calculate lattice enthalpies?
It cannot be determined directly
Which lattice enthalpy would point down in a born haber cycle?
formation
What’s always at the bottom row of a born haber cycle?
The solid compound
What’s always at the top row of a Born Haber cycle?
positive ions, electrons and gaseous atoms (which turn into negative ions next)
ΔatH(Cl2) is the same thing as…
the mean bond energy for Cl2
Explain 2 trends in lattice enthalpies
ion size: the larger the ions, the less -ve the ΔlatformH will be, the weaker the lattice will be
ion charge: the bigger the charge, the greater the attraction so the stronger the lattice enthalpy (more -ve values)
Perfect ionic model
assumed by theoretical lattice enthalpies
ions are 100% spherical and ionic
attractions are purely electrostatic
What 4 things cause covalent character?
Small +ve ion
multiple +ve charges on +ve ion
large -ve ion
multiple -ve charges on -ve ion
The more ________ the bigger the difference between the theoretical and born haber lattice enthalpies
covalent character
Why would the born haber value be larger than the theoretical value for a compound?
if it has some covalent character
it tends towards giant covalent structure
lattice + is stronger than if it was 100% ionic
What is different in the shapes of purely ionic ions and ions with covalent character?
purely ionic- 100% spherical
covalent character- electron charge cloud is distorted
Substances with more ways of arranging their atoms and energy (more disordered) have a higher/lower entropy?
higher
Why do solids have lower entropies than gases?
as the solid increases in temperature its entropy increases as particles vibrate more
gases move around even more–> more disordered
At 0K substances have ______ entropy and the reason why
0
no disorder
particles are stationary
When do significant increases in entropy occur?
change of state from s to l to g
increase in no of molecules from r to p
entropy unit
JK−1mol−1
ΔS =
Sprod - Sreact
Gibbs free energy purpose
combines effect of entropy and enthalpy into 1 number
ΔG=
ΔH-TΔS
ALWAYS USE S/1000
ΔG units
KJmol-1
ΔG has to be ____ve for a reaction to occur
-
What is ΔG for melting/boiling? Why?
0
physical phase changes are equillibria
If the reaction involves an increase in entropy what does increasing temperature do to ΔG?
makes it more likely that ΔG is negative and the reaction occurs
If the reaction involves a decrease in entropy what would an increase in temperature do to ΔG
Less likely that ΔG is negative and the reaction is less likely to occur
ΔG = ΔH - TΔS as a graph’s y, m, x and c values
ΔG as y
T as x
ΔH as c
-ΔS as m
ΔG = ΔH - TΔS positive gradient means
ΔS is negative
