1.7 thermodynamics Flashcards
what is enthalpy change? units?
heat energy transferred in a reaction at constant pressure. KJmol-1
what is lattice formation enthalpy? △Hr
eg
endo or exo?
enthalpy change when 1mol of a solid ionic compound is formed from its gaseous ions
Na+(g) + Cl- (g)-> NaCl (s)
exo
what does △H^⦵ mean?
what do subscripts r,c and f mean?
△H is enthalpy change and ⦵ means substances were in standard states and measurements were made under standard conditions (100KPa and stated temp).
r-enthalpy change is for a reaction
c-enthalpy change is for combustion
f-enthalpy change is for formation
what is lattice dissociation enthalpy? △Hled
eg
endo or exo?
enthalpy change when 1 mol of a solid ionic compound is completely dissociated into its gaseous ions
NaCl(g)-> Na+(g) + Cl-(g)
endo
what is the relationship between the values of △H lattice and △H diss?
they are the same number but dissociation is + and formation is -
what is lattice enthalpy a measure of?
ionic bond strength
page 21
what is enthalpy change of formation △Hf
eg
exo or endo?
enthalpy change when 1 mol of a compound is formed from its elements in standard states
Ca(s) + Cl2(g) -> CaCl2(s)
exo for most
what is bond dissociation enthalpy △Hdiss
eg
exo or endo?
enthalpy change when all bonds of same type in 1 mol of gaseous molecules are broken
Cl2(g)-> 2Cl(g)
endo
what is enthalpy change of atomisation of an element? △Hat
eg
exo or endo?
enthalpy change when 1 mol of gaseous atoms is formed from an element in its standard state
1/2cl2(g) -> cl(g)
endo
what is enthalpy change of atomisation of a compound? △Hat
eg
exo or endo?
enthalpy change when 1 mol of a compound in its standard state is converted to gaseous atoms
NaCl(s) -> Na(g) + Cl(g)
endo
what is first ionisation energy? △Hie1
eg
exo or endo?
enthalpy change when 1 mol of gaseous 1+ ions is formed from 1 mol of gaseous atoms
Mg(g) -> Mg+(g) +e-
endo
what is second ionisation energy? △Hie2
eg
exo or endo?
enthalpy change when 1 mol of gaseous 2+ ions is formed from 1 mol of gaseous 1+ ions
Mg+(g) -> Mg2+(g) +e-
endo
what is first electron affinity? △Hea1
eg
exo or endo?
enthalpy change when 1 mol of gaseous 1- ions is made from 1 mol gaseous atoms
O(g) + e- -> O2- (g)
exo for many nonmetals
what is second electron affinity? △Hea2
eg
exo or endo?
enthalpy change when 1 mol of gaseous 2- ions is made from 1 mol gaseous 1- ions
O(g) + e- -> O2- (g)
endo as adding -e to - ion
what is enthalpy change of hydration? △Hhyd
eg
exo or endo?
enthalpy change when 1 mol of aqueous ions is formed from gaseous ions
Na+(g) -> Na+(aq)
exo
what is the enthalpy change of solution?
△Hsolution
eg
exo or endo?
enthalpy change when 1 mol of an ionic substance dissolves in enough solvent to form an infinitely dilute solution
MgCl2(s) +aq ->Mg^2+(aq) + 2Cl-(aq)
varies
what is enthalpy of combustion? △Hc
eg
exo or endo?
enthalpy change when 1 mol of a substance undergoes complete combustion in oxygen with all substances in standard states
H2(g) + 1/2O2(g) -> H2O(l)
exo
what is enthalpy of neutralisation? △Hneut
eg
exo or endo?
enthalpy change when 1 mol of water is formed in a reaction between an acid and alkali under standard conditions
1/2H2SO4(aq) + NaOH(aq) -> 1/2Na2SO4(aq) +H2O(l)
exo
what is enthalpy of vapourisation? △Hvap
eg
endo or exo?
enthalpy change when 1 mol of liquid is turned into a gas
H2O(l)-> H2O(g)
endo
what is enthalpy of fusion? △Hfus
eg
endo or exo?
enthalpy change when 1 mol of a solid is turned into a liquid
Mg(s)-> Mg(l)
endo
what is a born-haber cycle
used to figure out enthalpy change if you take a less direct route as you cant measure it directly
what goes where on a born-haber cycle
enthalpy of formation, atomisation enthalpies, ionisation energy for positive ion, electron affinity of negative ion, lattice enthalpy of formation of ionic compound
how does a born-haber cycle change for a group 2 element?
include second ionisation enthalpy too
multiply atomisation enthalpy of non-group 2 element by no of moles
multiply electron affinity of - ion by no of mol ions formed
what is △Hbond dissociation equal to?
2 x the △Hat
what does the purely ionic model of a lattice assume?
all the ions are spherical and have their charge evenly distributed around them
what is evidence for ionic compounds usually having some covalent character?
the experimental lattice enthalpy from the born-haber cycle is different to the theoretical lattice enthalpy (that is found out by doing calculations based on purely ionic model)
what is covalent character?
positive and negative ions in a lattice arent usually exact spheres. + ions polarise neighbouring - ions to diff extents
more polarisation= more covalent the bonding will be
what does it show if the experimental and theoretical lattice enthalpies for compounds are very different?
it shows that the compound has lots of covalent character
vice versa
which factors give the most covalent character?
small, highly charged, positive ions
large, highly charged, negative ions
what factors affect lattice enthalpy?
size of ion- smaller means more strongly attracted to d+ or d- charge of the h2o molecules are to the ions, the larger the enthalpy
charge of ion- larger charge, larger enthalpy of hydration
what 2 things happen when a solid ionic lattice dissolves in water?
bonds between ions break to give gaseous ions- endo (lattice enthalpy of diss)
bonds between ions and water are made- exo (enthalpy change of hydration
how do you calculate enthalpy of solution?
draw an enthalpy cycle
1) put ionic lattice and dissolved ions on top then connect them by change of solution
2) connect ionic lattice to gaseous ions by lattice enthalpy of dissociation (+ number)
3) connect gaseous ions to dissolved ions by hydration enthalpies of each
what is entropy?
tells you how much disorder there is
measure of number of ways that particles can be arranged and the no of ways energy can be shared out between particles.
substances like disorder so particles move to increase entropy
entropy is represented by S
what factors affect entropy?
physical state- not much disorder in solids so they have low entropy. lots of disorder in gases as they move lots so highest entropy
dissolution- dissolving increases entropy as dissolved particles can move freely
no of particles- more particle= more entropy as theres more ways they can be arranged
eg N2O4 -> 2NO2 entropy increases as no of mol increases
how do you calculate entropy change?
units?
△S = S(products) - S (reactants)
JK-1mol-1
what is free energy change? △G
used to predict if a reaction is feasible (will carry onto completion once started without energy being supplied to it)
if its - or equal to 0, it is feasible
how do you work out free energy change?
△G = △H - T△S
Jmol-1, Jmol-1, K, JK-1mol-1
why may a reaction that is theoretically feasible not happen?
may have a high activation energy or be so slow you dont notice it
how does temp effect free energy?
exothermic + positive entropy change = △G always negative so always feasible
endothermic + negative entropy change = △G always positive so never feasible
exothermic + negative entropy change = △G negative at low temp but positive at high
endothermic + positive entropy change = △G positive at low temp but negative at high
How do you work out temperature of feasibility?
△G = △H - T△S
0 = △H - T△S
T△S = △H
T = △H / △S
how do you write the free energy equation as a straight line equation?
△G = (- △S x T) + △H
what do the different parts of the free energy graph mean?
△G is the y axis
T is the x axis
△H is y intercept
-△S is gradient (so △S is negative of gradient)
temp of feasibility is the x intercept
