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Chemistry > 1.7 thermodynamics > Flashcards

1.7 thermodynamics Flashcards

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1
Q

what is enthalpy change? units?

A

heat energy transferred in a reaction at constant pressure. KJmol-1

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2
Q

what is lattice formation enthalpy? △Hr
eg
endo or exo?

A

enthalpy change when 1mol of a solid ionic compound is formed from its gaseous ions
Na+(g) + Cl- (g)-> NaCl (s)
exo

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2
Q

what does △H^⦵ mean?
what do subscripts r,c and f mean?

A

△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

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3
Q

what is lattice dissociation enthalpy? △Hled
eg
endo or exo?

A

enthalpy change when 1 mol of a solid ionic compound is completely dissociated into its gaseous ions
NaCl(g)-> Na+(g) + Cl-(g)
endo

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4
Q

what is the relationship between the values of △H lattice and △H diss?

A

they are the same number but dissociation is + and formation is -

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5
Q

what is lattice enthalpy a measure of?

A

ionic bond strength

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6
Q

page 21

A
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7
Q

what is enthalpy change of formation △Hf
eg
exo or endo?

A

enthalpy change when i mol of a compound is formed from its elements in standard states
Ca(s) + Cl2(g) -> CaCl2(s)
exo for most

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8
Q

what is bond dissociation enthalpy △Hdiss
eg
exo or endo?

A

enthalpy change when all bonds of same type in 1 mol of gaseous molecules are broken
Cl2(g)-> 2Cl(g)
endo

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9
Q

what is enthalpy change of atomisation of an element? △Hat
eg
exo or endo?

A

enthalpy change when 1 mol of gaseous atoms is formed from an element in its standard state
1/2cl2(g) -> cl(g)
endo

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10
Q

what is enthalpy change of atomisation of a compound? △Hat
eg
exo or endo?

A

enthalpy change when 1 mol of a compound in its standard state is converted to gaseous atoms
NaCl(s) -> Na(g) + Cl(g)
endo

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11
Q

what is first ionisation energy? △Hie1
eg
exo or endo?

A

enthalpy change when 1 mol of gaseous 1+ ions is formed from 1 mol of gaseous atoms
Mg(g) -> Mg+(g) +e-
endo

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12
Q

what is second ionisation energy? △Hie2
eg
exo or endo?

A

enthalpy change when 1 mol of gaseous 2+ ions is formed from 1 mol of gaseous 1+ ions
Mg+(g) -> Mg2+(g) +e-
endo

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13
Q

what is first electron affinity? △Hea1
eg
exo or endo?

A

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

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14
Q

what is second electron affinity? △Hea2
eg
exo or endo?

A

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

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15
Q

what is enthalpy change of hydration? △Hhyd
eg
exo or endo?

A

enthalpy change when 1 mol of aqueous ions is formed from gaseous ions
Na+(g) -> Na+(aq)
exo

16
Q

what is the enthalpy change of solution?
△Hsolution
eg
exo or endo?

A

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

17
Q

what is enthalpy of combustion? △Hc
eg
exo or endo?

A

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

18
Q

what is enthalpy of neutralisation? △Hneut
eg
exo or endo?

A

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

19
Q

what is enthalpy of vapourisation? △Hvap
eg
endo or exo?

A

enthalpy change when 1 mol of liquid is turned into a gas
H2O(l)-> H2O(g)
endo

20
Q

what is enthalpy of fusion? △Hfus
eg
endo or exo?

A

enthalpy change when 1 mol of a solid is turned into a liquid
Mg(s)-> Mg(l)
endo

21
Q

what is a born-haber cycle

A

used to figure out enthalpy change if you take a less direct route as you cant measure it directly

22
Q

what goes where on a born-haber cycle

A

enthalpy of formation, atomisation enthalpies, ionisation energy for positive ion, electron affinity of negative ion, lattice enthalpy of formation of ionic compound

23
Q

how does a born-haber cycle change for a group 2 element?

A

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

24
Q

what is △Hbond dissociation equal to?

A

2 x the △Hat

25
Q

what does the purely ionic model of a lattice assume?

A

all the ions are spherical and have their charge evenly distributed around them

26
Q

what is evidence for ionic compounds usually having some covalent character?

A

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)

27
Q

what is covalent character?

A

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

28
Q

what does it show if the experimental and theoretical lattice enthalpies for compounds are very different?

A

it shows that the compound has lots of covalent character
vice versa

29
Q

which factors give the most covalent character?

A

small, highly charged, positive ions
large, highly charged, negative ions

30
Q

what factors affect lattice enthalpy?

A

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

31
Q

what 2 things happen when a solid ionic lattice dissolves in water?

A

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

32
Q
A

Chemistry (16 decks)

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