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Chemistry > Lattice enthalpy and entropy and gibbs free energy > Flashcards

Lattice enthalpy and entropy and gibbs free energy Flashcards

1
Q

lattice enthalpy definition

A

is the enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions

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2
Q

lattice enthalpy basic info

A

only in ionic compounds (not covalent as no ions)

strength of ionic bond directly linked to lattice enthalpy

always exo as bond formed

more exo = stronger attraction between ions

cant be directly measure as impossible to form 1 mole of ionic compound

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3
Q

How do we determine lattice enthalpy

A

born-haber cycle and hess’ law

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4
Q

sig of lattice enthalpy value

A

more negative = more exo

stronger attraction between opp charged ions

stronger ionic bonds in giant ionic lattice

higher melting point

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5
Q

symbol for lattice enthalpy

A

DeltaLEH

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6
Q

factors affecting lattice enthalpy

A

ionic size-
NOT ATOMIC RADIUS BUT IONIC RADIUS

smaller ions are more strongly attracted to larger ones

greater the diff stronger the attraction and greater DeltaLEH

Ionic charge-
ions of greater charge more strongly attract than ions of smaller charge

Mg 2+ attracts stonger than Na+

Mg in compound has higher DeltaLEH

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7
Q

Standard enthalpy change of formation

KEY

exo or endo

A

enthalpy change when 1 mole of a compound is formed from its elements in standard states and conditions

usually exo

e.g. Mg(s) + Cl2(g) –> MgCl2(s)

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8
Q

standard enthalpy change of atomisation

exo or endo

A

enthalpy change when 1 mole of gaseous atoms is formed from its elements in standard states and conditions

endo as bond are always broken

e.g. 1/2 Cl2(g) –> Cl(g)

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9
Q

First ionisation energy

KEY

exo or endo

A

enthalpy change when one electron is removed from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions

endo as energy needed to overcome nuclear attraction

e,g Na(g) —> Na+(g) + e-

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10
Q

second ionisation energy

exo or endo

A

enthalpy change when 1 electron is removed from every ion in 1 mole of gaseous 1+ ions to form 1 mole of gaseous 2+ ions

Endo (more than 1st as greater proton : e- so stronger nuclear attraction)

Ca+(g) —> Ca2+(g) + e-

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11
Q

first electron affinity
deltaEA1H

exo or endo

A

enthalpy change when 1 electron is added to each atom in one mole of gaseous atoms to form 1 mole of gaseous 1- ions

exo because electron is attracted to the nucleus

Cl(g) + e- —> Cl-(g)

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12
Q

second electron affinity
DeltaEA2H

exo or endo

A

enthalpy change when 1 electron is added to each atom of 1 mole of gaseous 1- ions to form 1 mole of gaseous 2- ions

endo as electrons repel each other need to overcome

Cl-(g) + e- —> Cl2-(g)

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13
Q

Born-Haber cycle key features

A

elements in standard states have zero enthalpy (shown on datum line)

all DeltaH values pointing up = exo
pointing down = endo

clockwise=anticlockwise

if change includes 2 moles then * that section by 2

DO EXAM QUESTIONS

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14
Q

standard enthalpy change of solution
DeltaSolH

A

the enthalpy change when 1 mole of an compound dissolves completely in water under standard conditions

can be exo or endo

NaCl (s) –> Na+(aq) + Cl-(aq)

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15
Q

2 processes that occur when ionic compound dissolves

A

ionic lattice dissociates into its gaseous ions

the gaseous ions are hydrated with H20 molecules

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16
Q

`dissociation of ionic lattice

endo or exo

A

endo

energy needed to overcome attraction between opp charged ions

dissociation is opp of lattice enthalpy

NaCl (s) –> Na+(g) + Cl-(g)

17
Q

Hydrating gaseous ions

endo or exo

A

exo

energy releases when gasoues ions attract and bond to h20

+ attract slightly negative O
- attract slightly positive H

dipole dipole interaction (weak but many) form

18
Q

standard enthalpy change of hydration
DeltahydH

KEY

exo or endo

A

enthalpy change when 1 mole of isolated gaseous ions is dissolved in H2O forming 1 mole of aqueous ions under standard conditions

Na+(g) + aq —> Na+(aq)

exo

19
Q

Factors affecting Hydration value

A

Ionic size-
smaller ions attract + bond with H20 more strongly
more exo as stronger bond

ionic charge-
higher charged ions attract and bond with H2O more strongly
more exo as stronger bond

20
Q

Enthalpy

A

measure of the heat content of a chemical system

21
Q

entropy

A

measure of the dispersal of energy in a system

The entropy is greater when the system is more disordered

22
Q

Basic facts of entropy

A

all substance have some degree of disorder (as particles in constant motion)

entropy= always +

more disorder = more entropy

entropy of pure substance increases with increasing temp

units= JK-1mol-1

23
Q

how do we know a system is becoming more disordered? and why does this matter?

A

if entropy change (DeltaS) is positive

system becomes more energetically stable as it becomes more disordered + entropy helps explain spontaneous processes as entropy tends towards max (2nd law of thermodynamics)

24
Q

Entropy of diff states

A

more disordered = greater entropy

entropy increases ——>
solid liquid gas

entropy increases during changes in state that cause more disorder

in exam questions answer in terms of moles of gas (if present)

25
Q

standard entropy change of reaction

A

entropy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard states and conditions

26
Q

entropy change calc

A

entropy of products - entropy of reactants

more disorder= entropy change is +

less disordered= entropy change is -

27
Q

when is a process feasible?

A

a process is spontaneous/feasible is chem system becomes more stable and overall energy decreases

depends on=
temp
entropy change
enthalpy change of surrounding

28
Q

Gibbs free energy change

A

is the balance between the enthalpy change, entropy change and temp of a process

29
Q

Gibbs free energy equ

A

DeltaG = DeltaH - TDeltaS

units for DeltaG usually KJmol-1
units for T = K
units for DeltaS = KjK-1mol-1 (convert by / by 1000)

30
Q

why do most exo reaction occur spontaneously at room temp

A

enthalpy content of system decreases during reaction and energy is released to surroundings

increases stability

31
Q

how do some endo reactions occur at room temp

A

enthalpy increases during reaction

for it to be spontaneous overall energy must decrease

so must be a decrease in free energy via the entropy change
TDeltaS= greater than DeltaH

more stable

32
Q

when is a reaction feasible?

A

when DeltaG = less than 0

if = 0 then in equilibrium (e.g. state change)

33
Q

calc min temp for feasibility

A

calc DeltaS plus convert to KjK-1mol-1

Calc DeltaH

then set DeltaG to 0

0=DeltaH - TDeltas
T=DeltaH/DeltaS = min temp

convert to C if needed (-273)

34
Q

limitations of predictions of feasibility

A

some have - DeltaG but still not spontaneous

not considered reaction kinetics
rate of reaction so slow (Ea too high) so reaction not observed

not thermodynamically feasible

Chemistry (18 decks)

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