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
standard entropy change of reaction
entropy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard states and conditions
26
entropy change calc
entropy of products - entropy of reactants more disorder= entropy change is + less disordered= entropy change is -
27
when is a process feasible?
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
Gibbs free energy change
is the balance between the enthalpy change, entropy change and temp of a process
29
Gibbs free energy equ
DeltaG = DeltaH - TDeltaS units for DeltaG usually KJmol-1 units for T = K units for DeltaS = KjK-1mol-1 (convert by / by 1000)
30
why do most exo reaction occur spontaneously at room temp
enthalpy content of system decreases during reaction and energy is released to surroundings increases stability
31
how do some endo reactions occur at room temp
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
when is a reaction feasible?
when DeltaG = less than 0 if = 0 then in equilibrium (e.g. state change)
33
calc min temp for feasibility
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
limitations of predictions of feasibility
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