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Chemistry > 22) Enthalpy and entropy > Flashcards

22) Enthalpy and entropy Flashcards

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1
Q

Define lattice enthalpy

A

the enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions

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2
Q

ΔLEH⦵ is an _ (exothermic / endothermic) change therefore, ΔH is _ (positive / negative)

A

exothermic

negative

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3
Q

What is lattice enthalpy a measure of?

A

the strength of ionic bonding

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4
Q

Lattice enthalpy cannot be measured directly therefore, what is required?

A

a Born-Haber cycle for indirect determination

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5
Q

Define standard enthalpy change of atomisation ΔatH⦵

A

the enthalpy change that takes place when one mole of gaseous atoms forms from the element in its standard state

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6
Q

ΔatH⦵ is always _ (exothermic / endothermic) ?

A

endothermic - bonds broken

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7
Q

Define first ionisation energy ΔIEH⦵

A

the energy required to remove 1 e- from each atom in one mole of gaseous atoms of an element to form one mole of gaseous 1+ ions

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8
Q

ΔIEH⦵ is _ (exothermic / endothermic) ?

A

endothermic - overcome nuclear attraction

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9
Q

What is first ionisation energy a measure of?

A

ability to lose e-

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10
Q

Define first electron affinity ΔEAH⦵

A

the enthalpy change that takes place when 1e- is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions

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11
Q

What is first electron affinity a measure of?

A

ability to gain e-

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12
Q

ΔEAH⦵ is _ (exothermic / endothermic) ?

A

exothermic - e- added is attracted towards nucleus

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13
Q

What 2 things are important to remember about between each horizontal level of a Born-Haber cycle?

A

only one species has changed

all the species are balanced

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14
Q

Give the alternative path for lattice enthalpy on a Born-Haber cycle

A

(gaseous ions) - first electron affinity - first ionisation energy (gaseous atoms) - enthalpy change of atomisation (elements in standard states) + enthalpy change of formation (ionic lattice)

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15
Q

Define second electron affinity

A

the enthalpy change that takes place when 1e- is added to each ion in one mole of gaseous 1- ions to form one mole of gaseous 2- ions

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16
Q

Second electron affinity is _ (exothermic / endothermic) ?

A

endothermic - negative ion repels e-

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17
Q

When are successive electron affinities required?

A

when an anion has a greater charge than 1-

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18
Q

What will you need to do if more than 1 atom of an element is involved throughout a Born-Haber cycle?

A

multiply anything involved by a factor of the no. of atoms

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19
Q

Define enthalpy change of solution

A

the enthalpy change that takes place when one mole of a compound is completely dissolved in water under standard conditions

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20
Q

How can enthalpy change of solution be determined experimentally?

A

using q = mcΔT
calculate moles dissolved
(if 1mol -> aqueous ions) 1 mole would gain energy q in kJ / mol
uses the mass of solution - not the mass of water alone!

21
Q

Give two features of the dissolving process, when a solid ionic compound dissolves in water

A

ionic lattice breaks up (opposite of ΔLEH⦵)

water molecules are attracted to and surround the ions

22
Q

Define enthalpy change of hydration

A

the enthalpy change that takes place when one mole of isolated gaseous ions is dissolved in water forming one mole of aqueous ions under standard conditions

23
Q

Give the alternative path for hydration on a Born-Haber cycle

A

(gaseous ions) + lattice enthalpy (ionic lattice) + enthalpy change of solution (aqueous ions)

24
Q

What can lattice enthalpy be a good indicator for?

A 
melting point
(other factors e.g. packing of ions, needs to be considered also)
25
Q

How does increasing ionic radius affect lattice enthalpy and hydration, and therefore, melting point?

A

attraction between ions / ions and water molecules decreases
lattice enthalpy and hydration energy is less negative
melting point decreases

26
Q

How does increasing ionic charge affect lattice enthalpy and hydration, and therefore, melting point?

A

attraction between ions / ions and water molecules increases
lattice enthalpy and hydration energy is more negative
melting point increases

27
Q

What will happen if the sum of hydration enthalpies > magnitude of lattice enthalpy?

A

the overall energy change (enthalpy of solution) will be exothermic and the compound should dissolve

28
Q

How are many compounds with endothermic enthalpy changes of solution still soluble?

A

solubility also depends on temperature and entropy

29
Q

Define entropy S

A

term used for the dispersal of energy and disorder within the chemicals making up the chemical system

30
Q

As entropy increases, the dispersal of energy _ and the disorder _

A

increases

increases

31
Q

What happens if a system changes to be more random?

A

energy can be spread out more so ΔS is positive

32
Q

What happens if a system changes to be less random?

A

energy becomes more concentrated so ΔS is negative

33
Q

Give 2 changes that would result in a system becoming more random

A

solid -> liquid -> gas

increase in the number of gaseous molecules

34
Q

Define standard entropy S⦵

A

the entropy of one mole of a substance, under standard conditions
(every substance has one, a positive value with the units JK-1 mol-1)

35
Q

entropy change of a reaction ΔS⦵ = ?

A

∑S⦵(products) - ∑S⦵(reactants)

36
Q

Define free energy change ΔG

A

the overall energy change during a chemical reaction; the balance between enthalpy, entropy and temperature

37
Q

What is the Gibbs’ equation?

A

ΔG = ΔH - TΔS

free energy change = enthalpy change with surroundings - temp. in K x entropy change of system

38
Q

What should you remember about ΔS in the Gibbs’ equation?

A

must be changed to kJ K-1 mol-1 by x 10^-3 to match units of ΔH

39
Q

What must there be for a reaction to be feasible?

A

a decrease in free energy

ΔG < 0

40
Q

ΔH has a _ than TΔS - however as temperature increases TΔS becomes _?

A

larger magnitude

more significant

41
Q

Feasibility is supported when ΔH is _ and ΔS is _ - but as T increases the significance of _?

A

negative
positive
ΔS

42
Q

Is a reaction feasible when ΔH is negative and ΔS is positive?

A

yes, ΔG is negative

43
Q

Is a reaction feasible when ΔH is negative, ΔS is negative and temperature is low?

A

yes, ΔG is negative

44
Q

Is a reaction feasible when ΔH is negative, ΔS is negative and temperature is high?

A

no, ΔG is positive

45
Q

Is a reaction feasible when ΔH is positive and ΔS is negative?

A

no, ΔG is positive

46
Q

Is a reaction feasible when ΔH is positive, ΔS is positive and temperature is low?

A

no, ΔG is positive

47
Q

Is a reaction feasible when ΔH is positive, ΔS is positive and temperature is high?

A

yes, ΔG is negative

48
Q

Give the two equations for minimum temperature for a reaction to be feasible

A 
ΔG = ΔH - TΔS = 0
T = ΔH / ΔS
49
Q

What are the limitations of predictions made for feasibility?
Why do many reactions have a negative ΔG and not seem to take place?

A

ΔG indicates thermodynamic feasibility but takes no account of kinetics or rate of reaction - so a reaction may not seem to take place if it has a very large Ea or very slow rate of reaction

Chemistry (31 decks)

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