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A-Level Chemistry > Thermodynamics > Flashcards

Thermodynamics Flashcards

1
Q

define enthalpy change of formation

A

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

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2
Q

define lattice enthalpy of dissociation

A

enthalpy change when 1 mole of a solid ionic compound is dissociated into its gaseous ions under standard conditions

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3
Q

define lattice enthalpy of association

A

enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions under standard conditions

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4
Q

define bond dissosciation enthalpy

A

the enthalpy change when 1 mole of covalent bonds is broken into 2 gaseous atoms (or free radicals)

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5
Q

define enthalpy change of atomisation

A

the enthalpy change when 1 mole of gaseous atoms is made from an element in its standard state

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6
Q

define the enthalpy change of 1st ionisation

A

enthalpy change when 1 mole of electrons are removed from 1 mole of atoms in their gaseous states to produce 1 mole of 1+ ions also in their gaseous states

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7
Q

define the enthalpy change of 2nd ionisation

A

enthalpy change when 1 mole of electrons are removed from 1 mole of 1+ ions in their gaseous states to produce 1 mole of 2+ ions also in their gaseous states

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8
Q

define 1st electron affinity

A

enthalpy change when 1 mole of gaseous 1- ions are made from 1 mole of gaseous atoms

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9
Q

define 2nd electron affinity

A

enthalpy change when 1 mole of gaseous 2- ions are made from 1 mole of gaseous 1- ions

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10
Q

what are born-haber cycles used for? why?

A
  • used to calculate lattice enthalpies
  • can’t calculate from experiments
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11
Q

what is the formula to calculate lattice enthalpy of dissociation?

A

-ΔH°f + ΔH°at + In + Ea

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12
Q

describe the perfectly ionic model

A
  • ions that are perfectly spherical
  • charge is evenly distributed in this sphere (point charges)
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13
Q

what is the perfectly ionic model used for?

A

calculating theoretical lattice enthalpies

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14
Q

what does it mean if the experimental lattice enthalpy of a compound is different to the theoretical lattice enthalpy?

A
  • compound doesn’t follow the perfectly ionic model so has some covalent characteristics
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15
Q

why might a compound not follow the perfectly ionic model?

A
  • positive ion polarises the negative ion (distorts the charge distribution in the negative ion)
  • more polarisation = more covalent character
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16
Q

what is entropy a measure of?

A

disorder in a system

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17
Q

what symbol represents entropy?

A

S

18
Q

what is entropy?

A

number of ways energy can be shared between particles

19
Q

state the link between disorder and entropy

A

more disorder = higher level of entropy

20
Q

list 4 factors that increase entropy

A
  • inc number of molecules
  • inc temperature
  • inc randomness of molecule (s -> l -> g)
  • inc complexity of molecule
21
Q

what does enthalpically unfavourable mean?

A

endothermic

22
Q

will a reaction tend to more or less disorder? what effect does this have on entropy?

A

more disorder + therefore inc entropy

23
Q

give an equation for measuring entropy change

A

entropy change = entropy of products - entropy of reactants
ΔS = Sproducts - Sreactants

24
Q

what is entropy change measured in?

A

J K-1mol-1

25
Q

what does it mean if the entropy of a reaction is positive?

A

the reaction is entropically feasible

26
Q

what is the symbol for Gibbs Free Energy?

A

ΔG

27
Q

what does ΔG tell us?

A

whether or not a reaction is feasible

28
Q

give an equation to calculate Gibbs Free Energy

A

ΔG = ΔH - TΔS

gibbs free energy (J mol-1) = enthalpy change (J mol-1) - temp (K) x entropy change (J K-1 mol-1)

29
Q

what equation is used to calculate the temperature at which a reaction becomes feasible?

A

T = ΔH / ΔS

  • make sure ΔH is in J mol-1
30
Q

when is a reaction considered feasible in terms of ΔG?

A

reaction feasible if ΔG is negative or zero

31
Q

why may a reaction not be observed to be occuring, despite it being calculated to be feasible?

A
  • activation energy may be too high
  • rate of reaction may be very slow
32
Q

if ΔH value is negative and ΔS value is positive, what is the ΔG value? at what temp is the reaction considered feasible?

A
  • ΔG always negative
  • feasible at any temp
33
Q

if ΔH value is positive and ΔS value is negative, what is the ΔG value? at what temp is the reaction considered feasible?

A
  • ΔG always positive
  • never feasible at any temp
34
Q

if ΔH value is negative and ΔS value is negative, what is the ΔG value? at what temp is the reaction considered feasible?

A
  • ΔG negative at lower temps
  • feasible at lower temps
35
Q

if ΔH value is positive and ΔS value is positive, what is the ΔG value? at what temp is the reaction considered feasible?

A
  • ΔG negative at higher temps
  • feasible at higher temps
36
Q

define enthalpy change of solution

A

enthalpy change when 1 mole of an ionic substance is dissolved in the minimum amount of solvent to ensure no further enthalpy change is observed upon further dilution

37
Q

what conditions need to be met for a substance to dissolve?

A
  • substance bonds must break (endothermic)
  • new bonds form between solvent and substance (exothermic)
  • new bonds formed must have same strength or greater than those broken
38
Q

describe what happens when an ionic lattice in solid form is dissolved in water

A
  • substance bonds broken to create free moving ions
  • partially pos H is attracted to negative ions and partially neg oxygen is attracted to positive ions
  • structure starts to break down
  • water molecules surround the ions (hydration)
  • bonds formed between ions and water. the ions are hydrated
39
Q

are soluble substances more likely to have exothermic or endothermic enthalpies of solution? why?

A
  • exothermic
  • new bonds formed must have same strength or greater than those broken
40
Q

give a formula to calculate enthalpy change of solution

A

LEdiss + enthalpy change of hydration

41
Q

define enthalpy change of hydration

A

enthalpy change when 1 mole of aqueous ions is made from one mole of gaseous ions

A-Level Chemistry (23 decks)

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