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A2 Chem > Chemical Energetics > Flashcards

Chemical Energetics Flashcards

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1
Q

define lattice energy

A

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

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2
Q

is ^Hlatt exo or endo? explain why it is so

A

exo

because ions are combined to FORM BONDS in an ionic solid lattice

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3
Q

what does a large, negative value of ^Hlatt indicate

A

that the ionic compound is more stable than its gaseous

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4
Q

why is the ionic compound more stable than its gaseous ions

A

no electrosatic forces of attraction between them in gas phase so they are less stable
whereas strong electrostatic forces of attraction exist between +vely and -vely charged ions in lattice

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5
Q

how can ^Hlatt be determined

A

born haber cycle- type of energy cycle and experimental values

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6
Q

2 factors which affect lattice energy are …

A

charge and radiusof the ions which make up the crystalline lattice

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7
Q

lattice energy becomes _ as the ionic radius _. explain your answer too

A

less exo as radius increases.

this is bcos charge is more spread out over the ion as its larger.

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8
Q

as the ionic radius increases what happens to the lattice

A

the centres of the ions are further apart and so the electrostatic force of attraction between opp charged ions is weaker

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9
Q

as ionic charge _ lattice energy becomes _

A

increases

more exo

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10
Q

explain the connection between ionic charge and lattice energy

A

the greater the charge, the higher the charge density. this results in stronger electrostatic force of attraction between ions in the lattice. hence LE gets more exo

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11
Q

what is electron affinity

A

ability/strength of the nucleus to attract an incoming electron

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12
Q

what is the first electron affinity

A

the enthalpy change when 1 mole of electrons is added to 1 mole of gaseous atoms to form 1 mole of single negatively charged gaseous ion. this is ALWAYS exo

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13
Q

why are the successive electron affinities endo?

A

incoming electron is being added to an already negative ion; therefore, energy is required to overcome the repulsive forces

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14
Q

to use a born-haber cycle to calculate ^Hlatt what all must be known before hand

A
  • ^Hf
  • enthalpy changes when elements go from their std state to their gaseous ions ie: atomisation, ionisation for metals, electron affinities for non-metals
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15
Q

state hess’ law

A

enthalpy change in a chemical reaction is the same regardless of the route taken, as long as initial and final conditions of reactants and products are the same in each route

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16
Q

which 3 factors affect electron affinity

A
  • nuclear charge: more the nuc charge the stronger attractive forces
  • distance from nucleus: greater distance between outermost shell and nucleus the weaker the attraction
  • shielding: more the no. of shells the weaker the force of attraction
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17
Q

state the degree of exo of electron affinity across a period and down a group

A
  • more exo across pd

- less exo down grp

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18
Q

less effective the attraction of electrons to the nucleus the less _

A

exo

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19
Q

define ^H sol

A

enthalpy change when 1 mole of an ionic substance dissolves in sufficient water to form a very dilute solution

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20
Q

if an ionic compounds has exo? or endo? value of ^H sol it is likely to be soluble

A

^H sol should be either exo or only slight endo will it dissolve

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21
Q

if a compound has a large positive value of ^H sol it will be _

A

insoluble

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22
Q

define ^H hyd

A

enthalpy change when 1 mole of a specified gaseous ion dissolves in sufficient water to form a very dilute solution

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23
Q

explain how ^H hyd compensates for the remaining lattice energy after ^H sol

A
  • the ^H sol is used to dissolve the compound
  • but more energy is needed to break the attractive forces between the ions
  • this is compensated by the ^H hyd: ions form ion-dipole attractions with water molecules and in doing so release energy which can then be used to breal the lattice
24
Q

how to find the total ^H hyd

A

^H hyd of both anions and cations

25
Q

factors affecting ^H hyd

A

ionic radius and charge

26
Q

explain how ionic radius affects the value of ^H hyd

A

as ionic radius decreases (ie down the group) the ions have greater charge density and this results in stronger ion-dipole attractions (smaller=more exo)
therefore more energy is released in forming these as ions become hydrated thus the value of ^H hyd becomes more exo

27
Q

explain how ionic charge affects the value of ^H hyd

A

as charge increases (across period) the ions have greater charge density and this results in stronger ion-dipole attractions
therefore more energy is released in forming these as ions become hydrated thus the value of ^H hyd becomes more exo

28
Q

define the ^H at

A

enthalpy change when 1 mole of gaseous atoms is formed from its element under std conditions

29
Q

state std conditions

A

298K 101kPa

30
Q

enthalpy change sign for ^H at is always _ because …

A

positive because it is endothermic as energy is needed to break any in the element into its gaseous atoms

31
Q

what makes the 1st electron affinity exo

A

attractive forces between the electron and the nucleus, electrostatic bond forms, energy is released

32
Q

describe the trend of electron affinities of non metals across the period

A

more exo (max at g7)

33
Q

describe the general trend of electron affinities of non metals down the group

A

less exo for successive elements going down except for the first ones ie O and F

34
Q

the more difficult it gets to add an electron due to the various factors _ energy is released upon adding it making electron affinity _

A

less

less exo

35
Q

why is flourine an exception and have a lower electron affinity thatn chlorine

A
  • it has a very small atomic radius
  • hence high electron density
  • hence more repulsion between the incoming and present electrons
  • thse repulsive forces reduce the attractive force between nucleus and incoming electron
  • hence electron affinity is less exo
36
Q

define entropy of a given system

A

number of possible arrangements of the particles and their energy in the given system
(how disordered it is)

37
Q

what increases entropy? and in turn, when entropy increases what increases?

A
  • disorder

- energetic stability

38
Q

order of entropy of the states of matter:

A

gas>liquid>solid

39
Q

why is a more disordered substance more energetically stable

A

bcos energy of the system is more spread out

40
Q

2 kinds of substances that have lower entropy than their corresponding opposites

A
  • simpler ones have lower S than complex

- harder ones have lower S tha softer ones of the same type

41
Q

when does entropy change

A

during change of state

42
Q

explain how entropy increases as sth melts

A
  • increase in temp causes more vibrating
  • the regularly arranged particles of the lattice become irregular
  • these are still close but can slide
  • therefore disorder
43
Q

explain how entropy increases as sth boils

A
  • particles are far apart and can easily move

- hence more disordered

44
Q

how does entropy decrease as sth condenses

A
  • particles come together in regular arrangement
  • ability to move decreases
  • fewer ways of arranging the energy
  • less disorder
45
Q

does entropy increase or decrease as a solid dissoves in a solvent

A

INcreases
because particles are ordered in lattice and slightly only vibrate but dissolved theyre more spread out and the energy can be arranged in a no of ways

46
Q

crystallization is related to _ in entropy. elaborate

A

DEcrease

-in solid form particles are less disordered

47
Q

more the gas particles more the entropy. why

A

more ways of arranging the gas molecules

more energetically stable

48
Q

formula to calculate entropy change of system

A

sum of ^Sproducts - sum of ^Sreactants

49
Q

what is Gibbs free energy/ significance

A

the energy change that takes into account both the entropy change and the enthalpy change to determind whether a reaction is feasible

50
Q

state the Gibbs equation

A

^G=^Hr-T^Ssys

51
Q

^H units are kJ/mol. ^Ssys units are in J/K/mol. therefore…

A

one must be converted by * or / by 1000

52
Q

how can the value of Gibbs free energy be used to determine whether a reaction is feasible or not

A

when ^G is negative, the rxn is feasiblle and likely to occur
when ^G is positive, the rxn is not feasible and unlikely to occur

53
Q

state a factor which affects feasibility

A

temp

54
Q

explain how temp affects feasibility of exothermic reactions

A
  • recall the gibbs equation; where ^H is -ve
  • if ^Ssys is positive then ^G will be negative hence always feasible
  • if ^Ssys is negative: if large ^G will be positive ie at high temp hence unfeasible. but if small it will be feasible ie at low temp
  • as per le chatliers too: in an exo reaction increase in temp favours the endo direction ie to reactants
  • meaning no product forms at high temp when ^S is negative
55
Q

explain how temp affects feasibility of endothermic reactions

A
  • recall the gibbs equation; where ^H is +ve
  • ^S is negative: never feasible because when added gives positive ^G
  • ^S is positive:more feasible at higher temp because second term is large enough to overcome the +ve ^H
  • also corresponds to le chatliers: in and endo rxn an increase in temp casuses equilibrium to shift to products
  • more product at high temp when ^S is positive

A2 Chem (4 decks)

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