Chemical Energetics Flashcards

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
factors affecting ^H hyd
ionic radius and charge
26
explain how ionic radius affects the value of ^H hyd
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
explain how ionic charge affects the value of ^H hyd
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
define the ^H at
enthalpy change when 1 mole of gaseous atoms is formed from its element under std conditions
29
state std conditions
298K 101kPa
30
enthalpy change sign for ^H at is always _ because ...
positive because it is endothermic as energy is needed to break any in the element into its gaseous atoms
31
what makes the 1st electron affinity exo
attractive forces between the electron and the nucleus, electrostatic bond forms, energy is released
32
describe the trend of electron affinities of non metals across the period
more exo (max at g7)
33
describe the general trend of electron affinities of non metals down the group
less exo for successive elements going down except for the first ones ie O and F
34
the more difficult it gets to add an electron due to the various factors _ energy is released upon adding it making electron affinity _
less | less exo
35
why is flourine an exception and have a lower electron affinity thatn chlorine
- 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
define entropy of a given system
number of possible arrangements of the particles and their energy in the given system (how disordered it is)
37
what increases entropy? and in turn, when entropy increases what increases?
- disorder | - energetic stability
38
order of entropy of the states of matter:
gas>liquid>solid
39
why is a more disordered substance more energetically stable
bcos energy of the system is more spread out
40
2 kinds of substances that have lower entropy than their corresponding opposites
- simpler ones have lower S than complex | - harder ones have lower S tha softer ones of the same type
41
when does entropy change
during change of state
42
explain how entropy increases as sth melts
- 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
explain how entropy increases as sth boils
- particles are far apart and can easily move | - hence more disordered
44
how does entropy decrease as sth condenses
- particles come together in regular arrangement - ability to move decreases - fewer ways of arranging the energy - less disorder
45
does entropy increase or decrease as a solid dissoves in a solvent
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
crystallization is related to _ in entropy. elaborate
DEcrease | -in solid form particles are less disordered
47
more the gas particles more the entropy. why
more ways of arranging the gas molecules | more energetically stable
48
formula to calculate entropy change of system
sum of ^Sproducts - sum of ^Sreactants
49
what is Gibbs free energy/ significance
the energy change that takes into account both the entropy change and the enthalpy change to determind whether a reaction is feasible
50
state the Gibbs equation
^G=^Hr-T^Ssys
51
^H units are kJ/mol. ^Ssys units are in J/K/mol. therefore...
one must be converted by * or / by 1000
52
how can the value of Gibbs free energy be used to determine whether a reaction is feasible or not
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
state a factor which affects feasibility
temp
54
explain how temp affects feasibility of exothermic reactions
- 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
explain how temp affects feasibility of endothermic reactions
- 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