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Chemistry - Module 5 > 22 - enthalpy and entropy > Flashcards

22 - enthalpy and entropy Flashcards

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1
Q

define lattice enthalpy

A

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

is lattice enthalpy positive or negative and why

A

negative

lattice enthalpy involves bond formation - exothermic

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3
Q

describe each step in route 1 + 2 - is it exothermic or endothermic

A

Route 1
* Forming gaseous atoms – endothermic as bond breaking
* Forming gaseous ions – endothermic
* Making solid ionic lattice - Lattice enthalpy + exothermic

Route 2
* Enthalpy change of formation – exothermic

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4
Q

define Standard enthalpy change of formation

A
  • Enthalpy change that takes place when one mole of a compound is formed from its element under standard conditions, with all reactants and products in standard states
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5
Q

is enthalpy change of formation exothermic or endothermic

A

exothermic - bond making

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6
Q

define Standard enthalpy change of atomisation

A
  • Enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions
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7
Q

is enthalpy change of atomisation exothermic or endothermic

A

Always endothermic – bonds are broken to form gaseous atoms

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8
Q

what do you need to be careful of when using enthalpy change of atomisation

A
  • When in standard state the bond enthalpy refers to bond being broken
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9
Q

define first ionisation energy

A
  • Enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions
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10
Q

is first ionisation energy exothermic or endothermic

A

Endothermic – energy required to overcome attraction between electron and nucleus

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11
Q

define first electron affinity

A
  • Enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions
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12
Q

is first electron affinity endothermic or exothermic

A
  • Exothermic – electron being added is attracted towards nucleus
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13
Q

is second electron affinity exothermic or endothermic

A

ENDOTHERMIC = electron being gained by negative ion which repels

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14
Q

construct this cycle

A
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15
Q

what do you need to be careful of when constructing a cycle with 2+ charges

A
  • Need first and second ionisation energies of 2+ ion
  • Two atoms of 1+ charge atom throughout – needs to be doubled
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16
Q

construct this cycle and calculate lattice enthalpy of magnesium chloride

A
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17
Q

define enthalpy change of solution

A
  • Enthalpy change that takes place when one mole of a solute dissolves in a solvent
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18
Q

enthalpy change of solution - what happens if solvent is water

A

ions from lattice end up surrounded by water molecules as aq ions because partial charges of water attracted towards ions

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19
Q

is enthalpy change of solution endothermic or exothermic

A

can be both

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20
Q

describe the experimental method of finding the enthalpy change of solution for KCl

A

Mass in equation – mass of substance changing temp

Number of moles of solute – definition says 1 mole so need to divide energy change by the number of moles

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21
Q

how does dissolving work

A
  • Ionic lattice breaks up forming separate gaseous ions

Opposite energy change to lattice energy

  • Water molecules are attracted to and surround the ions

Separate gaseous ions interact with polar water moles to form hydrated aq ions = enthalpy change of hydration

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22
Q

define enthalpy change of hydration

A
  • Enthalpy change that accompanies the dissolving of gaseous ions in water to form 1 mole of aqueous ions
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23
Q

draw a born harber cycle connecting lattice enthalpy, enthalpy change of hydration and enthalpy change of solution

A
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24
Q

calculate lattice enthalpy of sodium chloride

A
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25
general properties of ionic compounds
* High melting + boiling point * Soluble in polar solvents * Conduct electricity when molten or in aq solution
26
Factors affecting lattice enthalpy
ionic size ionic charge
27
how does ionic size affect lattice enthalpy
* Increase radius * Charge is more spread out over ion * Ions are further apart = bigger distance between centre of ions * Electrostatic forces of attraction between ions in lattice are weaker * Weaker forces = less exothermic = less negative lattice enthalpy less energy released upon its formation from gaseous ions
28
how does ionic charge affect lattice enthalpy
* Increase ionic charge * Greater charge density * Stronger electrostatic forces of attraction * More negative = more exothermic more energy released upon its formation from gaseous ions
29
factors affecting enthalpy change of hydration
ionic size ionic charge
30
how does ionic size affect enthalpy change of hydration
* Radius increases * Charge density decreases * Weaker attraction * Less energy released when they become hydrated = less exothermic
31
how does ionic charge affect enthalpy change of hydration
* Increase charge * Greater charge density * Stronger attractions between water molecules and ions in solution * More energy released when they become hydrated – more exothermic
32
correlation between lattice enthalpy and melting point
* More exothermic // more negative lattice enthalpy = higher melting point
33
what must be overcome to dissolve ionic compound in water
attraction between ions in ionic lattice must be overcome
34
how much energy is needed to overcome attraction between ions in an ionic lattice
* Requires an amount of energy equal to the lattice enthalpy
35
how to use enthalpy changes to predict solubility
* If sum of hydration enthalpies greater than magnitude of lattice enthalpy – overall enthalpy change will be exothermic and compound SHOULD dissolve
36
what is entropy
* Measure of how disordered a system is
37
units of entropy
JK-1mol-1
38
the greater the entropy value...
greater that energy is spread out per Kelvin per mole
39
what state has the smallest and largest entropy values in general
Solids – smallest entropies gases - greatest
40
symbol for entropy
41
what does 0K mean for entropy
no energy and all substances have entropy value of zero
42
what happens if a system becomes more or less random
* If system becomes more random – energy spread out more – positive entropy change * If system becomes less random – energy more concentrated – negative entropy change
43
how does going from solid - liquid - gas affect entropy change
* Entropy increases as goes from solid – liquid – gas * Melting / boiling increase randomness of particles * Energy spread out more + positive entropy change
44
what type of reaction can cause an increase in entropy
those that increase the number of gas molecules
45
if number of gas molecules increase or decrease...
* If number of gas molecules decrease – negative entropy change * If number of gas molecules increase – positive entropy change
46
predict entropy change of this reaction
47
what is standard entropy
* Entropy of one mole of a substance under standard conditions
48
is standard entropy always positive or negative
always positive
49
how to calculate entropy changes
- must take into account number of moles - multipy entropy change for a species by number of moles in equation
50
when does a reaction occur
if products have a lower overall energy than reactants
51
what is free energy
overall change in energy in a chemical reaction
52
symbol for gibbs free energy
53
what makes up gibbs free energy
- Enthalpy change – the heat transfer between chemical system and surroundings - Entropy change at the temp of the reaction T^S – dispersal of energy within the chemical system itself
54
what is the gibbs equation
55
whats the condition for feasibility
if the products have a lower free energy than the reactants change in G must be negative
56
determine feasibility
57
58
endothermic feasibility
59
limitations on these feasibility predictions
* Does not take into account kinetics / rate of reaction – may occur at an unobservable rate * might be a large energy barrier (Ea) which the reacting species have to overcome before a reaction can occur * some reactions are feasible since ∆G is negative, but kinetically not feasible since it just occurs too slowly * Only predict feasibility under standard conditions
60
whats the difference between standard enthalpy change of formation + lattice enthalpy
lattice enthalpy must start from gaseous ions to form an ionic compound formation must start from elements in standard states to form compound
61
explain whether water or ice would have the greater entropy
Water will have a greater entropy particles of water are able to move more freely compared to ice
62
units of entropy
J mol-1 K-1
63
if the question has not given you enthalpy values - how can you tell if a reaction is feasible
at room temp - T will be positive S value - look for number of gaseous moles on each side H - is it exothermic (negative ) or endothermic (positive) combustion - exo neutralisation - endo
64
how to calculate enthalpy change
products minus reactants
65
assumptions made when calculating feasibility

Chemistry - Module 5 (7 decks)

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