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A-Level Chemistry > Entropy > Flashcards

Entropy Flashcards

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1
Q

What is electron affinity?

A

The enthalpy change when 1 mole of electrons is added to 1 mole of atoms in the gaseous phase to form 1 mole of 1- ions

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2
Q

What is enthalpy of atomisation?

A

The enthalpy change that takes place when 1 mole of gaseous atoms form from the element in its standard state

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3
Q

What is lattice enthalpy?

A

The enthalpy change when 1 mole of an ionic substance is formed from its gaseous ions under STD conditions

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4
Q

What is the strength of an ionic bond related to?

A

Lattice enthalpy

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5
Q

If a compound has greater ionic bonding, will it be more EXO or ENDO?

A

More exo

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6
Q

Why can’t lattice enthalpy be measured directly?

A

As cannot form 1 mole of ionic lattice from gaseous ions

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7
Q

What factors affect lattice enthalpy?

A

Charge

Size

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8
Q

Why does charge affect lattice enthalpy?

A

Bigger ion = stronger attraction = more exo

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9
Q

Why does size affect lattice enthalpy?

A

Smaller = ions more tightly packed together = greater attraction = more exo

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10
Q

What happens to decomposition temperatures down a group?

A

Increase

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11
Q

Why do decomposition temperatures increase down a group?

A

Mg2+ = smaller than Ba2+
So charge occupies smaller vol
= Mg2+ has higher “charge density”
This distorts electron cloud within negative ion
Weakening ionic bonding = reduces decomp temp

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12
Q

What can Born-Haber cycles be used to calculate?

A

Ionic bond strength based on experimental data

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13
Q

What equation do you use to calculate a missing enthalpy change on a Born-Haber cycle?

A

Lattice enthalpy = formation - sum of rest

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14
Q

What is the cycle of a Born-Haber cycle?

A 
Formation
Atomisation of metal
Atomisation of non-metal
1st IE of metal
1st electron affinity of non-metal 
Lattice enthalpy of compound
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15
Q

What should you do to the enthalpy change when there is two moles of an element?

A

Multiply it by 2

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16
Q

Which ions have a more negative lattice enthalpy?

A

Smaller sized ions as they can get closer together = stronger attraction

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17
Q

What is enthalpy of solution?

A

The enthalpy change when 1 mole of ionic compound is completely dissolved in water under STD conditions

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18
Q

What happens when you dissolve an ionic compound?

A

Break up ionic lattice into gaseous ions

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19
Q

What is enthalpy of solution the reverse of?

A

Lattice enthalpy

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20
Q

What is enthalpy of hydration?

A

The enthalpy change when 1 mole of gaseous ions are dissolved in water forming 1 mole of aqueous ions under STD conditions

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21
Q

What should all hydration values be?

A

Exo

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22
Q

Why is enthalpy of hydration exo?

A

Because it is bond making

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23
Q

What equation must you use to calculate enthalpy of solution from a cycle?

A

L.E + enthalpy of solution = sum of enthalpies of hydration

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24
Q

What factors affect enthalpy of hydration?

A

Charge

Size

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25
Q

Why does charge affect enthalpy of hydration?

A

Higher charge on ion = greater attraction for H2O molecule = more exo

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26
Q

Why does size affect enthalpy of hydration?

A

Smaller sized ions have greater charge density compared to large ions = greater attraction for H2O = more exo

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27
Q

Why do exothermic chemical reactions happen?

A

A chemical reaction will proceed if products are energetically more stable than reactants

28
Q

What is entropy?

A

The measure of the dispersal of energy in a system

29
Q

What is more likely exist disordered or ordered states?

A

Disordered

30
Q

How is entropy related to spontaneous direction of change?

A

Spontaneous direction of change is from a less probable to more probable
Meaning entropy always INCEASES

31
Q

What is the symbol for entropy?

A

S

ΔS

32
Q

What number will entropy produce?

A

Always a positive number

33
Q

What are the units for entropy?

A

J K-1 mol-1

K = kelvin

34
Q

Out of a solid, liquid and gas which has the lowest entropy?

A

Solid = more order but still some entropy as particles vibrate in fixed position

35
Q

Out of a solid, liquid and gas which has the highest entropy?

A

Gas = least order

36
Q

What happens to entropy as temperature increases?

A

Entropy increases

37
Q

What happens to ΔS if a change makes the system more random?

A

+ΔS

38
Q

What happens to ΔS if a change makes the system less random (more order)?

A

-ΔS

39
Q

For chemical reactions we can predict entropy change to be +/-, by using what?

A

State symbols

40
Q

CaCO3 (s) —-> CaO (s) + CO2 (g)

A

+ΔS

41
Q

N2O4 (g) —-> 2NO2 (g)

A

+ΔS

As more moles on right side = more chance for disorder

42
Q

2CH3OH (l) + 3O2 (g) —-> 2CO2 (g) + 4H2O (l)

A

-ΔS

As less moles of gas on right side

43
Q

What is the definition for calculating entropy?

A

The entropy change that accompanies a reaction in molar quantities expressed in the equation, under STD conditions

44
Q

What is the equation for calculating entropy?

A

ΔS = sum of S products - sum of S reactants

45
Q

What are the steps for calculating entropy?

A

Decide whether +/-ΔS from symbol equation

Put values given in question in equation

46
Q

What type of reaction must it be for it to be spontaneous?

A

Exo

47
Q

Why must a reaction be exo to be spontaneous?

A

As the heat given out by the recations it increases the disorder (entropy) of the surroundings so in the universe disorder increases

48
Q

What is the equation of entropy of surroundings?

A

ΔS surroundings = - ΔH / Temp

49
Q

In the entropy of surroundings equation what must the ΔS be?

A

Positive

50
Q

Why must the ΔS be positive in the equation for entropy of surroundings?

A

For spontaneous change

51
Q

What is the Gibbs Free Energy equation?

A

ΔG = ΔH -TΔS <0

52
Q

What is the Gibbs Free Energy equation dictated by?

A

Temperature

53
Q

What mus you always do if you get an equation on the Gibbs Free Energy?

A

Write out the equation

54
Q

What must ΔG be for a reaction to proceed?

A

Negative

55
Q

If ΔG is positive what will happen?

A

Th reaction won’t be feasible

56
Q

What does ΔG= at the point of feasibility?

A

0

57
Q

What is the assumption made for the point of feasibility?

A

ΔH and ΔS do not vary with temperature

58
Q

What does temperature adjust in the GIbbs Free Energy equation?

A

The significance of ΔS system

59
Q

At low temperature what happens in Gibbs Free Energy equation?

A

ΔG= ΔH (-TΔS = negible)

So only reations with -ΔH can proceed so ΔG<0

60
Q

At high temperatures what happens in Gibbs Free Energy equation?

A

ΔG = -TΔS (ΔH = negible)

So any reactions with +ΔH can proceed as -TΔS<0

61
Q

What are the limitations of ΔG?

A

Even if ΔG is negative the reaction may not be feasible
Because AE might be too high
Or rate of reaction is very slow

62
Q

How do you calculate ΔG (feasibility)?

A

Calculate ΔS
Calculate ΔH
Divide ΔS by 1000 (for kJ)
Then put into ΔG= ΔH -TΔS

63
Q

What units must the temperature be in for Gibbs Free Energy equation?

A

Kelvin

64
Q

If you calculate the value of ΔG, and it is positive what does this mean?

A

The reaction is not feasible as it must be <0

65
Q

How would you then use the Gibbs Free Energy equation to calculate the temperature?

A

Assume feasibility = ΔG = 0
So ΔH -TΔS = 0
Rearrange to give ΔH/ ΔS = T
The convert answer to degrees C

66
Q

How do you calculate ΔS?

A

Products - reactants

67
Q

How do you calculate Δf?

A

Products - reactants

A-Level Chemistry (26 decks)

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