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Physical chemistry > Thermodynamics > Flashcards

Thermodynamics Flashcards

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1
Q

Define enthalpy change.

A

The heat energy change at constant pressure.

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2
Q

What are standard conditions?

A

298K, 100kPa

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3
Q

Define enthalpy of lattice dissociation.

A

The enthalpy change when one mole of an ionic compound is separated into its component gaseous ions.

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4
Q

Define enthalpy of lattice formation.

A

Enthalpy change when one mole of an ionic compound is formed from its constituent ions in the gaseous state.

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5
Q

Define enthalpy of formation.

A

Enthalpy change when one mole of a compound is formed from its elements when all reactants and products are in their standard states under standard conditions.

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6
Q

Define enthalpy of atomisation.

A

The enthalpy change for the formation of one mole of gaseous atoms from the element in its standard state.

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7
Q

Define bond dissociation enthalpy.

A

Enthalpy change when one mole of a covalent bond is broken under standard conditions in the gaseous state.

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8
Q

Define first ionisation enthalpy.

A

Enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to form one mole of gaseous ions with a single positive charge.

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9
Q

Define second ionisation enthalpy.

A

Enthalpy change when one mole of electrons is removed from one mole of gaseous ions with a single positive charge to form one mole of gaseous ions with a 2+ charge.

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10
Q

Define first electron affinity.

A

Enthalpy change when one mole of gaseous atoms forms one mole of negative ions with a single negative charge.

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11
Q

Define second electron affinity.

A

Enthalpy change when one mole of gaseous ions with a single negative charge forms one mole of gaseous ions with a double negative charge.

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12
Q

Define enthalpy of solution.

A

Enthalpy change when one mole of a solute dissolved in water.

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13
Q

Define enthalpy of hydration.

A

Enthalpy change when one mole of gaseous ions is converted into one mole of aqueous ions.

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14
Q

Why do smaller ions have more negative values of enthalpy of hydration?

A

They have higher charge densities so the negative charge on the ion attracts the &+ H of the water more strongly.

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15
Q

Why is the first electron affinity of oxygen exothermic?

A

There is attraction between the nucleus of the oxygen atom and the external electron.

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16
Q

Why is the second electron affinity of oxygen endothermic?

A

The electron and negative oxygen ion repel each other, so energy is required.

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17
Q

What is entropy?

A

A measure of disorder in a system.

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18
Q

What are the units for entropy?

A

J/K/mol

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19
Q

What is the entropy value if there is lots of disorder?

A

High.

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20
Q

What are standard entropy values?

A

Calculations for 1 mole of a substance based on a scale where the substance has an entropy of 0 at 0K.

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21
Q

Why do substances tend to disorder?

A

They become more energetically stable.

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22
Q

Why is entropy change bigger when changing to a gas from a liquid, rather than to a liquid from a solid?

A

Gases are a lot more disordered.

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23
Q

What is the formula for entropy change?

A

ΔS = S(products) - S(reactants)

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24
Q

Why do macromolecular substances have very low entropy values?

A

They have very regular macromolecular structures.

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25
Q

Why do metallic substances have low entropy values?

A

They have very ordered metallic lattices.

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26
Q

Why do ionic compounds have low entropy values?

A

They have ordered ionic lattices.

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27
Q

Why do molecular crystals have low entropy values?

A

They are regular ordered crystalline structures.

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28
Q

Generally, what happens to the entropy value as Mr of substance, in the same state, increases?

A

Increases.

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29
Q

What happens the entropy value as number of moles increases?

A

Increases.

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30
Q

What is a feasible reaction?

A

A reaction that can occur at a particular temperature.

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31
Q

Why might a feasible reaction not happen?

A

*the activation energy hasn’t been met
*the reaction is too slow to observe

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32
Q

What are units of Gibbs free energy?

A

KJ/mol

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33
Q

What is the equation for Gibbs free?

A

ΔG = ΔH - TΔS

34
Q

What are the units of enthalpy change?

A

KJ/mol

35
Q

What are the units of entropy change?

A

J/K/mol

36
Q

What must you do to the value of entropy change before using it in a calculation?

A

Divide by 1000 to get into KJ/K/mol.

37
Q

When calculating the temperature at which a reaction becomes feasible, what is ΔG assumed to be?

A

0

38
Q

What must the value of ΔG be for the reaction to be feasible?

A

Less than or equal to 0.

39
Q

What does it mean if ΔG = 0?

A

The system is in equilibrium.

40
Q

What is the feasibility of an exothermic reaction with positive entropy change?

A

-ΔG, so feasible.

41
Q

What is the feasibility of an endothermic reaction with positive entropy change?

A

Feasible when T is large (ΔG negative).

42
Q

What is the feasibility of an endothermic reaction with negative entropy change?

A

+ΔG, so never feasible.

43
Q

What is the feasibility of an exothermic reaction with negative entropy change?

A

Feasible when T is small (ΔG negative).

44
Q

What is temperature in Kelvin never less than?

A

0

45
Q

What is the value of ΔG when changing state?

A

0

46
Q

Is ΔG positive or negative for temperatures below melting point?

A

Positive.

47
Q

Why is ΔG positive for temperatures below melting point?

A

The reaction is not feasible (cannot react with a solid).

48
Q

Is ΔG positive or negative for temperatures above melting point and below boiling point.

A

Negative.

49
Q

If ΔG is plotted on a graph, what is the gradient?

A

-ΔS in KJ/K/mol

50
Q

If ΔG is plotted on a graph, what is the y intercept?

A

ΔH

51
Q

If ΔG is plotted on a graph, what is on the x axis?

A

Temperature.

52
Q

If ΔG is plotted on a graph, what is on the y axis?

A

ΔG

53
Q

On a positive ΔG graph, what is the x intercept?

A

Maximum temperature for a reaction to be feasible (ΔG=0)

54
Q

On a negative ΔG graph, what is the x intercept?

A

Minimum temperature for a reaction to be feasible (ΔG=0).

55
Q

How do you work out ΔS from the gradient?

A

Multiply the negative gradient by 1000.

56
Q

What are Born-Haber cycles used for?

A

To calculate lattice enthalpies as they can’t be calculated from experiments.

57
Q

What is the perfect ionic model?

A
  • ions are prefectly spherical
  • charge is evenly distibuted in spheres (point charges)
58
Q

How can theoretical lattice enthalpies be calculated?

A

From data assuming a perfectly ionic model.

59
Q

What does it mean if the experimental lattice enthalpy is higher than the theoretical?

A
  • compound doesn’t follow perfect ionic model
  • has some covalent characteristics
60
Q

How can an ionic compund have some covalent characteristics?

A
  • positive ion distors charge distribution in negative ion
  • positive ion polarieses negative ion
  • some electron pulled to positive ion
61
Q

What does more polarisation mean for covalent character?

A

More covalent character.

62
Q

What do lattice enthalpy values tell us?

A

How much a substance is purely ionic.

63
Q

Why is there a greater difference between theoretic and experimental lattice enthalpy values between +1 and +2 ion?

A
  • larger distortion by positive ion
  • more covalent character
64
Q

What must happen for a substance do dissolve?

A
  • substance’s bonds must break
  • new bonds must form between solvent + substance
  • new bonds must be same/greater strength than bonds broken
65
Q

Is bonds breaking endothermic or exothermic?

A

Endothermic

66
Q

Is bonds forming endothermic or exothermic?

A

Exothermic

67
Q

What happens when an ionic compound dissolves in a polar solvent?

A
  • delta + H atoms attracted to negative ions
  • delta - O atoms attracted to positive ions
68
Q

How can enthalpy of solution be calculated?

A

Using:
* lattice dissocation enthalpy
* enthalpy of hydration

69
Q

What is the symbol for enthalpy of hydration?

A

ΔhydH

70
Q

What is the symbol for enthalpy of solution?

A

ΔsolH

71
Q

What is the symbol for enthalpy change?

A

ΔH

72
Q

What is the symbol for enthalpy of formation?

A

ΔfH

73
Q

What is the formula for lattice enthalpy of formation?

A

ΔlattH

74
Q

What is the formula for lattice enthalpy of dissocation?

A

ΔlattH

75
Q

What is the formula for bond dissocation enthalpy?

A

ΔdisH
or
ΔBDEH

76
Q

What is the symbol for first ionisation enthalpy?

A

ΔIE1H

77
Q

What is the symbol for second ionisation enthalpy?

A

ΔIE2H

78
Q

What is the symbol for enthalpy of atomisation?

A

ΔatH

79
Q

What is the symbol for first electron affinity?

A

ΔEA1H

80
Q

What is the symbol for second electron affinity?

A

ΔEA2H

Physical chemistry (10 decks)

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