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Chemistry Exams > Enthalpy > Flashcards

Enthalpy Flashcards

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1
Q

What is the definition of lattice enthalpy?

A

The enthalpy change when one mole of an ionic solid is formed from its gaseous ions in standard conditions

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2
Q

Give an example of a lattice enthalpy reaction

A

Na+ (g) + Cl- (g) -> NaCl (s)

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3
Q

What factors result in a greater magnitude of lattice enthalpy?

A
  • Higher charges on the ions
  • Greater number of cation-anion interactions
  • Smaller distance between the centres of the two ions (inter-ionic distance)
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4
Q

What must all lattice enthalpy values be, and why?

A

All values are negative, as it is a formation reaction meaning energy is released

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5
Q

What is the definition of standard enthalpy change of atomisation?

A

The enthalpy change when one mole of GASEOUS ATOMS is formed from an element in its standard state

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6
Q

Give two examples of an atomisation reaction

A

C (s) -> C (g)

1/2H2 (g) -> H (g)

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7
Q

What is important to remember in atomisation?

A

The product must be gaseous

It is one mole of atoms, not one mole of molecules (e.g. not one mole of H2 but one mole of H)

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8
Q

What is the first electron affinity of an element?

A

The enthalpy change when each atom in one mole of gaseous atoms gains an electron to form a 1- ion

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9
Q

Give an example of an electron affinity reaction

A

Cl (g) + e- -> Cl- (g)

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10
Q

Is first electron affinity exothermic or endothermic?

A

Exothermic (negative value)

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11
Q

What is the exception to first electron affinity being exothermic?

A

For noble gases, first electron affinity is endothermic

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12
Q

Is second electron affinity exothermic or endothermic?

A

Endothermic (positive value)

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13
Q

Why is second electron affinity endothermic?

A

As energy has to be put in to overcome the forces of repulsion from the already negative ion

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14
Q

What is the definition of standard enthalpy of neutralisation?

A

The enthalpy change when an acid and an alkali react to form 1 mole of water, under standard conditions

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15
Q

What is the definition of standard enthalpy of combustion?

A

The enthalpy change when 1 mole of a substance is completely burned in oxygen, under standard conditions

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16
Q

What is the definition of standard enthalpy change of formation?

A

The enthalpy change when one mole of a compound is formed from its elements in their standard states, under standard conditions

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17
Q

Give an example of standard enthalpy of formation

A

Na (s) + 1/2Cl2 (g) -> NaCl (s)

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18
Q

What assumptions are made when find theoretical lattice enthalpy?

A
  • The ions are in contact with one another
  • The ions are perfectly spherical
  • The charge on each ion is evenly distributed around
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19
Q

What causes differences between theoretical and experimental lattice energy?

A

Covalent character in the ionic bonding

20
Q

What causes covalent character?

A

The polarisation of the anion by the cation

21
Q

What is the effect of covalent character in ionic bonding on the value of lattice enthalpy?

A

Greater covalent character = more negative experimental value compared to the theoretical value

22
Q

Why does the cation polarise the anion?

A

The positive cation will attract electrons from the anion, resulting in a distortion of the electron density of the anion. Some electrons will be drawn closer to the cation (covalent character)

23
Q

What two cation factors increase the level of polarisation?

A

The cation will have a greater polarising effect when it is smaller, and has a higher charge (i.e. has a higher charge density

24
Q

What two anion factors increase the level of polarisation?

A

The anion will be more easily polarised when it is larger, and has a higher charge

25
Q

What is the definition of enthalpy change of solution?

A

The enthalpy change when one mole of an ionic solid dissolves in water to form an infinitely dilute solution

26
Q

Give an example of an enthalpy change of solution reaction

A

NaCl(s) -> Na+ (aq) + Cl- (aq)

27
Q

What is an infinitely dilute solution?

A

A solution in which further dilution has no measurable effect on enthalpy of solution

28
Q

What is enthalpy change of hydration?

A

The enthalpy change when one mole of an ion in gaseous state is completely hydrated

29
Q

Give an example of an enthalpy change of hydration reaction

A

Cl- (g) -> Cl- (aq)

30
Q

What factors affect the enthalpy of hydration?

A
  • The charge on the ions

- The size of the ion

31
Q

How and why does the charge on the ions affect the enthalpy of hydration?

A

The greater the charge on the ions, the stronger the force of attraction between the ion and water, the more negative the value for enthalpy of hydration

32
Q

How and why does the size of the ions affect the enthalpy of hydration?

A

The smaller the size, the stronger the force of attraction between the ion and water, the more negative the value for enthalpy of hydration

33
Q

What is the relationship between enthalpy of solution, lattice enthalpy and enthalpy of hydration?

A

Enthalpy of solution = Enthalpy of hydration - Lattice enthalpy

34
Q

What is the relationship between entropy and disorder?

A

Increasing entropy = increasing disorder

35
Q

What is a spontaneous process?

A

A process that takes place without continuous intervention from us

36
Q

What is the natural direction of change in terms of entropy?

A

Toward increasing entropy

37
Q

What is entropy?

A

A property of matter that is associated with the degree of disorder/randomness of the particles

38
Q

What is total entropy change equal to?

A

Total entropy change = Entropy of system + Entropy of surroundings

39
Q

For a reaction to be spontaneous, what must total entropy change be?

A

Total entropy change must be positive for a reaction to be spontaneous

40
Q

What is the equation to work out entropy of system?

A

Entropy of system = Entropy of products - Entropy of reactants

41
Q

What is the equation to work out entropy of surroundings?

A

Entropy of surroundings = -Enthalpy change/Temperature

42
Q

In exothermic reactions, entropy of surroundings will always be…

A

Positive, as energy is released to the surroundings causing increased disorder

43
Q

In endothermic reactions, entropy of surrounding will always be…

A

Negative, as energy is taken in from the surrounding causing decreased disorder

44
Q

How does an increase in temperature impact the entropy of a cold object?

A

Supply heat energy to a cold object will increase the movement of molecules and the entropy significantly

45
Q

How does an increase in temperature impact the entropy of an already hot object?

A

Supply heat energy to an already hot object will still increase the movement of molecules and the entropy to some extent, but not as much as with a cold object. This is because relatively the increased degree of movement is less for hot objects

46
Q

What does entropy change with changes in state?

A

Entropy increases in the following order:

Solid

47
Q

How does a change in the number of moles from reactants to products affect entropy of system?

A

More moles = more particles = more ways in which the particles can be arranged = increased entropy

Chemistry Exams (14 decks)

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