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A-level Chemistry Yr 13 Mine > 22 - Enthalpy and Entropy > Flashcards

22 - Enthalpy and Entropy Flashcards

1
Q

When a solid ionic lattice dissolves in water what two things happen?

A
  1. Bonds between the ions break to give gaseous ions - this is endothermic. Opposite of lattice enthalpy.
  2. Bonds between the gaseous ions and the water are made - this is Exothermic, called enthalpy change of hydration.
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2
Q

Draw out an ionic lattice being dissolved into water.

A

The partially negative oxygen atoms are attracted to the postive ion and the partially negative hydrogen ion is attached to the negative ion.

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3
Q

3.73g of KCl is dissolved in 25.0 g of water. The temperature decreases by 6.5 °C. Calculate the enthalpy change of solution.

A

.

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4
Q

What type of reaction can enthalpy of solution be?

A

It can be Exothermic or endothermic.

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5
Q

What is the definition of Lattice Enthalpy?

A

The enthalpy change when one mole of an ionic lattice is formed from its gaseous ions under standard conditions. (298 K and 100 kPa)

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6
Q

What is lattice enthalpy a measure of?

A

Is a measure of strength of ionic bonding in a giant ionic lattice.

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7
Q

How is lattice enthalpy given off?

A

Ionic compounds (K+ and Cl-) can form regular structures called giant ionic lattices. The positive and negative ions are held in place by electrostatic attraction. When gaseous ions combine to make a solid lattice energy is given out called lattice enthalpy.

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8
Q

Give an example of an equation for lattice enthalpy.

Give a suitable guess for the standard lattice enthalpy.

A

Lattice Enthalpy = must be negative (Exothermic)

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9
Q

What is the lattice enthalpy value always? And why?

A

Negative.

Because it’s an Exothermic change, as energy is given out when the ions combine to form the lattice.

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10
Q

What is the rule, if the lattice enthalpy is more negative then…

A

The stronger the bonding.

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11
Q

What also effects lattice enthalpy?

A
  • The higher the charge on the ions.
  • The smaller the ionic radii.
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12
Q

How does a higher ionic charge on the ions affect Lattice Enthalpy?

A

The higher the charge on the ions, the more energy is released in the forming in the ionic lattice. Due to stronger electrostatic forces between the ions.

The more energy released means the lattice enthalpy will be more negative, more Exothermic.

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13
Q

Why is NaCl lattice enthalpy -787 kjmol-1 but the lattice enthalpy of MgCl2 is -2526 kjmol-1?

A

Mg and S both make 2+ or 2- ions realises more energy as they are more ionic than Na and Cl which create 1+ and 1- ions. In the giant ionic lattice that they form.

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14
Q

How does the size of the ionic radii (size of ions) affect Lattice Enthalpy?

A

The smaller the ionic radii the more Exothermic the reaction, so the more negative the lattice enthalpy.

This is because smaller ions have a higher charge density and their smaller ionic radii means that the ions sit closer together in the lattice, so the attraction of the ions are stronger.

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15
Q

Draw an example of a lattice enthalpy equation.

A
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16
Q

What does Hess’s law state?

A

The total enthalpy change of a reaction is always the same, no matter which route is tacken, known as the conservation of energy.

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17
Q

How do you find lattice enthalpy? And why

A

Through a Born-Haber cycle.

You can not calculate lattice enthalpy directly, so you must use an energy cycle to work it out. As for ing samples of gaseous ions and reacting them together is very difficult.

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18
Q

Draw a general Born-Haber cycle.

A
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19
Q

Explain this general born-harbour cycle.

A

Route 1:
Formation of gaseous atoms - standard states to gaseous atoms, change is endothermic as bond-breaking.
Formation of gaseous ions - gaseous atoms into positive and negative ions, change is endothermic.
Lattice formation - gaseous ions into the solid ionic lattice, lattice enthalpy always endothermic.

Route 2:
The enthalpy chang of formation as it coverts the elements in their standard states directly to the ionic lattice, this Exothermic.

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20
Q

What is the standard enthalpy change of atomisation?

A

Is the 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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21
Q

Draw out an example of an equation for standard enthalpy change of atomisation?

Use a suitable value.

A

Value must be endothermic

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22
Q

What type of reaction must the enthalpy change of atomisation must always be and why?

A

Endothermic as bonds are broken to form gaseous ions.

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23
Q

What is the standard enthalpy change of formation?

A

The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions.

24
Q

Give an example of standard enthalpy chance of formation.

A

Enthalpy change can be Exothermic and endothermic.

25
Q

What is first ionisation energy?

A

The enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one moles of gaseous 1+ ions.

26
Q

What type of reaction is ionisation energies?

A

Endothermic.

Energy required to overcome the attraction between the negative electron and positive nuclei.

27
Q

What is electron affinity?

A

The opposite to ionisation energy, measures the energy to gain electrons.

28
Q

What is the definition for first electron affinity?

A

The 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.

29
Q

Give an example of first electron affinity equation?

Giving a suitable value.

A

Value must be Exothermic

30
Q

What type of reaction of first electron affinities must be and why?

A

Exothermic

The electron being added is attracted towards the nucleus.

31
Q

Give an example of second (successive) electron affinity equation?

Giving a suitable value

A

Value must be endothermic

32
Q

What type of reaction are second electron affinities must be and why?

A

Endothermic. A second electrons is being gained by a negative ion, which repels the electron away. So energy must be put in to force the negative-charged electron onto the negative ion.

33
Q

.

A

A

34
Q

How does the ionic charge effect the enthalpy of hydration?

A

As ionic charge increases.
Attraction with water molecules increases.
This means more energy is energy is released when the bonds are made giving them a more Exothermic, more negative enthalpy of hydration.

35
Q

How does the ionic size/radius effect the enthalpy of hydration?

A
  • Ionic radius increases
  • Attraction between ion and water molecules decreases.
  • Therefore hydration energy less negative, less Exothermic.
36
Q

What must you be carful for when drawing Born-Haber cycle for group 2 Elements? Use MgCl as an example.

A
  • Group 2 elements form 2+ ions so must include the second ionisation energy.
  • There are 2 moles of chlorine ions in each moles of MgCl2 - so must double the atomisation energy of chlorine and double the first electron affinity of chlorine.
37
Q

Draw a Born-Haber cycle for Magnesium Chloride (MgCl2).

A
38
Q

How can we predict solubility with enthalpies?

A

If the sum of the hydration enthalpies is larger than the magnitude of the lattice enthalpy, the overall enthalpy change (enthalpy change of solution) will be Exothermic and the compound should dissolve

39
Q

What is the enthalpy change of Solution?

A

Is the enthalpy change that takes place when 1 mole of solute dissolves in a solvent. (Usually water)

40
Q

What is the enthalpy change of hydration?

A

The enthalpy change when one mole of gaseous ions dissolves in water

41
Q

What decides whether the enthalpy change of solution is Exothermic or endothermic?

A

The relative sizes of the lattice enthalpy and the enthalpy changes of hydration.

42
Q

Draw a general Born-Haber cycle for lattice enthalpy, enthalpy of hydrations and enthalpy of solution.

A

Dissolving = enthalpy of solution remember can be Exothermic and endothermic

43
Q
A
44
Q
A
45
Q

What does entropy tells us?

A

How much disorder there is.

More disorder and the greater the dispersal, leads to a higher the entropy .

46
Q

What are the units of entropy?

A

J K-1 Mol-1

47
Q

What do the units of J K-1 Mol-1 entropy units mean?

A

The greater the energy value, the greater the energy is spread out per kelvin per mole.

48
Q

What is entropy related to with energy?

A

Entropy is related to the natural tendency for energy to spread out rather than be concentrated in one place.

49
Q

What is the standard entropy of a substance?

A

Is the entropy of one mole of a substance, under standard conditions (100 kPa and 298K).

50
Q

What value are standard entropy always?

A

Alway POSITIVE values.

51
Q

Why are standard entropy’s of substances always positive?

A

At 0 k, there would be no energy and all substances would have an entropy of zero.
Above 0k (can only go above not below), energy becomes dispersed among the particles and all substances have positive entropy.

52
Q

What does entropy measure?

A

Measure of the number of ways that particles can be arranged and the number of ways that the energy can be shared out between the particles.

53
Q

What affects the entropy?

A
  1. Physics State
  2. More Particles means more entropy
54
Q

How does physical state affect entropy?

A

Solid Particles just wobble in a fixed position - hardly any randomness, so have lowest entropy.
The. Gasses have greater entropies than solids but less than liquids.
Gas have greatest entropies, free to move. So have got the most random arrangements of particles, so have the highest entropies.

55
Q

How does more particles mean more entropy?

A

A

A-level Chemistry Yr 13 Mine (12 decks)

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