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A Level Chemistry > Topic 13: Energetics II > Flashcards

Topic 13: Energetics II Flashcards

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1
Q

What is lattice energy?

A
  • ionic compounds can form regular structures, giant ionic lattices where the positive and negative ions are held together by electrostatic attractions
  • when gaseous ions combine to make a solid lattice, energy is given out
  • this energy is lattice energy
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2
Q

What is the definition of standard lattice energy?

A
  • the standard lattice energy, deltaLEHø, is the energy change when 1 mole of an ionic solid is formed from its gaseous ions under standard conditions
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3
Q

What is standard lattice a measure of?

A
  • it measures ionic bond strength
  • the more negative the lattice energy, the stronger the bonding
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4
Q

How does ionic charge and size affect lattice energy?

A
  • the higher the charge on the ions, the more energy is released when an ionic lattice forms
  • this is due to the stronger electrostatic forces between ions
  • more energy released means that lattice energy will be more negative
  • compounds with 2+ or 2- ions are more exothermic
  • the smaller the ionic radiiof the ions involved, themore exothermic the lattice energy
  • smaller ions have a higher charge density and their smaller ionic radii means that the ions can sit closser tofether in the lattice, making attraction between the ions stronger
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5
Q

How do you draw a Born-Haber cycle?

A
  • General order:
  • Enthalpy of formation
  • Atomisation enthalpy
  • Ionisation energy
  • Electron affinity
  • Lattice energy
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6
Q

What are the two ways to work out lattice energy?

A
  • experimentally: using experimental enthalpy values in a Born-Haber cycle
  • theoretically: doing some calculations based on the purely ionic model of a lattice
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7
Q

Why does polarisation lead to covalent character in some ionic lattices?

A
  • e.g. magenisum halides have more covalent character in their ionic bonds than sodium halides
  • in a sodium halide, the cation Na+, has only a small charge so it can’t really pull electrons from the anion towards itself
  • so the charge is distributed evenly around the ions and there’s almost no polarisation
  • magnesium halides don’t fit the ionic moodel quite so well because charge isn’t evenly distributed around the ions
  • the cation Mg2+, has a bigger charge, so it can pull electrons from the anion towards itself, polarising the bond
  • in general, the greater the charge density of the cation, the poorer the match will be between experimental and theoretical values for LE
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8
Q

What is polarisation?

A
  • when the positive charge on the cation attracts elecrons towards it from the anion
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9
Q

Which cations are most polarising and which anions are more polarisable?

A
  • small cations with a high charge are very polarising because they have high charge density
  • so the catioin can pull electrons towards itself
  • large anions with a high charge are polarised more easily than smaller ones with a lower charge
  • this os because their electrons are further away from the nucleus and there is more repulsion between electrons, so electrons can be pulled away more easily
  • if a compound contains a cation with a high polarising ability and an anion that is easily polarised, some of the anion’s electron charge cloud will be dragged towards the positive cation
  • if the compound is polarised enough, a partially covalent bond is formed
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10
Q

What happens when a solid ionic lattice dissolves in water?

A
  • the bonds between the ions break - this is endothermic
  • the enthalpy change is the opposite of the lattice enthalpy
  • Bonds between the ions and the water are made - this is exothermic
  • the enthalpy change here is called the enthalpy change of hydration
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11
Q

What is the definition of enthalpy change of hydration

A
  • the enthalpy change of hydration deltahydH is the enthalpy change when 1 mole of gaseous ions dissolves in water
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12
Q

What is the definition of enthalpy change of solution?

A
  • the enthalpy change of solution, deltasolH, is the enthalpy change when 1 mole of solute dissolves in water
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13
Q

How to calculate enthalpy change of solution using a cycle?

A
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14
Q

How does ionic charge and ionic radius affect the enthalpy of hydration?

A

Ions with a greater charge have a greater enthalpy of formation:

  • ions with a higher charge are better at attracting water molecules than those with lower charges: the elecstrostatic attraction between the ion and the water molecules is stronger
  • this means more energy is released when the bonds are made, giving them a more exothermic enthalpy of hydration

Smaller ions have a greater enthalpy of hydration:

  • smaller ions have a higher charge density than bigger ions
  • they attract the water molecules better and have more exothermic enthalpy of hydration
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15
Q

What is entropy?

A
  • a measure of the disorder of a system
  • it tells you the number of ways that particles can be arranged and the number of ways that the energy can be shared out between the particles
  • the more disordered the particles are, the higher the entropy is
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16
Q

What affects entropy?

A

Physical state:

  • gas particles have the highest entropy because they have the most random arrangements of particles

Dissolving:

  • dissolving a solid also increases iits entropy
  • dissolved particles can move freely as theyre no longer held in one place

More particles means more entropy:

  • the more particles you have, the more ways they and their energy can be arranged
17
Q

Why do particles try to increase their entropy?

A
  • substances are more energetically stable when there’s more disorder
18
Q

How to calculate the entropy change of a system

A
  • deltaSsystem = Sproducts - Sreactants
  • a positive entropy change means reaction is likely to be feasiible
19
Q

What is the total entropy change and how do you calculate it?

A
  • total entropy change is the sum of the entropy of changes of the system and surroundings
  • delta Stotal = delta Ssystem + delta Ssurroundings
20
Q

How do you calculate entropy of the surroundings?

A
  • delta Ssurroundings = - deltaH (enthalpy change J mol-1) / T (temperature in K)
21
Q

Calculation example 1

A
22
Q

Calculation example 2

A
23
Q

Calculation example 3

A
24
Q

Definition of standard enthalpy change of formation

A
  • the standard enthalpy change of formation of a compound, deltafHØ is the enthalpy change when one mole of the compoud forms from its elements under standard conditions with the elements and the compound in their standard states
25
Q

Definition of the standard enthalpy change of atomisation

A
  • the standard enthalpy change of atomisation of an element is the energy change needed to produce one mole of gaseous atoms of the elemtent
26
Q

What is the first ionisation energy?

A
  • the first ionisation energy of an element is the energy needed to remove one electron from each atom in one mole of gaseous atoms of the element under standard conditions
27
Q

What is the first electron affinity?

A
  • the first electron affinity of an element is the energy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous ions with a single negative charge

A Level Chemistry (19 decks)

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