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OCR A A-Level Inorganic Chemistry > Chapter 22: Enthalpy > Flashcards

Chapter 22: Enthalpy Flashcards

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1
Q

What is lattice enthalpy, and its symbol?

A
  • The enthalpy change when 1 mole of an ionic lattice is formed from its gaseous ions
  • ∆LEH
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2
Q

Write the equation for lattice enthalpy of potassium chloride, and is this enthalpy change endothermic or exothermic, and why?

A
  • K+ (g) + Cl- (g) -> KCl (s)
  • Exothermic as bonds are being formed
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3
Q

What do lattice enthalpy values show?

A
  • The more exothermic the lattice enthalpy, the stronger the ionic bonding in a giant ionic lattice
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4
Q

How is lattice enthalpy found?

A
  • It cannot be measured directly (heating would give gaseous ion pairs), so it is calculated indirectly using a type of Hess cycles called Born-Haber cycles, which break lattice enthalpy up into different steps
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5
Q

Generally describe the steps lattice enthalpy can be broken into.

A
  1. Splitting the ionic compound into its elements in their standard states
  2. Turning the elements in their standard states to gaseous atoms
  3. Turning the metal atom into a cation
  4. Turning the non-metal atom into an anion
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6
Q

What is the first step known as, and what is its definition and symbol?

A
  • The standard enthalpy of formation
  • The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states
  • ΔfH⦵
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7
Q

Write the equation for the enthalpy of formation of potassium chloride, and is it endothermic or exothermic, and why?

A
  • K (s) + 1/2 Cl2 (g) -> KCl (s)
  • It is exothermic for salts
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8
Q

What is the second step known as, and what is its definition and symbol?

A
  • The standard enthalpy of atomisation
  • The enthalpy change that takes place when 1 mole of gaseous atoms is formed from the element in its standard state
  • ΔatH⦵
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9
Q

Write the equations for the enthalpy of atomisation of potassium chloride, and are values for this enthalpy change endothermic or exothermic, and why?

A
  • K (s) -> K (g)
  • 1/2 Cl2 (g) -> Cl (g)
  • Endothermic because bonds are broken to form gaseous atoms
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10
Q

What is the third step known as, and what is its definition and symbol?

A
  • Ionisation energy
  • First ionisation energy: the energy required for the removal of 1 mole of electrons from 1 mole of gaseous atoms of an element
  • ΔIE1H⦵
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11
Q

Write the equation for the ionisation energy in potassium chloride, and are values for this enthalpy change endothermic or exothermic, and why?

A
  • K (g) -> K+ (g) + e-
  • Endothermic because energy is required to overcome the attraction between the positive nucleus and the negative electron
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12
Q

What is the fourth step known as, and what is its definition and symbol?

A
  • Electron affinity
  • First electron affinity: the enthalpy change when 1 electron is added to each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1- ions
  • ΔEA1H⦵
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13
Q

Write the equation for the electron affinity in potassium chloride.

A
  • Cl (g) + e- -> Cl- (g)
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14
Q

Are electron affinities endothermic or exothermic, and why?

A
  • The first electron affinity is exothermic as the first electron is attracted by the nucleus more strongly than it is repelled by the electrons
  • After this, the negative ion repels electrons, so the second and successive electron affinities are endothermic
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15
Q

How are these other enthalpy changes used to calculate lattice enthalpy? Give 8 details.

A
  • You draw a Born-Haber cycle with:
  • the cations, electrons and non-metal atoms at the top
  • the ionic solid at the bottom
  • on the right is the lattice enthalpy
  • on the left are all the other enthalpies with the exception of electron affinities
  • arrows point down for exothermic energy changes
  • arrows point up for endothermic energy changes
  • because of this, electron affinities have to go on the right above lattice enthalpy
  • Lattice enthalpy = the total of all of the other enthalpy changes (follow the arrows)
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16
Q

What are 4 things to be careful of when using Born-Haber cycles?

A
  • If you have multiples of the atoms/ ions, do not forget to multiply the enthalpy changes
  • Remember to show all of the ionisation energies where there are more than 1
  • If given values of bond dissociation enthalpy instead of enthalpy change of atomisation, remember to adapt for 1 mole of product
  • Second electron affinities
17
Q

What limitation is there on the use of average bond enthalpy to find the enthalpy change of atomisation?

A
  • It only works if the standard state of the element is gaseous and the element has covalent bonds
  • This is what the definition of average bond enthalpy specifies
18
Q

How are Born-Haber cycles drawn when there are 2 electron affinities?

A
  • The ions are the highest level on the right
  • The first electron affinity is shown below this
  • Between the 2 levels on the left is the cation, electrons and non-metal atom
  • This way, there is an arrow down for the first electron affinity (exothermic) and an arrow up for the second (endothermic)
19
Q

What is another way of calculating lattice enthalpy?

A
  • Using the enthalpy changes of solution and hydration
20
Q

What is the definition of the standard enthalpy change of solution and its symbol?

A
  • The enthalpy change that takes place when 1 mole of a solute dissolves in a solvent
  • ΔsolH⦵
21
Q

What is the definition of the standard enthalpy change of hydration and its symbol?

A
  • The enthalpy change that takes place when 1 mole of gaseous ions are dissolved in water
  • ΔhydH⦵
22
Q

What happens when ionic compounds dissolve in water?

A
  • The lattice breaks up
  • The ions are separated and react with polar water molecules to form hydrated ions
  • These hydrated ions are held together by ion-dipole forces
23
Q

Write the equation for the enthalpy of solution of sodium chloride, and is it endothermic or exothermic, and why?

A
  • NaCl (s) -> Na+ (aq) + Cl- (aq)
  • It can be either endothermic or exothermic as the lattice breaks up apart (endothermic) and ion-dipole forces form (exothermic), and the sum of these gives the enthalpy of solution
24
Q

Write the equations for the enthalpy of hydration of sodium chloride, and is it endothermic or exothermic, and why?

A
  • Na+ (g) -> Na+ (aq)
  • Cl- (g) -> Cl- (aq)
  • It is exothermic since ion-dipole forces are being formed
25
Q

How are the enthalpies of solution and hydration used to find lattice enthalpy?

A
  • In a Hess cycle
  • Using an energy level diagram
26
Q

How would you use a Hess cycle to find lattice enthalpy if given the values for the enthalpies of hydration and solution?

A
  • Lattice enthalpy would go at the top
  • The aqueous ions would go at the bottom at the bottom
  • As a result, the enthalpy of hydration would be on the left, and the enthalpy of solution on the right
  • Both equations of hydration would be combined into one step
27
Q

What decides which way energy level diagrams to find lattice enthalpy from the enthalpies of solution and hydration should be drawn?

A
  • Whether the enthalpy of solution is endothermic or exothermic
28
Q

What do all energy level diagrams have in common?

A
  • The gaseous ions go at the top
  • Lattice enthalpy goes on the left and is exothermic (arrow goes down)
  • The enthalpies of hydration go on the right (order doesn’t matter) and are also exothermic (arrows go down)
29
Q

How are energy level diagrams drawn when the enthalpy of solution is endothermic?

A
  • The ionic solid is at the bottom
  • The enthalpy of solution goes on the right and is the final step
  • It is endothermic, and therefore points up
30
Q

How are energy level diagrams drawn when the enthalpy of solution is exothermic?

A
  • The ionic solid is on the left
  • The aqueous ions are at the bottom
  • The enthalpy of solution is on the left so that it can point down without clashing
31
Q

What factors affect lattice enthalpy?

A
  • Ionic radius
  • Ionic charge
32
Q

How does ionic radius affect lattice enthalpy?

A
  • As the radius increases:
  • the attraction between the ions decreases
  • therefore the lattice enthalpy becomes less exothermic
33
Q

How does ionic charge affect lattice enthalpy?

A
  • As the charge of ions increases:
  • the attraction between the ions increases
  • the lattice enthalpy becomes more exothermic
34
Q

What does it mean if a compound has a more exothermic lattice enthalpy?

A
  • The more exothermic the lattice enthalpy of an ionic compound, the higher its melting point is
35
Q

What factors affect enthalpy of hydration?

A
  • Ionic radius
  • Ionic charge
36
Q

How does ionic radius affect enthalpy of hydration?

A
  • As the radius increases:
  • the attraction between the ion and the (polar) water molecules decreases
  • the enthalpy of hydration becomes less exothermic
37
Q

How does ionic charge affect enthalpy of hydration?

A
  • As the ionic charge increases:
  • the attraction with the water molecules increases
  • the enthalpy of hydration becomes more exothermic
38
Q

How does the enthalpy of hydration of a compound help determine its solubility?

A
  • To dissolve ionic compounds, the attraction between the ions has to be overcome
  • This therefore requires a quantity of energy equal to the lattice enthalpy
  • If the sum of the enthalpies of hydration is greater than or equal to the lattice enthalpy, this indicates that the compound is soluble

OCR A A-Level Inorganic Chemistry (15 decks)

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