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Chemistry A - Level > Topic 13.1 - Energetics II: Lattice Energy > Flashcards

Topic 13.1 - Energetics II: Lattice Energy Flashcards

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1
Q

What is the definition of a lattice?

A

A regular 3 - dimensional arrangement of atoms or ions in a crystal

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2
Q

The standard enthalpy change of formation definition

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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3
Q

The standard enthalpy change of combustion definition

A

The enthalpy change when one mole of a compound is completely burned in excess oxygen under standard conditions

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4
Q

Ionisation Enthalpy definition

A

The amount of energy required to remove an electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous +1 cations under standard conditions

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5
Q

Electron affinity definition

A

The energy change per mole when an electron is added to a gaseous atom to form a gaseous anion under standard conditions

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6
Q

Atomisation energy definition

A

The enthalpy change when an element in its standard state is converted into one mole of free gaseous atoms under standard conditions

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7
Q

Lattice enthalpy definition

A

The enthalpy change/ energy released when one mole of a solid ionic lattice is formed from its constituent ions in the gaseous state (under standard conditions)

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8
Q

What are the standard conditions?

A
  • 100kPa
  • 298K (or specified temperature)
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9
Q

Write the equation for the enthalpy of formation of C2H4(g)

A

2C(s) + 2H2(g) -> C2H4(g)

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10
Q

Write the equation for enthalpy change of combustion of C2H4(g)

A

C2H4(g) + 3O2 -> 2CO2(g) + 2H2O (l)

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11
Q

Write the equation for the 1st ionisation energy of sodium?

A

Na(g) -> Na(+) + e(-)

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12
Q

Write the equation for the 1st electron affinity of chlorine?

A

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

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13
Q

Write the equation for the 2nd electron affinity of nitrogen

A

N(-)(g) + e(-) -> N(2-)(g)

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14
Q

Write the equation for the enthalpy change of atomisation of chlorine?

A

1/2Cl2(g) -> Cl(g)

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15
Q

Write the equation for the standard enthalpy of atomisation of Ozone?

A

1/3O3(g) -> O(g)

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15
Q

Write the equation for the lattice enthalpy of NaCl

A

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

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16
Q

Write the first electron affinity of fluorine?

A

F(g) + e(-) -> F(-)(g)

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16
Q

What enthalpy change does the equation Cl2(g) -> 2Cl(g) represent?

A

2 x enthalpy change of atomisation of chlorine

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16
Q

What enthalpy change does the equation S(g) + 2e(-) -> S(2-)(g) represent?

A

The sum of the first and second electron affinities of sulphur

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16
Q

What is an energy level diagram?

A

Y-axis: Enthalpy (energy content)
X-axis: Course of Reaction
- Shows you reactants, products, the (delta)H of the reaction and the Activation Energy

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16
Q

What enthalpy change does the equation 2Al(s) + 3N2(g) -> 2Al2N3 (s) represent?

A

2x the enthalpy of formation of Al2N3

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16
Q

What is a reaction profile?

A

Y-axis: Enthalpy (energy content)
X-axis: Course of Reaction
- Shows you reactants, products and the (delta)H of the reaction

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17
Q

As Lattice enthalpy increases…

A

the strength of the electrostatic forces between the ions in the ionic solid increases, therefore, the stability of your ionic solid increases

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17
Q

What is Lattice enthalpy a measure of?

A

The strength of the electrostatic forces between the ions in an ionic solid

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18
Q

Is Lattice enthalpy exothermic or endothermic? Why?

A

Exothermic - as you are making new bonds which releases energy to the surroundings

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19
Q

If a reaction is endothermic (delta)H is…?

A

+ve

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20
Q

If a reaction is exothermic (delta)H is…?

A

-ve

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21
Q

What factors affect the strength of an ionic bond?

A
  • Ionic radius
  • Ionic charge
22
Q

If the ionic radius of an ion decreases how does this affect ionic bond strength and lattice enthalpy?

A
  • Ionic bond strength increases as the ions in the lattice are closer together, therefore, exert stronger electrostatic forces of attraction on each other
  • Lattice enthalpy increases (becomes more exothermic) and you produce a more stable ionic compound
23
Q

If the ionic charge of an ion increases how does this affect ionic bond strength and lattice enthalpy?

A
  • Ionic bond strength increases as the ions exert stronger electrostatic forces of attraction on each other
  • Lattice enthalpy increases (becomes more exothermic) and you produce a more stable compound
24
Q

The compounds with the most negative lattice enthalpies are those that have…ions?

A

small and highly charged

25
Q

What four points do you have to include in questions comparing the lattice enthalpy of two different ionic compounds?

A
  • Compare lattice enthalpy (more or less exothermic)
  • Compare ionic charge/radius (smaller or bigger)
    NB: Always mention both even if they are the same
  • Compare how close they are
  • Compare attraction
    NB: Don’t need to name ions just write Cl(-), Na(+) etc.
26
Q

Describe how, and explain why, the lattice enthalpy of magnesium chloride differs from that of magnesium iodide.

A
  • The Cl- ion in MgCl2 has a smaller ionic radius in comparison to the I- ion in MgI2. The cation in both has the same charge and ionic radius.
  • This means that the ions in MgCl2 are closer together than in MgI2
  • Therefore, there are stronger electrostatic forces of attraction between the the Mg(2+) and Cl(-) ions in MgCl2 in comparison to the Mg(2+) and I(-) ions in MgI2.
  • Therefore, the lattice enthalpy of MgCl2 is more exothermic than that of MgI2
27
Q

Describe how, and explain why, the lattice enthalpy of sodium chloride differs from that of magnesium chloride.

A
  • The Mg(2+) ion in MgCl2 has a larger ionic charge, 2+, and smaller ionic radius, than the Na+ ion in NaCl.
  • This means that the ions in MgCl2 are closer together than the ions in NaCl
  • Therefore, there are stronger electrostatic forces of attraction between the Mg(2+) and Cl(-) ions in MgCl2 in comparison to the Na(+) and Cl(-) ions in NaCl
  • Therefore, the lattice enthalpy of MgCl2 is more exothermic, more negative, than that of NaCl
28
Q

Why can you not directly determine the enthalpy change of a thermal decomposition reaction?

A

Thermal Decomposition reactions need to be heated to high temperatures to occur. This means you don’t know how much energy you are putting into the reaction so you have to determine it indirectly, using a Hess Cycle.

29
Q

The standard enthalpy change of neutralisation definition

A

The enthalpy change when one mole of water is produced from solutions of an acid and an alkali reacting together

30
Q

The standard enthalpy change of reaction definition

A

The enthalpy change that occurs when equation quantities of materials react in their standard states under standard conditions

31
Q

Why might a (delta)H of reaction differ from the value stated in the data booklet?

A
  • Reaction not done at standard conditions
  • Heat from the reaction lost to the surroundings
32
Q

State Hess’ Law

A

The enthalpy change of a physical or chemical process is the same whatever the path from the reactants to the products

33
Q

What are the steps, in order for a Born-Haber cycle, going clockwise from your ionic compound?

A
  • Formation (down)
  • Atomisation of both Cation and Anion (up)
  • Ionisation energy of Cation, however many required (up)
  • Electron affinity of Anion, however many required (down for the first electron affinity and then up for the rest)
  • Lattice enthalpy (down)
34
Q

Why is the second (and further) electron affinity endothermic (up)?

A

As the second electron is being forced into a small and, therefore, highly electron dense area so a lot of energy is required to overcome these repulsive forces

35
Q

What model do theoretical Lattice Enthalpy values follow?

A

A pure ionic model

36
Q

Will the theoretical Lattice Enthalpy values be lower or higher than the experimental values for the same compound?

A

Lower - as using a pure ionic model assumes that there has been no polarising of the anion by the cation and so the compound has no covalent character

37
Q

Why will the theoretical Lattice Enthalpy values be lower than the experimental values for the same compound?

A

Theoretical Lattice enthalpy values are based off of the pure ionic model which assumes that there has been no polarising of the anion by the cation and so the compound has no covalent character. In pretty much all compounds there is some polarising of the anion by the cation, therefore, the ionic compounds always form stronger bonds than the theoretical bonds predicts.

38
Q

As you go down the halogens the disparity between the Theoretical vs Experimental Lattice Enthalpy values will…? Why?

A

Increase (become more inaccurate) - As the size of the halogen increases it’s electron cloud is more easily distorted by the cation

39
Q

What happens to electron affinity values as you go down the halogens?

A

As you go down Group 7 the electron affinity values will become more positive. This is because the electron is further away from the nucleus, therefore, requires more energy to be added to the atoms.

40
Q

What is the definition of polarisation?

A

The distortion of the electron cloud in a molecule or anion by a nearby positive charge

41
Q

What is the definition of polarising power?

A

The ability of a cation to distort the electron cloud of a neighbouring anion

42
Q

What is the definition of polarisability?

A

An indication of the extent to which the electron cloud in a molecule or ion can be distorted by a nearby electric charge

43
Q

What type of enthalpy values does the Born-Haber cycle predict?

A

Theoretical

44
Q

What happens in the first step of an ionic solid dissolving in water?

A

The ionic lattice is broken down into gaseous ions

45
Q

What happens in the second step of an ionic solid dissolving in water?

A

The gaseous ions are hydrated

46
Q

Standard Enthalpy Change of Solution Definition

A

The enthalpy change when 1 mole of a compound dissolves to form a solution of infinite dilution under standard conditions

47
Q

Standard Enthalpy Change of Hydration Definition

A

The enthalpy change when 1 mole of isolated gaseous ions is dissolved in water to give an infinitely dilute solution under standard conditions

48
Q

What is the definition of an infinitely dilute solution?

A

Is a solution where there is so much water that there is no further energy change if more water is added

49
Q

Write an equation to show the enthalpy change of solution for NaCl

A

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

50
Q

Write an equation to show the enthalpy change of hydration for NaCl

A

Na+(g) + aq -> Na+(aq)
Cl-(g) +aq -> Cl-(aq)

51
Q

What is the enthalpy change given to an ionic compound being broken down into its gaseous ions?

A

-ve lattice enthalpy

52
Q

Describe what a Born-Haber cycle looks like for the dissolving of an ionic compound

A
  • -ve lattice enthalpy upwards
  • Enthlpy of hydration downwards
    • Enthalpy of solution can be +ve or -ve depending on the size of the enthalpy changes of the other two
53
Q

Write an equation that shows the -ve lattice enthalpy of NaCl

A

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

54
Q

Is -ve lattice enthalpy exothermic or endothermic? Why?

A

Endothermic - you are breaking bonds between the ions in the lattice which require a lot of energy to overcome

55
Q

Is enthalpy change of solution exothermic or endothermic? Why?

A

Depends, but generally endothermic as you require energy to break the bonds in the solid ionic lattice.

If the enthalpy of solution is very endothermic it is unlikely that the ionic solid is soluble as it is less energetically favourable for bonds between water and the ions to form comparatively to if the ions remained as a solid.

If the enthalpy of solution is only slightly endothermic or exothermic then the ionic solid is likely to be soluble.

56
Q

Is enthalpy change of hydration exothermic or endothermic? Why?

A

Exothermic - You are forming stronger bonds between the ions

57
Q

How do the ions in aqueous solutions form bonds with the water/ polar molecules that they are dissolved in?

A

+ve ions -> partially -ve O-atoms
-ve ions -> partially +ve H-atoms

58
Q

What are the two factors that effect the enthalpy change of hydration?

A
  • ionic radius
  • ionic charge
59
Q

When you decrease the ionic radius, the enthalpy change of hydration…? Why?

A

Increases, becomes more negative/exothermic. This is because small ions approach closer to the water molecules so will form stronger electrostatic forces of attraction between them which releases more energy.

60
Q

When you increase the ionic charge, the enthalpy change of hydration…? Why?

A

Increases, becomes more negative/exothermic. The highly charged ions with smaller ionic radii form stronger electrostatic forces of attraction with the water molecules, therefore, releasing more energy.

61
Q

Is using a Born-Haber cycle accurate fro determining a trend in the solubility of ionic compounds? Why/why not?

A

No - it is very inaccurate.

  • Since both LE and EoH are both affected by ionic radius and charge this reduces the likelihood of any clear trends in enthalpy changes of solution
  • The sign and magnitude of the enthalpy change of solution is not a reliable guide to whether or not an ionic solid will dissolve
  • The enthalpy change of solution is generally a small difference between two large enthalpy changes, neither of which can be measured directly. Therefore, even small errors in estimating trends in those values can lead to large percentage errors in the predicted enthalpy changes of solution.
62
Q

What are the assumptions that are made when using the equation Q = mc(delta)T?

A
  • The specific heat capacity (c) of your solution is the same as water
  • The density (p) of your solution is the same as water

Chemistry A - Level (26 decks)

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