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Thermodynamics > Born-Haber Cycles > Flashcards

Born-Haber Cycles Flashcards

1
Q

What is Enthalpy?

A

Heat energy stored in a chemical system.

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2
Q

What is Standard Enthalpy of a reaction?

A

Enthalpy change occurring when the amount shown in the reaction equation react in their standard states and under standard conditions.

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3
Q

What is Standard Molar Enthalpy of formation?

A

Enthalpy change occurring when 1 mole of a compound is formed from its elements with all substances in their standard states and under standard conditions.

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4
Q

What is Standard Molar Enthalpy of Combustion?

A

Enthalpy change occurring when 1 mole of a compound is burned completely in an excess of oxygen with all substances in their standard states and under standard conditions.

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5
Q

What is mean bond enthalpy?

A

Enthalpy change occurring when 1 mole of a covalent bond, measured in the gaseous state is broken, averaged over a range of compounds.

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6
Q

What is Hess’s Law?

A

The enthalpy change of reaction is independent of the route taken provided the initial and final conditions are the same.

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7
Q

How is mean enthalpy change calculated?

A

[Sum of bond enthalpies of the reactants]-[sum of bond enthalpies of the products]

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8
Q

What makes a chemical reaction exothermic?

A

If more energy is released when bonds are formed in the products than is absorbed when bonds are broken in the reactants.

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9
Q

What makes a reaction endothermic?

A

If more energy is absorbed when bonds are broken in the reactants than is released when bonds are formed in the products.

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10
Q
A
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11
Q

How is AHr calulated using enthalpy of formation data

A

[AHf products]-[AHf reactants]

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12
Q

How is AHr calculated using enthalpy of combustion data?

A

[AHf reactants]-[AHf products]

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13
Q

What is bond dissociation enthalpy?

A

Enthalpy change occurring when all the bonds of the same type in 1 mole of gaseous molecules is broken to form one mole of gaseous atoms.

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14
Q

What is Standard Molar Enthalpy of Atomisation of an element?

A

Enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state.

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15
Q

What is Standard Molar Enthalpy of Atomisation of an compound?

A

Enthalpy change occurring when 1 mole of a compound measured in its standard state is converted to gaseous atoms.

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16
Q

What is First Ionisation Energy?

A

Enthalpy change occurring when 1 mole of electrons is removed from 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions.

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17
Q

What is Second Ionisation Energy?

A

Enthalpy change occurring when 1 mole of electrons is removed from 1 mole of gaseous 1+ ions to form 1 mole of 1 mole of gaseous 2+ ions.

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18
Q

What is First Electron Affinity?

A

Enthalpy change occurring when 1 mole of gaseous atoms is converted to 1 mole of gaseous 1- ions.

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19
Q

What is Second Electron Affinity?

A

Enthalpy change occurring when 1 mole of gaseous 1- ions is converted into 1 mole of gaseous 2- ins.

20
Q

Why is First Electron Affinity exothermic?

A

There is an attraction between the added electron and the nucleus. So energy is released when the electron is gained.

21
Q

Why is Second Electron Affinity endothermic?

A

Electron is being added to a negative ion. The negative ion repels the added electron.

22
Q

What is Lattice Formation Enthalpy?

A

Enthalpy change occurring when 1 mole of a solid ionic lattice is formed from its gaseous ions.

23
Q

What is Lattice Dissociation Enthalpy?

A

Enthalpy change occurring when 1 mole of an ionic substance is separated into its gaseous ions.

24
Q

In a Born Haber Cycle, how can lattice formation enthalpy be calculated?

A

Add all the enthalpy changes together.

25
Q

What are the two main factors which affect lattice enthalpy?

A

Ionic Radius
Ionic Charge

26
Q

What is lattice enthalpy a measure of?

A

Is a measure of the strength of the attractions between the ions in an ionic solid.

27
Q

The larger the ionic radius the smaller the lattice enthalpy.
Explain this trend.

A

When the ions have a larger ionic radius, the oppositely charged ions cannot get as close together. This means that the strength of the attractions between them is weaker resulting in longer and weaker ionic bonds.

28
Q

For ions of similar size, the lattice enthalpy increases with the increasing size of the charge. Explain this trend.

A

The higher the charge on the ion the smaller the ionic radius will be. This means that the ions are closer together in the lattice. Therefore the attractions between them are stronger, resulting in shorter and stronger ionic bonds.

29
Q

Explain why the lattice dissociation enthalpy for silver chloride is less endothermic than the lattice dissociation enthalpy for silver fluoride.

A

The chloride ion is larger than the fluoride ions.
So the attraction between the silver ion and the chloride ion is weaker than the attraction between the silver ion and the fluoride ion.

30
Q

What is the Perfect Ionic Model?

A

Assumes all ions are perfect spheres with purely ionic attractions ( no covalent character).

31
Q

What does it mean if the theoretical values for lattice formation enthalpies are different from the Born-Haber cycle values? (Born-Haber cycles use experimental data)

A

The ionic bond being formed has some covalent character.

32
Q

What is covalent Character?

A

Covalent character arises when the negative ion becomes distorted, i.e. polarised.

This is most likely when the cation is small with a high charge density and the anion is large.

33
Q

When bonding is completely ionic, the ions are spherical. What does this mean for the theoretical and Born-Haber enthalpies?

A

The theoretical and Born-Haber enthalpies are the same.

34
Q

When bonding includes some covalent character the charge cloud is distorted. What does this mean for the theoretical and Born-Haber enthalpies?

A

The theoretical and Born-Haber enthalpies are different.

35
Q

The values calculated using experimental data in the Born-Haber cycle are greater than those calculated using the theoretical perfect ionic model, Why is this?

A

Covalent bonding character puts extra energy into the ionic bond.

The covalent character makes the bond stronger so more energy is released when forming the lattice.

36
Q

Fluoride ions usually match the perfect ionic model and have a good agreement between theoretical and experimental values for lattice enthalpy.
Why is this?

A

Fluoride ion is very resistant to polarisation due to its small size.

37
Q

Suggest why an experimentally measured enthalpy of lattice dissociation can be greater than the theoretical value.

A

some ionic bonds can also have some covalent character. Therefore forces of attraction in the lattice are stronger than pure ionic attractions.

38
Q

What is enthalpy of solution?

A

Enthalpy change occurring when one mole of an ionic solid dissolves completely to form aqueous ions.

39
Q

When a solid ionic lattice dissolves, two steps occur. What are these two steps?

A

Bonds between the ions break ( Lattice Dissociation)

Bonds between the ions and water form (Hydration)

40
Q

What happens during dissolving of an ionic solid?

A

The ionic lattice is broken up

Solvent molecules then surround the ions.

41
Q

During dissolving positive ions are attracted to what?

A

The negative end of the polar solvent molecule (i.e the oxygen atom in water molecules)

42
Q

During dissolving Negative ions are attracted to what?

A

The positive end of the polar solvent molecules (i.e. the H atoms in water molecules)

43
Q

How is enthalpy of solution calculated?

A

Enthalpy Lattice Dissociation + Enthalpy of Hydration

44
Q

What is Enthalpy of Hydration?

A

Enthalpy change occurring when 1 mole of gaseous ions is surrounded by water molecules.

45
Q

Would you expect the enthalpy of hydration of chloride ion to be more or less exothermic than the enthalpy of hydration of bromide ion?

A

More exothermic.
Because chloride ions have a smaller ionic radius than bromide ions so the force of attraction between the chloride ion and the the partially positive hydrogen atom in water is stronger.

Thermodynamics (2 decks)

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