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Year 13 - Chemistry > C22 - Enthalpy and Entropy > Flashcards

C22 - Enthalpy and Entropy Flashcards

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1
Q

What is lattice enthalpy?

ΔLE H

A

A measure of the strength of ionic bonding in a giant ionic lattice.

Lattice enthalpy is the enthalpy change when one mole of an ionic compound is formed from its gaseous ions under standard conditions.

It is exothermic so will always be negative.

ΔLE H

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2
Q

What are the 2 routes of the Born-Haber cycle?

A

Route 1: indirect. Elements in standard states -> (formation of gaseous atoms) gaseous atoms -> (formation of gaseous ions) gaseous ions -> (lattice enthalpy/lattice formation from gaseous ions) ionic lattice.

Route 2: direct. Lattice formation from elements to form ionic lattice.

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3
Q

How is lattice enthalpy found?

A

It can’t be measured directly so must be done indirectly with other known energy changes.
This is done with the Born-Haber cycle.

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4
Q

What occurs in route one of the Born-Haber cycle?

A

There are 3 different processes:

1) Formation of gaseous atoms
- Changing the elements in their standard states into gaseous atoms.
- It’s endothermic and involves bond breaking.

2) Formation of gaseous ions
- Changing the gaseous atoms into + and - gaseous ions.
- It’s overall endothermic.

3) Lattice formation:
- Changing the gaseous ions into the solid ionic lattice.
- This is the lattice enthalpy and is exothermic.

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5
Q

What occurs in route two of the Born-Haber cycle?

A

It converts the elements in their standard states directly to the ionic lattice.

There’s only one enthalpy change; enthalpy change of formation, ΔfH, which is exothermic.

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6
Q

What is the standard enthalpy change of formation, ΔfH?

A

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

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7
Q

What is the standard enthalpy change of atomisation, ΔatH?

A

The enthalpy change when one mole of gaseous atoms are formed from the element in its standard state under standard conditions.

It’s always endothermic as bonds are being broken to form gaseous atoms.

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8
Q

What’s the first ionisation energy, ΔIE H?

A

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

Ionisation energies are endothermic as energy is required to overcome the attraction between the e- and nucleus.

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9
Q

What’s the first electron affinity, ΔEA H?

A

The enthalpy change when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions.

These are exothermic as the e- added is attracted towards the nucleus.
However, the second is endothermic as a second e- is being gained by a negative ion so energy must be put in to overcome repulsion.

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10
Q

What’s the standard enthalpy change of solution, Δsol H?

A

The enthalpy change when one mole of a solute dissolves dissolves in a solvent.

This can be endothermic or exothermic.

If the solvent is water, ions from the ionic lattice end up surrounded its water molecules as aqueous ions.

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11
Q

What happens when a solid ionic compound dissolves in water?

A
  1. The ionic lattice is broken up forming separate gaseous ions. (Opposite of lattice enthalpy)
  2. The separate gaseous ions interact with polar water molecules to form hydrated aqueous ions. This uses the enthalpy change of formation.
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12
Q

What is the enthalpy of hydration, Δhyd H?

A

The enthalpy change when one mole of aqueous ions are formed when gaseous ions dissolve in water.

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13
Q

What are the two routes that can be taken when an ionic compound is dissolved in water?

A

Route 1: An ionic lattice is formed from gaseous ions (lattice enthalpy used), then they dissolve to form aqueous ions.

Route 2: The gaseous ions are hydrated to form aqueous ions.

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14
Q

What two factors affect lattice enthalpy?

A

Ionic size/radius

Ionic charge

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15
Q

How does ionic size affect lattice enthalpy?

A

As ionic radius increases, attraction between ions decreases.
Lattice energy becomes less negative and melting point decreases.

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16
Q

How does ionic charge affect lattice enthalpy?

A

As ionic charge increases, attraction between ions increases.
Lattice energy becomes more negative and melting point increases.

17
Q

How do ionic size and ionic charge affect hydration enthalpies?

A

As ionic radius increases, attraction between ions and water molecules decreases.
Hydration energy is less negative.

As ionic charge increases, attraction with water molecules increases and hydration energy becomes more negative.

18
Q

What enables a compound to dissolve?

A

If the sum of the hydration enthalpies is larger than the size of the lattice enthalpy, then the overall enthalpy change should be exothermic (or slightly endothermic) and the compound should dissolve.

19
Q

What’s the symbol for entropy?

A

S, measured in J/Kmol

20
Q

What’s the entropy value for solids, liquids and gases?

A

Solids < liquid < gas

21
Q

What is entropy?

A

A measure of disorder.

The greater the entropy, the greater the dispersal of energy and the greater the disorder.

22
Q

What happens when a substance changes from solid to liquid to gas?

A

Melting and boiling point increase the randomness of particles.

Energy is spread out more and the change in S, entropy, is positive.

23
Q

What happens when there is a change in the number of gas molecules and a reaction produces more gas?

A

Production of gas / increase in the number of moles of gas increases the disorder of particles.

Energy is spread out more and the change in S, entropy, is positive.

24
Q

How is entropy change calculated?

A

ΔS = ∑ΔS (products) - ∑ΔS (reactants)

25
Q

What is free energy change, ΔG?

A

The overall change in energy during a chemical reaction.

It’s made up of 2 types of energy:
- Enthalpy change ΔH. This is the heat transfer between the chemical system and the surroundings.

  • Entropy change at temperature of reaction TΔS. This is the dispersal of energy within the chemical system itself.
26
Q

What is the Gibbs’ equation?

A

ΔG = ΔH - TΔS

Where:
ΔG is free energy change
ΔH is enthalpy change with surroundings
T is temperature in K
ΔS is entropy change of system
27
Q

What does the feasibility of a reaction depend on?

A

The Gibbs’ free energy value and balance between ΔH and TΔS

If ΔG < 0, the reaction will be spontaneous.

28
Q

Why may reactions not take place, even though they are spontaneous/feasible?

A

They have a high Ea.

29
Q

What enthalpy changes are used in a born-haber cycle to determine enthalpy of solution?

A

Lattice enthalpy (gaseous ions to ionic lattice)

Enthalpy of hydration (gaseous ions of each element into aqueous ions)

Enthalpy of solution (ionic lattice to aqueous ions)

Year 13 - Chemistry (12 decks)

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