Thermodynamics (Physical) (complete)

Question 1

Define Ionisation enthalpy (first + second)

• Exothermic or Endothermic?

Answer 1

• First: The enthalpy change when each atom in one mole of gaseous atoms loses one electron to form one mole of gaseous 1+ ions. (Endothermic)
• Second: The enthalpy change when each ion in one mole of gaseous 1+ ions loses one electron to form one mole of gaseous 2+ ions. (Endothermic)

Question 2

Define Electron affinity (first + second)

• Exothermic or Endothermic?

Answer 2

• First: The enthalpy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous 1- ions. (Exothermic)
• Second: The enthalpy change when each ion in one mole of gaseous 1- ions gains one electron to form one mole of gaseous 2- ions. (Endothermic)

Question 3

Define Enthalpy of atomisation. (Exo/Endo?)

Answer 3

• The enthalpy change when one mole of gaseous atoms is produced from an element in its standard state.
  (Endothermic)

Question 4

Define Enthalpy of hydration. (Exo/Endo?)

Answer 4

• The enthalpy change when one mole of gaseous ions becomes hydrated.
  (Exothermic)

Question 5

Define Enthalpy of solution. (Exo/Endo?)

Answer 5

• The enthalpy change when one mole of an ionic solid dissolves in an amount of water large enough so that the dissolved ions are separated enough and don’t react with each other.

(Exo/Endo - varies)

Question 6

Define Bond dissociation enthalpy. (Exo/Endo?)

Answer 6

• The enthalpy change when one mole of covalent bonds is broken in the gaseous phase.
  (Endothermic)

Question 7

Define Lattice enthalpy of formation. (Exo/Endo?)

Answer 7

• The enthalpy change when one mole of a solid ionic compound is formed from its constituent ions in the gas phase.
  (Exothermic)

Question 8

Define Lattice enthalpy of dissociation. (Exo/Endo?)

Answer 8

• The enthalpy change when one mole of a solid ionic compound is broken up into its constituent elements in the gaseous phase.
  (Endothermic)

Question 9

Define Enthalpy of vapourisation. (Exo/Endo?)

Answer 9

• Enthalpy change when one mole of a liquid is turned into a gas.
  (Endothermic)

Question 10

Define Enthalpy of fusion. (Exo/Endo?)

Answer 10

• The enthalpy change when one mole of a solid is turned into a liquid.
  (Endothermic)

Question 11

Define Enthalpy of formation. (Exo/Endo?)

Answer 11

• The enthalpy change when one mole of a substance is formed from its constituent elements in standard states under standard conditions.
  (Exothermic)

Question 12

Define Enthalpy of combustion. (Exo/Endo?)

Answer 12

• The enthalpy change when one mole of a substance is burned completely with oxygen in standard states under standard conditions.

Question 13

Define Enthalpy Change

Answer 13

The heat energy of a reaction under constant pressure.

Question 14

Define Enthalpy Change

Answer 14

The heat energy of a reaction under constant pressure.

Question 15

Give the cycle for enthalpy of solution calculations.

Answer 15

Ionic solid —> Aqueous ions
<— Gas ions —>

(ionic solid—>Aqueous ions = enthalpy of solution)
(ionic solid<—Gas ions = Lattice enthalpy of formation)
(Aqueous ions<—Gas ions = enthalpy of hydration)

Question 16

What can the Lattice enthalpy of a compound show about that compound.

What is Lattice enthalpy affected by?

Answer 16

• The strength of the ionic bonding. (the greater the magnitude of the lattice enthalpy, the stronger the bonding.)
• Ion size and Charge size.
  • The smaller the ion size and larger the charge, the stronger the attractions so greater lattice enthalpy.

Question 17

Explain how a Born-Haber cycle is set out. (start at the bottom level and work left)

How would you calculate Enthalpy of formation?

Answer 17

Solid Ionic Compound<—(enthalpy of formation)—Elements in Normal States—(atomisation of metal and non-metal)—>Gaseous Ions—(ionisation energies of metal atom(s))—>Gaseous Metal Ions and Electrons and Gaseous Non-metals atoms (top level)—(electron affinities of non-metal atoms)—>Gaseous Ions—(lattice enthalpy of formation)—>Solid Ionic Compound (bottom level - complete)

Enthalpy of formation = sum of all other enthalpy values

Question 18

How can energetic stability of elements/compounds be compared/used to predict likely products?

Answer 18

• The more negative the enthalpy of formation of a compound, the more exothermic, so the more energy released when formed. Therefore the compound is more stable, as it requires more energy to form. The more stable the compound, the more likely it is to be formed as a likely product.

Question 19

Describe how a:
- Experimental lattice enthalpy value
- Theoretical lattice enthalpy value
: is calculated.
(which is the real value?)

Answer 19

• Experimental - using a Born-Haber cycle, where all other values are found by accurate measurements in experiments.
• Theoretical - by a theoretical calculation that considers the size, charge and arrangement of ions in the lattice. This assume the structure is perfectly ionic.
• The Experimental value is the ‘real’ value.

Question 20

Define Covalent Character.
Describe the trends with an ions ability to be distorted/distort.

Answer 20

• There is often some distortion of the ions in ionic compounds, therefore the ions are not perfectly spherical. If there is lots of distortion, the ions are said to have some covalent character.
• Positive ions that are small and/or highly charged are very good at distorting negative ions. Due to less nuclear shielding, more nuclear attraction, higher charge.
• Negative ions that are larger and or highly charged are easier to distort. Die to more nuclear shielding, less nuclear attraction, higher charge.

Question 21

How can Covalent Character affect:
- Conductivity
- Melting and Boiling Points
- Solubility

Answer 21

• Weaker conductivity
• Lower m.p and b.p
• Lower solubility

Question 22

What can comparison between the Experimental and Theoretical lattice enthalpy values of an ionic compound show?

How would you calculate percentage difference?

Answer 22

• The larger the different between the two values, the greater the covalent character of an ionic compound.
• If the difference is small, the compound will have almost spherical ions.
• ((E-T) / T ) x 100

Question 23

Define Entropy. (Units?) (Give the equation for calculating entropy)

Name the 2nd and 3rd Law of thermodynamics.
Describe the trend in entropy during the state changes.

Answer 23

• A measure of the disorder of a reaction. (the more disorder something is, the greater the entropy)
• Units = Jmol-1K-1 Entropy = (Sum of entropies of products) — (Sum of entropies of reactants)

2nd Law - over time, entropy will naturally increase
3rd Law - the entropy of a substance is zero at absolute zero and increases with temperature.

• From solid to liquid, the entropy change is large, but not as larger as from liquid to gas due to the large amount of disorder in gases compared to solids and liquids.

Question 24

Give the equation for calculating Gibbs Free Energy Change.
Name the three conditions required for a reaction to be feasible. (not including negative Gibbs energy)

Why may a feasible reaction not actually take place?

Answer 24

Gibbs Free Energy = (Enthalpy change) — (Temperature x Entropy change)

1- Small, -ve enthalpy change.
2- High temperature.
3- Large, +ve Entropy change.

• The reaction may have too large of an activation energy, so the reaction doesn’t occur.