[ 1 ] 3.1.8 — Thermodynamics

Question 1

What are the three stages in the formation of an ionic compound from its elements?

Answer 1

Formation of gaseous atoms, formation of ions, formation of the ionic lattice.

Question 2

What is atomisation enthalpy?

Answer 2

The energy required to form 1 mole of gaseous atoms from an element in its standard state.

Question 3

What is bond dissociation enthalpy?

Answer 3

The energy required to break 1 mole of covalent bonds in the gas phase.

Question 4

What is first ionisation energy?

Answer 4

The energy needed to remove one electron from each atom in 1 mole of gaseous atoms.

Question 5

What is first electron affinity?

Answer 5

The energy released when one electron is added to each atom in 1 mole of gaseous atoms.

Question 6

What is lattice enthalpy of formation?

Answer 6

Energy released when 1 mole of an ionic solid forms from gaseous ions.

Question 7

What is lattice enthalpy of dissociation?

Answer 7

Energy required to separate 1 mole of an ionic solid into gaseous ions.

Question 8

What is the Born-Haber cycle used for?

Answer 8

To calculate lattice enthalpies or electron affinities using Hess’ Law.

Question 9

Are lattice formation enthalpies endothermic or exothermic?

Answer 9

Exothermic (negative).

Question 10

Are second electron affinities exothermic or endothermic?

Answer 10

Endothermic (repulsion of negative charge).

Question 11

Why is MgCl2’s Born-Haber cycle more complex than NaCl’s?

Answer 11

It includes second ionisation energy and 2 moles of chloride ions.

Question 12

What is hydration enthalpy?

Answer 12

Energy released when a gaseous ion dissolves in water.

Question 13

What is lattice dissociation enthalpy?

Answer 13

Energy required to separate ions in a solid lattice to gaseous form.

Question 14

What is enthalpy of solution?

Answer 14

The enthalpy change when 1 mole of a substance dissolves in water.

Question 15

How is enthalpy of solution calculated?

Answer 15

ΔH(solution) = Σ(ΔH hydration) + ΔH lattice dissociation.

Question 16

What factors increase hydration energy?

Answer 16

Higher ionic charge and smaller ion radius.

Question 17

What factors increase lattice energy?

Answer 17

Smaller ionic size and higher ionic charge.

Question 18

What determines ionic compound solubility?

Answer 18

Balance between hydration energy and lattice energy.

Question 19

What is entropy?

Answer 19

A measure of disorder in a system.

Question 20

What state has the highest entropy?

Answer 20

Gases > liquids > solids.

Question 21

What is the trend in entropy values for C, CO₂, and CO?

Answer 21

CO₂ > CO > C(s)

Question 22

What is the formula for entropy change?

Answer 22

ΔS = Σ(S products) - Σ(S reactants)

Question 23

What does a positive entropy change indicate?

Answer 23

The reaction increases disorder and is more likely to proceed.

Question 24

What is Gibbs free energy?

Answer 24

A value combining enthalpy and entropy to predict spontaneity: ΔG = ΔH - TΔS

Question 25

When will a reaction definitely be spontaneous?

Answer 25

When ΔG < 0

Question 26

Can a reaction be spontaneous even if it's endothermic?

Answer 26

Yes, if TΔS outweighs ΔH, making ΔG negative.

Question 27

What is the critical temperature?

Answer 27

Temperature at which ΔG = 0; T = ΔH/ΔS

Question 28

If both ΔH and ΔS are positive, when is the reaction spontaneous?

Answer 28

Only at high temperatures.

Question 29

If both ΔH and ΔS are negative, when is the reaction spontaneous?

Answer 29

Only at low temperatures.

Question 30

If ΔH is negative and ΔS is positive, is the reaction spontaneous?

Answer 30

Yes, at all temperatures.

Question 31

If ΔH is positive and ΔS is negative, is the reaction spontaneous?

Answer 31

No, at any temperature.