3.1.8 Thermodynamics

Question 1

What thermodynamic cycle is used to determine lattice enthalpy from several measurable enthalpy changes?

Answer 1

Born–Haber cycle

It applies Hess’s law to relate formation enthalpy to other enthalpy changes in ionic compounds.

Question 2

What is the enthalpy change when one mole of an ionic solid is formed from its gaseous ions?

Answer 2

Lattice enthalpy of formation

This process is exothermic because strong electrostatic attractions form between oppositely charged ions.

Question 3

What is the lattice enthalpy of dissociation?

Answer 3

The enthalpy change required to separate one mole of an ionic lattice into its gaseous ions.

This process is endothermic, as energy is absorbed to overcome the electrostatic forces between the ions.

Question 4

Fill in the blank:

The enthalpy change when one mole of gaseous atoms is formed from an element in its standard state is called the enthalpy of _______.

Answer 4

atomisation

For example, Na(s) → Na(g) represents atomisation of sodium.

Question 5

What is the enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to form gaseous ions?

Answer 5

First Ionisation energy

The first ionisation energy forms a +1 ion; successive ionisation energies remove further electrons.

Question 6

True or False:

Electron affinity is the enthalpy change when a gaseous atom gains an electron to form a negative ion.

Answer 6

True

This process is usually exothermic because the atom attracts the incoming electron.

Question 7

What term describes the enthalpy change when one mole of gaseous ions becomes hydrated by water molecules?

Answer 7

Enthalpy of hydration

Hydration releases energy as ion–dipole attractions form between ions and water molecules.

Question 8

Fill in the blank:

Born–Haber cycles are based on the principle that total enthalpy change is independent of the _______.

Answer 8

route

This is an application of Hess’s law.

Question 9

True or False:

A large difference between theoretical and experimental lattice enthalpies can indicate covalent character in an ionic compound.

Answer 9

True

Partial covalent bonding means the perfect ionic model is less accurate.

Question 10

What term describes the formation of one mole of a compound from its elements in their standard states?

Answer 10

Enthalpy of Formation

This value is often used as the starting point in Born–Haber cycles.

Question 11

Fill in the blank:

The energy required to break one mole of a specific covalent bond in gaseous molecules is called bond _______.

Answer 11

enthalpy

Bond enthalpies are average values measured across many compounds.

Question 12

Which two enthalpy terms are combined to calculate the enthalpy of solution of an ionic compound?

Answer 12

1. Lattice enthalpy
2. Hydration enthalpy

Enthalpy of solution = lattice dissociation enthalpy + hydration enthalpies of the ions.

Question 13

What thermodynamic quantity measures the degree of disorder in a system?

Answer 13

Entropy

Entropy increases when particles become more dispersed or randomly distributed.

Question 14

What symbol represents the change in entropy during a reaction?

Answer 14

∆S

Positive ∆S indicates increased disorder in the system.

Question 15

What thermodynamic quantity determines whether a reaction is feasible?

Answer 15

Gibbs free-energy change

It combines enthalpy, entropy and temperature effects.

Question 16

Fill in the blank:

The relationship linking enthalpy, entropy and Gibbs free energy is ____=_____.

Answer 16

∆G = ∆H – T∆S

Temperature must be in kelvin when using this equation.

Question 17

True or False:

A reaction is feasible when the value of ∆G is negative.

Answer 17

True

A negative Gibbs free-energy change indicates a thermodynamically favourable process.

Question 18

What value of ∆G corresponds to a reaction that is at equilibrium?

Answer 18

Zero

At equilibrium there is no overall driving force for the reaction.

Question 19

True or False:

Enthalpy change alone can always predict whether a reaction will occur.

Answer 19

False

Entropy and temperature also influence feasibility.

Question 20

What unit is used for temperature in the equation ∆G = ∆H – T∆S?

Answer 20

kelvin

Temperature must be converted from °C to K before calculations.

Question 21

Fill in the blank:

The entropy change of a reaction can be calculated from the _______ entropy values of reactants and products.

Answer 21

absolute

∆S = ΣS(products) − ΣS(reactants).

Question 22

True or False:

Increasing temperature can change whether a reaction is feasible.

Answer 22

True

The T∆S term increases with temperature and can change the sign of ∆G.

Question 23

When ∆H is positive and ∆S is positive, how does increasing temperature affect feasibility?

Answer 23

Increases

A larger T∆S term can make ∆G negative at high temperature.

Question 24

What type of graph can be used to determine ∆H and ∆S from thermodynamic data?

Answer 24

∆G versus T graph

The intercept and gradient allow determination of ∆H and ∆S.

Question 25

On a ∆G versus T graph, what does the **y intercept represent**?

Answer 25

ΔH

Question 26

On a ∆G versus T graph, what does the **gradient represent**?

Answer 26

∆S

Question 27

What is the term for the enthalpy change when one mole of a substance is **completely burned in oxygen**? ## Footnote Assume standard conditions, with all reactants and products in their standard states.

Answer 27

Enthalpy of Combustion