Chapter 22 - Enthalpy and entropy

Question 1

What is lattice enthalpy?

Answer 1

The enthalpy change that accompanies the formation of ONE MOLE of an IONIC COMPOUND from its GASEOUS IONS.

The amount of energy released when ions come together to form a lattice.

It is a measure of ionic bond strength in a giant ionic lattice.

Question 2

What is the standard enthalpy change of atomisation?

Understanding is required. Definition not required.

Answer 2

The enthalpy change for the formation of ONE MOLE of GASEOUS atoms from the element in its standard state under standard conditions.

Question 3

What is first electron affinity?

Understanding is required. Definition not required.

Answer 3

The enthalpy change for the ADDITION of one electron to each atom in ONE MOLE of GASEOUS atoms to form ONE MOLE of GASEOUS 1- ions.

Question 4

What method is used to calculate lattice enthalpy?

Answer 4

Lattice enthalpy cannot be measured directly and is calculated indirectly using a Born-Haber cycle.

Question 5

Describe what happens to the ions when a solid ionic compound dissolves in water.

Answer 5

• The ionic lattice breaks down to form gaseous ions.

- The gaseous ions bond to water molecules to form aqueous ions.

Question 6

What is the enthalpy change of hydration?

Answer 6

The enthalpy change when ONE MOLE of gaseous ions dissolve in water to form aqueous ions.

e.g. Na+(g) + aq → Na+(aq)

Question 7

What is the enthalpy change of solution?

Answer 7

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

e.g. NaCl(s) + aq → Na+(aq) + Cl-(aq)

Question 8

REVISE BORN-HABER CYCLES AND ENERGY CYCLE FOR DISSOLVING!

Answer 8

REVISE BORN-HABER CYCLES AND ENERGY CYCLE FOR DISSOLVING!

Question 9

What factors affect lattice enthalpy?

Answer 9

• Ionic size (As IS↑, LE becomes less exothermic)

- Ionic charge (As IC↑, LE becomes more exothermic)

Question 10

What factors affect hydration ethalpy?

Answer 10

Ionic size and charge.
As ionic size decreases and ionic charge increases:
- Attraction between ion and water molecules increases
- Enthalpy change of hydration becomes more exothermic.

Question 11

What is a spontaneous reaction?

Answer 11

A reaction in which free energy is released (ΔG < 0) and the system moves to a lower, more thermodynamically stable energy state.

A reaction which happens without outside intervention.

Question 12

What is entropy, S?

What is the unit?

Answer 12

A measure of the dispersal of energy in a system which is greater, the more disordered a system is.
Unit: JK-1mol-1

Question 13

Entropy is always ______.

Answer 13

Entropy is always positive.

Question 14

Explain the trend in entropy in the following transition,

Solid → Liquid → Gas.

Answer 14

• Particles are arranged more randomly
• Energy becomes more spread out
• Entropy increases and ΔS is positive

Question 15

An increase in the _____ of gaseous molecules increases the entropy.

Answer 15

An increase in the moles of gaseous molecules increases the entropy.

Question 16

Wha is the equation for ΔS of a system?

Answer 16

ΔS = ΣS(products) - ΣS(reactants)

Question 17

What is the free energy change, ΔG?

Answer 17

The overall energy change in a reaction.

ΔG = ΔH - TΔS
where S has the unit kJK-1mol-1 (JK-1mol-1 / 1000).

Question 18

What must you do to calculate the minimum temperature for a reaction to be feasible?

Answer 18

Make ΔG = 0 in the free energy change equation.

Question 19

For a reaction to be feasible…

Answer 19

ΔG < 0

Question 20

What are the limitations of predictions made by ΔG about feasibility?

Answer 20

ΔG takes no account of the kinetics or rate of reaction.

e.g. Although ΔG < 0 for a certain reaction, this reaction does not appear to take place at room temperature. The reaction has a large activation energy resulting in a very slow rate of reaction.