Chapter 22 - Enthalpy and Entropy

Question 1

Define lattice enthalpy

Answer 1

The enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions.

Question 2

What kind of energy change will lattice enthalpy always be?

Answer 2

Exothermic

Enthalpy change is negative

Question 3

What is the Born-Haber cycle?

Answer 3

A way of indirectly calculating lattice enthalpy from other known energy changes.

Question 4

What is the formation of gaseous atoms and what energy change is it?

Answer 4

Changing elements in their standard states into gaseous atoms.
Endothermic as it involves bond breaking.

Question 5

What is the formation of gaseous ions and what energy change is it?

Answer 5

Changing gaseous atoms into positive and negative gaseous ions.
Endothermic.

Question 6

What is the lattice formation and what energy change is it?

Answer 6

Changing gaseous ions into the solid ionic lattice.

Exothermic.

Question 7

Define standard enthalpy change of formation

Answer 7

The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states.

Question 8

Define standard enthalpy change of atomisation

Answer 8

The enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions.

Question 9

Define first ionisation energy

Answer 9

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.

Question 10

Why is ionisation energy always endothermic?

Answer 10

Energy is required to overcome the attraction between a negative electron and a positive nucleus.

Question 11

Define first electron affinity

Answer 11

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

Question 12

Why is first elctron affinity always exothermic?

Answer 12

Electron being added is attracted in towards the nucleus.

Question 13

Why are successive electron affinities endothermic?

Answer 13

The negative ion repels the additional electron, so energy is required to force the electron onto it.

Question 14

Define standard enthalpy change of solution

Answer 14

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

Question 15

What processes take place when a solid ionic compound dissolves in water?

Answer 15

Ionic lattice breaks up

Water molecules are attracted to and surround the ions.

Question 16

What are the energy changes involved in these processes of a solid ionic compound dissolving in water?

Answer 16

Ionic lattice is broken up forming separate gaseous ions.
Separate gaseous ions interact with polar water molecules to form hydrated aqueous ions.
Called enthalpy change of hydration.

Question 17

Define enthalpy change of hydration

Answer 17

The enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions.

Question 18

Describe the effect of ionic size on lattice enthalpy and melting point

Answer 18

Ionic radius increases
Attraction between ions decreases
Lattice enthalpy becomes less negative
Melting point decreases

Question 19

Describe the effect of ionic charge on lattice enthalpy and melting point

Answer 19

Ionic charge increases
Attraction between ions increases
Lattice enthalpy becomes more negative
Melting point increases

Question 20

Describe the effect of ionic size on hydration enthalpy

Answer 20

Ionic radius increases
Attraction between ion and water molecules decreases
Hydration energy less negative

Question 21

Describe the effect of ionic charge on hydration enthalpy

Answer 21

Ionic charge increases
Attraction with water molecules increases
Hydration energy becomes more negative

Question 22

Why is hydration enthalpy an inaccurate method of predicting solubility?

Answer 22

Many compounds with endothermic enthalpy changes of solution are soluble.

Question 23

Define entropy

Answer 23

A measure of the dispersal of energy within the chemicals in a chemical system.

Question 24

What are the units of entropy?

Answer 24

J/K/mol

Question 25

How does entropy relate to physical states?

Answer 25

In order of size:
Gases > liquids > solids
for entropy value

Question 26

Define standard entropy

Answer 26

The entropy of one mole of a substance, under standard conditions.
(Always positive)

Question 27

How is an entropy change calculated for a reaction?

Answer 27

Change in entropy is the sum of the entropies of the products minus the sum of the entropies of the reactants.

Question 28

What is feasibility?

Answer 28

Whether a reaction is able to happen and is energetically feasible (or spontaneous).

Question 29

What is the equation for free energy change?

Answer 29

ΔG = ΔH - TΔS

Question 30

What is the requirement for the free energy change such that a reaction is feasible?

Answer 30

ΔG < 0

A decrease in free energy.

Question 31

Even though a reaction may have a negative value for ΔG, why might the reaction not take place at room temperature?

Answer 31

High activation energy resulting in a very slow rate.