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

Define standard enthalpy change of atomisation.

Answer 2

The enthalpy change when one mole of gaseous atoms is formed from the element in its standard state under standard conditions.

Question 3

Define first electron affinity.

Answer 3

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

Question 4

Define first ionisation energy.

Answer 4

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 5

Define standard enthalpy change of formation.

Answer 5

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

Question 6

Is ΔatmH endothermic or exothermic?

Answer 6

It is ALWAYS endothermic because bonds are broken to form gaseous atoms.

Question 7

Is first electron affinity endothermic or exothermic?

Answer 7

First is exothermic because the electron being added is attracted in towards the nucleus.

Question 8

Is lattice enthalpy endothermic or exothermic?

Answer 8

It is ALWAYS exothermic because it involves the formation of ionic bonds.

Question 9

What is formation of gaseous atoms and is it endothermic or exothermic?

Answer 9

It is changing elements from their standard states into gaseous atoms and it is endothermic as it involves bond breaking.

Question 10

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Answer 10

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Question 11

What is formation of lattice and is it endothermic or exothermic?

Answer 11

It is changing gaseous ions into the solid ionic lattice and it is exothermic as ionic bonds are formed.

Question 12

Is ionisation energy endothermic or exothermic?

Answer 12

It is ALWAYS endothermic because energy is required to overcome attraction between electron and nucleus.

Question 13

Why are successive electron affinities endothermic and not exothermic?

Answer 13

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

Question 14

What is a Born-Haber cycle?

Answer 14

A way of calculating lattice enthalpy using other known energy changes.

Question 15

Define standard enthalpy change of solution.

Answer 15

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

Question 16

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

Answer 16

The ionic lattice breaks up

So water molecules are attracted to and surround the ions.

Question 17

What are the energy changes involved in the process of a solid ionic lattice breaking up in water?

Answer 17

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

Question 18

Define enthalpy change of hydration.

Answer 18

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

Question 19

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

Answer 19

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

Question 20

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

Answer 20

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

Question 21

Describe the effect of increasing ionic size on hydration enthalpy.

Answer 21

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

Question 22

Describe the effect of increasing ionic charge on hydration enthalpy.

Answer 22

Ionic charge increases
Attraction between ion and water molecules increases
Hydration enthalpy becomes more negative

Question 23

Why is hydration enthalpy an inaccurate method of predicting solubility?

Answer 23

If overall enthalpy change of solution is exothermic, then compounds should dissolve, but many compounds with endothermic enthalpies of solutions are also soluble, so hydration enthalpy is not accurate.

Question 24

Define entropy.

Answer 24

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

Question 25

What are the units of entropy?

Answer 25

J K-1 mol-1

Question 26

How does entropy relate to physical states?

Answer 26

Gases > Liquids > Solids

Question 27

Define standard entropy

Answer 27

The entropy of one mole of a substance, under standard conditions.
100kPa and 298K

Question 28

How is entropy calculated for a reaction?

Answer 28

ΔS = (sum of products) - (sum of reactants)

Question 29

What is feasability?

Answer 29

Whether a reaction is able to happen and is energetically feasible.

Question 30

What is the equation for free energy change?

Answer 30

ΔG = ΔH - TΔS

ΔG = free energy change
ΔH = enthalpy change
T = temperature (K)
ΔS = entropy change

Question 31

What is the requirement of free energy change so that the reaction is feasable?

Answer 31

ΔG has to be negative indicating a decrease in free energy.