MODULE 5: Enthalpy and Entropy

Question 1

Entropy symbol:

Answer 1

S

Question 2

Order of highest to lowest entropy:

Answer 2

Gas, liquid, solid

Question 3

Entropy:

Answer 3

A measure of the dispersal of energy in a system which is greater, the more disordered a system

Question 4

Units of entropy:

Answer 4

J -1 K -1 mol -1

Question 5

Changes in entropy:

Answer 5

Changes to become more random = positive ΔS

Changes to become less random = negative ΔS

Question 6

Standard entropy:

Answer 6

Entropy of one mole of a substance, under standard conditions

Question 7

Calculating entropy change:

Answer 7

ΔS = ΔS (products) - ΔS (reactants)

Question 8

Feasibility:

Answer 8

Whether a reaction is able to occur and is energetically feasible (spontaneous)

Question 9

Free energy change:

Answer 9

Overall change in energy during a chemical reaction, made up of enthalpy change and entropy

Question 10

Gibbs equation:

Answer 10

ΔG = ΔH - TΔS

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

Question 11

Value of ΔG for a reaction to be feasible:

Answer 11

ΔG < 0

Question 12

Limitations of predicting feasibility:

Answer 12

Gibbs free energy only takes into account a reaction’s feasibility, but not activation energy or rate of reaction- which may cause a reaction to not occur or be very slow

Question 13

Lattice enthalpy: ΔlattH

Answer 13

Enthalpy change when one mole of an ionic compound is formed from its constituent gaseous ions under standard conditions

Question 14

Use of lattice enthalpy:

Answer 14

Measure strength of an ionic bond in a giant ionic lattice

Question 15

Standard enthalpy of formation: ΔfH

Answer 15

Enthalpy change when one mole of compound is formed from its constituent elements at standard state (state in standard conditions)

Question 16

Standard enthalpy change of atomisation: ΔatH

Answer 16

Enthalpy change when one mole of gaseous atoms are formed in standard conditions

Question 17

First ionisation energy: ΔIEH

Answer 17

Enthalpy change when one mole of electrons are removed from one mole of gaseous atoms to form one mole of gaseous 1+ ions

Question 18

First electron affinity: ΔEAH

Answer 18

Enthalpy change when one mole of electrons is added to one mole of gaseous atoms to form one mole of gaseous 1- ions

Question 19

Enthalpy change of solution: ΔsolH

Answer 19

Enthalpy change when one mole of solute dissolves in a solvent

Question 20

Enthalpy change of hydration: ΔhydH

Answer 20

Enthalpy change when gaseous ions dissolve in water to form one mole of aqueous ions

Question 21

Factors effecting lattice enthalpy and enthalpy change of hydration:

Answer 21

Ionic size and ionic charge

Question 22

Effect of ionic size on lattice enthalpy:

Answer 22

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

Question 23

Effect of ionic charge on lattice enthalpy:

Answer 23

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

Question 24

Effect of ionic size on enthalpy change of hydration:

Answer 24

• Ionic radius increases
• Attraction between ion and water molecules increases
• Enthalpy change of hydration less negative

Question 25

Effect of ionic charge on enthalpy change of hydration:

Answer 25

• Ionic charge increases
• Attraction to H2O increases
• Enthalpy change of hydration becomes more negative