Topic 17: Thermodynamics

Question 1

Entropy

Answer 1

The measure of the level of disorder in a thermodynamic system

Question 2

Entropy units

Answer 2

JK^-1mol^-1

Question 3

Entropy from entropy calcs

Answer 3

sum of entropy of products - sum of entropy of reactants

Question 4

Gibbs free equation

Answer 4

ΔG = ΔH -TΔS
= change in enthalpy - temp x change in entropy

Question 5

Gibbs free units

Answer 5

KJmol^-1 so convert the enthalpy and entropy because they’re in JK^-1mol^-1

Question 6

Feasible reaction

Answer 6

Negative gibbs free

Question 7

Exothermic reaction with positive entropy change

Answer 7

-ive (-) +ive
so always feasible

Question 8

Exothermic reaction with negative entropy change

Answer 8

-ive (-) -ive
Only feasible at low temps

Question 9

Endothermic reaction with positive entropy change

Answer 9

+ive (-) -ive
Only feasible at high temps

Question 10

Endothermic reaction with negative entropy change

Answer 10

+ive (-) -ive
Never feasible

Question 11

Enthalpy change

Answer 11

Heat change at constant pressure

Question 12

Enthalpy of combustion

Answer 12

Enthalpy change when one mole of substance is completely burnt in oxygen

Question 13

Enthalpy of atomisation

Answer 13

Enthalpy change when one mole of gaseous atoms is produced from its elements in standard states and standard conditions

Question 14

First ionisation energy

Answer 14

Enthalpy change when one mole of gaseous atoms is converted to one mole of gaseous ions each with a single positive charge.

Question 15

First electron affinity

Answer 15

Enthalpy change when one mole of gaseous atoms is converted to one mole of gaseous ions each with a single negative change.

Question 16

Lattice enthalpy of formation

Answer 16

Enthalpy change when one mole of solid ionic compound is formed from its gaseous ions.

Question 17

Lattice enthalpy of formation exo or endo

Answer 17

Exo, new bonds are always formed in this process so ΔH is always negative

Question 18

Lattice enthalpy of dissociation

Answer 18

Enthalpy change when one mole of solid ionic compound dissociates into its gaseous ions

Question 19

Enthalpy of hydration

Answer 19

Enthalpy change when water molecules surround one mole of gaseous ions

Question 20

Enthalpy of solution

Answer 20

Enthalpy change when one mole of an ionic compound is dissolved in a solvent to form an infinitely dilute solution

Question 21

Elements in standard standard states enthalpy

Answer 21

zero

Question 22

Born-Haber Cycle

Answer 22

Thermochemical cycle that includes all the enthalpy changes involved in the formation of an ionic compound. Start with elements in standard states

Question 22

Lattice enthalpy and ion size

Answer 22

Larger ions have smaller lattice enthalpies
- opposite charges don’t approach each other as closely if the ions are larger understand this concept later

Question 22

Lattice enthalpy and size of charge

Answer 22

Lattice enthalpy increases the large the charge as ions with double the charge give out roughly twice as much energy when they come together understand concept later

Question 23

Enthalpy of solution equation

Answer 23

Enthalpy of dissociation + lattice enthalpy (Think about breaking the bonds dissociating and then hydrating both the cations and anions)

Question 23

Covalent character shown

Answer 23

Large difference between calculated value and born-haber cycle experimental value. Extra bonding is present.

Question 24

How covalent character arises

Answer 24

Small cation approaches large anion and distorts the electron cloud, more electrons than expected concentrated between the cation and anion's nuclei The anion is polarised

Question 25

Factors increasing polarisation

Answer 25

Cations - small size high charge Anions - large size, high charge

Question 26

Kinetics and thermodynamics

Answer 26

A reaction can be feasible but it might take 10 years to complete

Question 27

Enthalpy of formation

Answer 27

Enthalpy change when one mole of compound is formed from its elements in standard states and conditions.