1.8 Thermodynamics

Question 1

What is a Born-Haber cycle?

Answer 1

An application of Hess’ law

Where there are multiple steps in one leg of the cycle

Question 2

How are ionic solids produced?

Answer 2

Between ions in the gas phase

Question 3

What is exothermic and endothermic in terms of bonds? Arrow representation?

Answer 3

Exothermic - bond forming (down arrow)

Endothermic - bond breaking (up arrow)

Question 4

What is significant about the ionic solid?

Answer 4

The ions in an ionic lattice are electrostatically bonded by ionic bonds

The standard lattice enthalpy of formation is a measure of the strength of an ionic bond

Question 5

What is standard enthalpy of formation? ΔHθf

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

What is standard enthalpy of combustion? ΔHθc

Answer 6

The enthalpy change when one mole of a substance reacts completely in excess oxygen under standard conditions with all reactants and products in their standard states

Question 7

What is ionisation enthalpy? ΔHθi

Answer 7

The standard enthalpy change when one mole of GASEOUS ATOMS is converted into one mole of GASEOUS IONS with a single POSITIVE charge

Question 8

What is bond dissociation enthalpy? ΔHθdiss

Answer 8

The enthalpy change when one mole of GASEOUS molecules each breaks a covalent bond to form 2 free radicals, averaged over a range of compounds

Question 9

What is enthalpy of atomisation? ΔHθat

Answer 9

The standard enthalpy change that accompanies the formation of one mole of GASEOUS ATOMS from the element in its standard state under standard conditions

Question 10

What is electron affinity? ΔHθea

Answer 10

The standard enthalpy change when one mole of GASEOUS ATOMS is converted into one mole of GASEOUS IONS, each with a single NEGATIVE charge

Question 11

What is the lattice enthalpy of formation? ΔLHθ

Answer 11

The standard enthalpy change that accompanies the formation of one mole of a solid ionic compound from its gaseous ions

Exothermic

Question 12

What is the enthalpy of lattice dissociation? ΔLHθ

Answer 12

The standard enthalpy change that accompanies the separation of one mole of a solid ionic compound into its gaseous ions

Endothermic

Question 13

How can we tell the difference between ΔLHθ with the formation and dissociation of lattices?

Answer 13

We can only tell from the positive or negative values due to bond forming/making therefore exo/endo

Question 14

What is the enthalpy of hydration? ΔhydHθ

Answer 14

The standard enthalpy change for the process where one mole of gaseous ions are surrounded by a maximum of water molecules to produce one mole of hydrated ions

Question 15

What is enthalpy of solution? ΔsolHθ

Answer 15

The standard enthalpy change that occurs when one mole of an ionic solid dissolves completely in sufficient solvent to form a solution in which the molecules or ions are far enough apart not to interact with each other

Question 16

In ionisation enthalpy what must you specify?

Answer 16

Whether it is first or second

Both are endothermic

Question 17

In electron affinity what must you specify?

Answer 17

Whether it is first or second electron affinity

First - exothermic
Second - endothermic

Question 18

Why is the first and second electron affinity exothermic and then endothermic?

Answer 18

First is exothermic as an attraction is formed between a neutral atom and an electron

Second is endothermic as energy needs to be put in to over come the repulsion of two negative ions

Question 19

What are the intermediates in a simple hess cycle for Enthalpy of lattice formation/dissociation?

Answer 19

Elements
The ionic solid

At the top the gaseous ions of the elements

Question 20

What are lattice enthalpy values based on?

Answer 20

The Born-Haber cycles

They are experimental values

Question 21

What is the perfect ionic model?

Answer 21

They are theoretical values

Question 22

What does the perfect ionic model presume?

Answer 22

Assumes there is 100% ionic bonding

The ions are perfect spheres with a uniform distribution of ion charge

Question 23

Why are experimental values and theoretical values of lattice enthalpy different?

Answer 23

In reality ions are perfectly spherical due to different charge densities and electrons aren’t always completely donated
Therefore there can be a degree of covalency
= not perfect ionic bonding

Mor discrepancy = greater degree of covalency

Question 25

What happens to a positive ion in water? (Enthalpy of solution)

Answer 25

The δ- oxygen of the polar water molecule is electrostatically attracted to the positive metal ion

Question 26

What happens to a negative ion in water? (Enthalpy of solution)

Answer 26

The δ+ hydrogen of the polar water molecule is electrostatically attracted to the negative metal ion

Question 27

What is significant about water surrounding ions in solution?

Answer 27

As soon as the gas ions are surrounded they are classed as aqueous But there is a limit to the number of water molecules to surround the ion = infinite dilution

Question 28

Is Enthalpy of solution exothermic or endothermic?

Answer 28

It could be either | Depends on the reaction

Question 29

How does solubility of a solution related to being exo/endothermic?

Answer 29

More exothermic = more soluble More endothermic = less soluble

Question 30

What are the intermediates in a simple hess cycle for Enthalpy of solution?

Answer 30

The ionic solid The gaseous ions The aqueous ions Check the arrow direction!!!

Question 31

Why does enthalpy of hydration decrease down a group?

Answer 31

Ionic radius increases = charge density decreases = the electrostatic attraction to the δ- oxygen of the polar molecule decreases = less exothermic as the attraction is weaker

Question 32

What is entropy?

Answer 32

A measurement of disorder The more disordered something is the more entropy it has Nature likes disorder

Question 33

How does entropy change with states?

Answer 33

Solid (low entropy) Liquid Gas (high entropy)

Question 34

What is the explanation for the entropy graph when increasing temperature- changing state?

Answer 34

Increase in temp Increases kinetic energy Particle start to move/vibrate And there is more disorder But there is greater disorder going from liquid to gas as ALL the particles separate completely

Question 35

How can entropy increase?

Answer 35

More particles from reactants to products eg 2 -> 7 Changing state towards a gas A molecule with more electrons (as they are moving around)

Question 36

What is entropy change?

Answer 36

∆S system = ΣS products - ΣS reactants If ∆S is positive - increase in entropy = increase in disorder

Question 37

What is significant about spontaneity, enthalpy and entropy?

Answer 37

Exothermic changes are spontaneous Endothermic aren’t normally but if they have a high entropy they can be spontaneous

Question 38

When does something have zero entropy?

Answer 38

At absolute 0 (0 kelvin) | Particles have zero energy so entropy is also zero

Question 39

What is Gibbs free energy?

Answer 39

∆G Combines the influence of both enthalpy and entropy The energy available to do useful work

Question 40

What is the Gibbs free energy equation?

Answer 40

∆G = ∆H - T∆S ∆G - Gibbs free energy ∆H - enthalpy change T - Temperature ∆S - entropy change Always write an equation out!

Question 41

How does the value of ∆G affect spontaneity and feasibility?

Answer 41

∆G less than 0 Spontaneous and feasible ∆G = zero Can be feasible or unfeasible ∆G more than 0 Not spontaneous, needs more energy to be feasible

Question 42

When is ∆G = zero?

Answer 42

During a change of state

Question 43

What can we assume when ∆G = zero?

Answer 43

∆H = T∆S So T = ∆H/∆S

Question 44

What do you need to remember with all Gibbs free energy equations?

Answer 44

``` ∆H = KJ mol-1 ∆S = J mol-1 ``` You need to convert ∆S to KJ!!!!

Question 45

How can we link y=mx+c to Gibbs free energy equation?

Answer 45

Y = M x +c ∆G = -∆S T +∆H