1.8 Thermodynamics Flashcards
What is a Born-Haber cycle?
An application of Hess’ law
Where there are multiple steps in one leg of the cycle
How are ionic solids produced?
Between ions in the gas phase
What is exothermic and endothermic in terms of bonds? Arrow representation?
Exothermic - bond forming (down arrow)
Endothermic - bond breaking (up arrow)
What is significant about the ionic solid?
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
What is standard enthalpy of formation? ΔHθf
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
What is standard enthalpy of combustion? ΔHθc
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
What is ionisation enthalpy? ΔHθi
The standard enthalpy change when one mole of GASEOUS ATOMS is converted into one mole of GASEOUS IONS with a single POSITIVE charge
What is bond dissociation enthalpy? ΔHθdiss
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
What is enthalpy of atomisation? ΔHθat
The standard enthalpy change that accompanies the formation of one mole of GASEOUS ATOMS from the element in its standard state under standard conditions
What is electron affinity? ΔHθea
The standard enthalpy change when one mole of GASEOUS ATOMS is converted into one mole of GASEOUS IONS, each with a single NEGATIVE charge
What is the lattice enthalpy of formation? ΔLHθ
The standard enthalpy change that accompanies the formation of one mole of a solid ionic compound from its gaseous ions
Exothermic
What is the enthalpy of lattice dissociation? ΔLHθ
The standard enthalpy change that accompanies the separation of one mole of a solid ionic compound into its gaseous ions
Endothermic
How can we tell the difference between ΔLHθ with the formation and dissociation of lattices?
We can only tell from the positive or negative values due to bond forming/making therefore exo/endo
What is the enthalpy of hydration? ΔhydHθ
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
What is enthalpy of solution? ΔsolHθ
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
In ionisation enthalpy what must you specify?
Whether it is first or second
Both are endothermic
In electron affinity what must you specify?
Whether it is first or second electron affinity
First - exothermic
Second - endothermic
Why is the first and second electron affinity exothermic and then endothermic?
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
What are the intermediates in a simple hess cycle for Enthalpy of lattice formation/dissociation?
Elements
The ionic solid
At the top the gaseous ions of the elements
What are lattice enthalpy values based on?
The Born-Haber cycles
They are experimental values
What is the perfect ionic model?
They are theoretical values
What does the perfect ionic model presume?
Assumes there is 100% ionic bonding
The ions are perfect spheres with a uniform distribution of ion charge
Why are experimental values and theoretical values of lattice enthalpy different?
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
What happens to a positive ion in water? (Enthalpy of solution)
The δ- oxygen of the polar water molecule is electrostatically attracted to the positive metal ion
What happens to a negative ion in water? (Enthalpy of solution)
The δ+ hydrogen of the polar water molecule is electrostatically attracted to the negative metal ion
What is significant about water surrounding ions in solution?
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
Is Enthalpy of solution exothermic or endothermic?
It could be either
Depends on the reaction
How does solubility of a solution related to being exo/endothermic?
More exothermic = more soluble
More endothermic = less soluble
What are the intermediates in a simple hess cycle for Enthalpy of solution?
The ionic solid
The gaseous ions
The aqueous ions
Check the arrow direction!!!
Why does enthalpy of hydration decrease down a group?
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
What is entropy?
A measurement of disorder
The more disordered something is the more entropy it has
Nature likes disorder
How does entropy change with states?
Solid (low entropy)
Liquid
Gas (high entropy)
What is the explanation for the entropy graph when increasing temperature- changing state?
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
How can entropy increase?
More particles from reactants to products eg 2 -> 7
Changing state towards a gas
A molecule with more electrons (as they are moving around)
What is entropy change?
∆S system =
ΣS products - ΣS reactants
If ∆S is positive - increase in entropy = increase in disorder
What is significant about spontaneity, enthalpy and entropy?
Exothermic changes are spontaneous
Endothermic aren’t normally but if they have a high entropy they can be spontaneous
When does something have zero entropy?
At absolute 0 (0 kelvin)
Particles have zero energy so entropy is also zero
What is Gibbs free energy?
∆G
Combines the influence of both enthalpy and entropy
The energy available to do useful work
What is the Gibbs free energy equation?
∆G = ∆H - T∆S
∆G - Gibbs free energy
∆H - enthalpy change
T - Temperature
∆S - entropy change
Always write an equation out!
How does the value of ∆G affect spontaneity and feasibility?
∆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
When is ∆G = zero?
During a change of state
What can we assume when ∆G = zero?
∆H = T∆S
So
T = ∆H/∆S
What do you need to remember with all Gibbs free energy equations?
∆H = KJ mol-1 ∆S = J mol-1
You need to convert ∆S to KJ!!!!
How can we link y=mx+c to Gibbs free energy equation?
Y = M x +c
∆G = -∆S T +∆H
