17 - Thermodynamics Flashcards
Standard molar enthalpy of formation
Enthalpy change when one mole of substance is formed from its constituent elements under standard conditions, all reactants and products in standard states.
Standard molar enthalpy of combustion
Enthalpy change when one mole of substance is completely burnt in oxygen under standard conditions, all reactants and products in standard states.
What are the standard conditions?
100kPa (1 atmosphere)
298K
Standard enthalpy of atomisation
Enthalpy change due to the formation of one mole of gaseous atoms from the element in its standard state under standard conditions.
1st electron affinity
Standard enthalpy change when a mole of gaseous atoms is converted to a mole of gaseous ions, each with a single negative charge.
2nd electron affinity
Enthalpy change when a mole of electrons is added to a mole of gaseous ions each with a single negative charge to form ions each with two negative charges.
Lattice enthalpy of formation
The standard enthalpy change when one mole of solid ionic compound is formed from its gaseous ions.
Enthalpy of hydration
Standard enthalpy change when water molecules surround one mole of gaseous ions.
First ionisation energy
Standard enthalpy change when one mole of gaseous atoms is converted into one mole of gaseous ions each with a single positive charge.
Second ionisation energy
The standard enthalpy change when one mole of electrons are lost from a mole of singly positive charged ions, forming one of mole of two positive charge.
Enthalpy of solution
Give equation.
Standard enthalpy change when one mole of solute dissolves completely in sufficient solvent to form a solution in which the molecules/ions are far enough apart not to interact with each other.
∆H(sol) = ∆H(lattice diss) + Σ∆H(hydration)
Lattice enthalpy of dissociation
Standard enthalpy change when one mole of solid ionic compound dissociates into its gaseous ions.
Mean bond enthalpy
Standard enthalpy change when one mole of gaseous molecules each breaks a covalent bond to form free radicals, averages over a range of compounds.
What do lattice enthalpies depend on?
- Size of ions (larger = weaker attraction as further apart = less negative)
- Charges on ions (bigger = greater attraction = stronger lattice enthalpy of dissociation = more negative)
Why are Born Haber calculated values (experimental) larger than theoretical values?
Covalent character in bond
What do theoretical lattice enthalpies assume?
Perfect ionic model, with 100% spherical ions and purely electrostatic attraction
Define a spontaneous process.
Words not equation
A process which will proceed on its own without any external influence.
In terms of entropy and order, what do things tend too?
Higher entropy
More disordered
What is entropy a measure of?
The disorder of something, or the number of ways it can be arranged.
Which state has the highest entropy?
Gas, as they are the most disordered.
When does a reaction involve an increase in entropy?
Solid/liquid to gas
Increase in number of molecules
Give an equation for ∆S?
∆S = ΣS(products) - ΣS(reactants)
When does a molecule/atom have 0 entropy?
At 0K
What is the unit of entropy?
J K-1 mol-1
Give Gibbs free energy equation.
What units are these in?
∆G = ∆H - T∆S
kJ mol-1, kJ mol-1, K, J K-1 mol-1
According to Gibbs, when is a reaction feasible?
∆G < 0
How can feasibility points be determined if given enthalpy and entropy changes for a reaction?
∆G = 0
Solve for T.
What can be known about an insoluble substance’s DeltaHSolution? Why?
Lattice enthalpy (to break up molecule) > hydration enthalpy (to coat molecule in water)
Not energetically favourable.
Endothermic DeltaHSolution
