Thermodynamics Flashcards
Standard molar enthalpy of formation
The enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions, all reactants and products in their standard states.
(Exotherimic)
Standard enthalpy of atmoisation
The enthalpy change which accompanies the formation of one mole of gaseous atoms from the element in its standard state under standard conditions. (Endothermic)
First ionisation energy
The standard enthalpy change when one mole of gaseous atoms is converted into a mole of gaseous ions each with a single positive charge. (Endothermic)
Second ionisation energy
Loss of a mole of electrons from a mole of singly positively charged ions. (Endothermic)
The first electron affinity
The standard enthalpy change when a mole of gaseous atoms is converted to a mole of gaseous ions, each with a single negative charge. (Exothermic)
The second electron affinity
The 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. (Endothermic)
Lattice formation enthalpy
The standard enthalpy change when one mole of solid ionic compound is formed from its gaseous ions. (Exothermic)
Enthalpy of lattice dissociation
One mole of ionic compound separates into its gaseous ions. (Endothermic)
Enthalpy of hydration
Standard enthalpy change when water molecules surround one mole of gaseous ions.(Exothermic)
Enthalpy of solution
Standard enthalpy change when one mole of solute 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. (Endothermic)
Mean bond enthalpy
The enthalpy change when one mole of gaseous molecules each breaks a covalent bond to form two free radicals, averaged over a range of compounds.
Why can lattice enthalpies obtained by calculation differ from those from a Born-Haber cycle?
Bond has covalent character
What factors increase polarisation?
- Cation : small size, high charge
2. Anion : large size, high charge
Describe the process of polarisation
- When the small cation approaches the electron clouds of the anion, it distorts the electrons attracting them towards it
- The distortion means that there are more electrons than expected conc. between the nuclei’s
- This represents a degree of covalency which accounts for the lattice enthalpy discrepancy
- The anion is said to be polarised
Why is the anion easy to distort?
Large size means the electrons are far from the nucleus. (maybe double charge)
Lattice enthalpy measures
strength of ionic bond
Bond enthalpies measure
strength of covalent bonding
Define Entropy
The randomness of a system expressed mathematically
Units of Entropy
JK-1 mol -1
Entropy =
Entropy of Products - Reactants
The Gibbs free energy change
The enthalpy change and the entropy change which combined govern the feasibility of a chemical reaction.
G =
H - TS
What happens at G = 0?
The reaction is just feasible. In a closed system an equilibrium exists around this temperature in which both products and reactants are present.
How can G be used to calculate entropy?
When there is a change of state eg. melting point G = 0 so enthalpy and the temperature at melting point will tell you entropy change
What is a good way of extracting metals and why?
Heat them with carbon. Coke is cheap. CO2 diffuses away so there’s no problem of separating it from the metal.
∆H(solution) =
Σ(∆H(hydration)) - ∆H(lattice formation)
or
Σ(∆H(hydration))+ ∆H(lattice dissociation)
Why do many endothermic reactions proceed spontaneously under normal conditions?
because there is an increase in entropy. Some exothermic reactions do not proceed spontaneously because there is a decrease in entropy.
