1.8 Thermodynamics Flashcards
Define Enthalpy change of formation.
The standard enthalpy change of formation of a compound is the energy transferred when 1 mole of the compound is formed from its elements under standard conditions (298 K and 100 kpa), all reactants and products being in their standard states
Define Enthalpy of atomisation
The enthalpy of atomisation of an element is the enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state
Define Bond dissociation enthalpy (bond energy)
The bond dissociation enthalpy is the standard molar enthalpy change when one mole of a covalent bond is broken into two gaseous atoms (or free radicals).
For diatomic molecules the dissH of the molcule is the same as 2x atH of the element
Define First ionisation enthalpy
The first ionisation enthalpy is the enthalpy change required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous ions with a +1 charge
Define Second ionisation enthalpy
The second ionisation enthalpy is the enthalpy change to remove 1 mole of electrons from one mole of gaseous 1+ ions to produces one mole of gaseous 2+ ions.
Define First electron affinity
The first electron affinity is the enthalpy change that occurs when 1 mole of gaseous atoms gain 1 mole of electrons to form 1 mole of gaseous ions with a –1 charge
The first electron affinity is exothermic for atoms that normally form negative ions. This is because the ion is more stable than the atom, and there is an attraction between the nucleus and the electron.
Define Second electron affinity
The second electron affinity is the enthalpy change when one mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions.
The second electron affinity for oxygen is endothermic because it take energy to overcome the repulsive force between the negative ion and the electron.
Define Enthalpy of lattice formation
The enthalpy of lattice formation is the standard enthalpy change when 1 mole of an ionic crystal lattice is formed from its constituent ions in gaseous form.
Define Enthalpy of lattice dissociation
The enthalpy of lattice dissociation is the standard enthalpy change when 1 mole of an ionic crystal lattice form is separated into its constituent ions in gaseous form.
Define Enthalpy of hydration hyd H
Enthalpy change when one mole of gaseous ions become aqueous ions.
This always gives out energy (exothermic, -ve) because bonds are made between the ions and the water molecules.
Define Enthalpy of solution
The enthalpy of solution is the standard enthalpy change when one mole of an ionic solid dissolves in a large enough amount of water to ensure that the dissolved ions are well separated and do not interact with one another.
Draw the born haber cycle for: Sodium Chloride
Chem revise page 2
Draw the born haber cycle for: magnesium chloride
Chem revise page 2
Draw the born haber cycle for: Calcium oxide
Chem revise page 3
What does the strength of an enthalpy of lattice formation depend on?
The sizes of the ions
The larger the ions, the less negative the enthalpies of lattice formation (i.e. a weaker lattice). As the ions are larger the charges become further apart and so have a weaker attractive force between them.
The charges on the ion
The bigger the charge of the ion, the greater the attraction between the ions so the stronger the lattice enthalpy (more negative values).
