Thermodynamics

Question 1

define enthalpy change of formation

Answer 1

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 (298K and 100kpa), all
reactants and products being in their standard states

Question 2

define enthalpy of atomisation

Answer 2

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

Question 3

define bond dissociation enthalpy (bond energy)

Answer 3

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)

Question 4

define enthalpy of sublimation

Answer 4

The enthalpy change for a solid metal turning to
gaseous atoms can also be called the Enthalpy of
sublimation and will numerically be the same as the
enthalpy of atomisation

Question 5

define first ionisation ethalpy

Answer 5

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

Question 6

define second ionisation enthalpy

Answer 6

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.

Question 7

define first electron affinity

Answer 7

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

Question 8

define second electron affinity

Answer 8

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.

Question 9

define enthalpy of lattice formation

Answer 9

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.

Question 10

define enthalpy of lattice dissocation

Answer 10

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

Question 11

define enthalpy of hydration

Answer 11

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.

Question 12

define enthalpy of solution

Answer 12

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

Question 13

why do we use born Haber cycles?

Answer 13

The lattice enthalpy cannot be determined directly. We calculate it indirectly by making use of changes for
which data are available and link them together in an enthalpy cycle the Born-Haber cycle

Question 14

what do you do if the question gives you the enthalpy of lattice dissociation instead of formation?

Answer 14

This Born Haber cycle has been constructed using a lattice enthalpy of formation.
Sometimes questions will give the enthalpy of lattice dissociation which has the opposite sign and the arrow points in the opposite direction.
This changes the calculation.

Question 15

write the formula for calculating the lattice enthalpy of formation

Answer 15

ΔfH =ΔatH (Na) + ΔIEH(Na)+ ΔatH(Cl) + ΔeaH(Cl) + ΔLattH