Enthalpy Definitions

Question 1

Define and give the symbol for Enthalpy of Formation, and state whether it’s endo/exothermic

Answer 1

• (∆H*f)
• Enthalpy change when one mole of a
  substance is formed from its constituent
  elements with all substances in their
  standard states
• Exothermic

Question 2

Define and give the symbol for Enthalpy of combustion, and state whether it’s endo/exothermic

Answer 2

• (∆H*c)
• Enthalpy change when one mole of a
  substance undergoes complete
  combustion in oxygen.
• Exothermic

Question 3

Define and give the symbol for Ionisation Enthalpy, and state whether it’s endo/exothermic

Answer 3

• (∆H*i)
• The first ionisation energy is the
  enthalpy change when one mole of
  gaseous atoms loses one electron per
  atom to produce gaseous 1+ ions.
• endothermic (1st &2nd)

Question 4

Define and give the symbol for electron affinity, and state whether it’s endo/exothermic

Answer 4

• (∆H*ea)
• The first electron affinity is the
  enthalpy change when one mole of
  gaseous atoms gains one electron per
  atom to produce gaseous 1- ions.
• 1st exothermic, 2nd endothermic

Question 5

Define and give the symbol for Enthalpy of atomisation, and state whether it’s endo/exothermic

Answer 5

• (∆H*a)
• Enthalpy change when one mole of
  gaseous atoms is produced from an
  element in its normal state.
• endothermic

Question 6

Define and give the symbol for hydration enthalpy, and state whether it’s endo/exothermic

Answer 6

• (∆H*hyd)
• Enthalpy change when one mole of
  gaseous ions become hydrated
  (dissolved in water / surrounded by
  water molecules).
• exothermic

Question 7

Define and give the symbol for enthalpy of solution, and state whether it’s endo/exothermic

Answer 7

• (∆H*sol)
• Enthalpy change when one mole of an
  ionic solid dissolves in an amount of
  water large enough so that the
  dissolved ions are well separated and do
  not interact with each other.
• endothermic

Question 8

Define and give the symbol for bond dissociation enthalpy, and state whether it’s endo/exothermic

Answer 8

• (∆H*dis)
• Enthalpy change when one mole of
  covalent bonds is broken in the gaseous
  state.
• endothermic

Question 9

Define and give the symbol for lattice enthalpy of formation, and state whether it’s endo/exothermic

Answer 9

• (∆H*L) (OR LEF)
• Enthalpy change when one mole of a
  solid ionic compound is formed from
  into its constituent ions in the gas
  phase
• Exothermic

Question 10

Define and give the symbol for lattice enthalpy of dissociation, and state whether it’s endo/exothermic

Answer 10

• (∆H*L) (or LED)
• Enthalpy change when one mole of a
  solid ionic compound is broken up into
  its constituent ions in the gas phase
• endothermic

Question 11

Why is second electron affinity endothermic when first is exothermic?

Answer 11

• the ion is already negative with a 1- charge
• this charge repels the negatively charged electron it needs to gain
• therefore requires a lot of energy to add the electron