thermodynamics definitions

Question 1

Enthalpy change of formation (ΔH f)

Answer 1

• (exothermic mostly) enthalpy change when one mole of a substance is formed from its constituent elements in their standard states.
  2Na(s) + 0.5O2(g) → Na2O(s)

Question 2

Enthalpy change of combustion (ΔH°c)

Answer 2

-(exothermic) enthalpy change when one mole of a substance is burned completely in oxygen with all substances in their standard states.
H2(g) + 0.5O2(g) → H2O(g)

Question 3

Enthalpy change of neutralisation (ΔneutH)

endo or exo?

Answer 3

• (exothermic) enthalpy change when 1 mole of a substance is formed in a reaction between an acid and alkali under standard conditions
  0. 5 H2SO4(aq) + NaOH(aq) → 0.5Na2SO4(aq) + H2O(l)

Question 4

ionisation enthalpy ΔieH

Answer 4

First ionisation energy: (endothermic) enthalpy change when one mole of gaseous atoms loses an electron to form a mole of gaseous 1+ ions
Second ionisation energy: (endothermic)
enthalpy change when each ion in one mole of gaseous 1+ ion loses one electron to form one mole of gaseous 2+ ion
Mg(g) → Mg+(g) + e-
Mg(g) → Mg2+ (g) + e-

Question 5

electron affinity ΔeaH ( 1 + 2 )

endo or exo

Answer 5

First electron affinity: (exothermic) enthalpy change when each atom in one mole of gaseous atoms gains an electron to form one mole of gaseous 1- ions
Second electron affinity: (endothermic) enthalpy change when each ion in one mole of gaseous 1- ions gains one electron to form one mole of gaseous 2- ions
O(g) + e- → O- (g)
O-(g) + e- → O2- (g)

Question 6

enthalpy of atomisation ΔatH

endo or exo?

Answer 6

(endothermic) enthalpy change when one mole of gaseous atoms is produced from an element in standard state
0. 5I2(s) → I(g)

Question 7

Hydration enthalpy ΔhydH

endo or exo?

Answer 7

(exothermic)
enthalpy change when one mole of gaseous ions become hydrated (dissolved in water)
Mg2+(g) + aq → Mg2+(aq)

Question 8

enthalpy of solution ΔsolH

Answer 8

(varies) 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 eachother
MgCl2(s) + aq → Mg2+(aq) + 2Cl- (aq)

Question 9

Lattice enthalpy of formation Δ LEFH

Answer 9

enthalpy change when one mole of a solid ionic compound is formed into its constituent ions in the gas phase
Mg2+(g) + 2Cl-(g) → MgCl2(s)

Question 10

Enthalpy of vaporisation ΔvapH

Answer 10

enthalpy change when one mole of a liquid is turned into a gas
H2O(l) → H2O(g)

Question 11

Bond dissociation enthalpy ΔLEDH

Answer 11

enthalpy change when one mole of a solid ionic compound is broken up into its constituent ions in the gas state
I2(g) → 2 I(g)

Question 12

enthalpy of fusion

ΔfusH

Answer 12

enthalpy change when one mole of a solid is turned into a liquid

Mg(s) → Mg(l)

Question 13

mean bond enthalpy

Answer 13

the average amount of energy needed to break a specific type of bond, measured over a wide variety of different molecules

Question 14

enthalpy change

Answer 14

the heat change at a constant pressure

Question 15

bond enthalpy

Answer 15

the energy required to break a particular covalent bond in one mole of molecule in a gaseous state.