thermodynamics

Question 1

What does Hess’s Law state?

Answer 1

• the enthalpy change for a reaction is independent of the route taken .

Question 2

Define standard enthalpy of formation. (ΔHf)

Answer 2

• the enthalpy change when one mole of a compound is formed from its constituent elements in standard conditions, with all products + reactants in standard states.
• e.g = Na (s) + 1/2Cl2 (g) —> NaCl (s)

Question 3

Define standard enthalpy of combustion. (ΔH°c)

Answer 3

• the enthalpy change when one mole of a substance is completely burnt in (excess) oxygen.

Question 4

Define enthalpy of atomisation. (ΔHat)

Answer 4

• the enthalpy change when one mole of gaseous atoms is formed from a compound in its standard state in standard conditions (endothermic .)

e.g = Na (s) —> Na (g)

Question 5

Define first ionisation energy. (ΔHi)

Answer 5

the enthalpy change when one mole of electrons is removed from one mole of singly charged ions —> produce doubly charged ions - second ionisation energy.

1st = M(g) —> M(g) + e-
2nd = M (g) —> M^2+ (g) + e-

Question 6

Define the first electron affinity. (EA1)

Answer 6

• enthalpy change when one mole of gaseous atoms gain one mole of electrons to form one mole of gaseous 1- ions.

e.g - Cl (g) + e- —> Cl- (g)

Question 7

Define second electron affinity. (EA2)

Answer 7

• enthalpy change when one mole of gaseous 1- ions gain one mole of electrons to form one mole of gaseous 2- ions.

Question 8

Define lattice enthalpy of formation. (ΔH ⦵ LE)

Answer 8

• enthalpy change when one mole of solid ionic compound/ crystalline compound formed from its constituents gaseous ions
• scattered infinite distance apart
  M^+ (g) + X^- (g) —> MX (s)

Question 9

Define lattice enthalpy of dissociation. (ΔH°)

Answer 9

• enthalpy change when one mole of solid ionic lattice is broken up to produce gaseous ions in an infinite distance apart .
• e.g = MX (s) —> M^+ (g) + X^- (g)

Question 10

what does the value of a lattice enthalpy depend on?

Answer 10

• the charges on the ions
• the size of the ions
• the type of lattice formed (pattern which they pack together)

Question 11

Define mean bond dissociation enthalpy

Answer 11

enthalpy change when one mole of (a certain type of) covalent bonds is broken, with all species in the gaseous state.

Question 12

Define mean bond dissociation enthalpy

Answer 12

enthalpy change when one mole of (a certain type of) covalent bonds is broken, with all species in the gaseous state.