Terms

Question 1

Enthalpy Change (ΔH) def

Answer 1

Amount of heat transferred during a chemical reaction at constant pressure

Question 2

Enthalpy of Formation def

Answer 2

ΔHfꝋ is the enthalpy change when 1 mole0 of a compound is formed from its elements under standard conditions.

By definition, enthalpy of formation of elements in their standard states is 0.

Exo or endo, because sum of bonds broken and formed.

Na(s) + ½Cl2 (g) → NaCl (s) ΔHfꝋ = -411 kJ mol -1

Question 3

What are standard conditions

Answer 3

• 100kPa
• 298K

Question 4

Ionisation enthalpy def

Answer 4

ΔHie ꝋ of an element is the amount of energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to produce 1 mole of gaseous 1+ ions.

Na (g) → Na+ (g) + e– ΔHieꝋ = +500 kJ mol-1

Question 5

Enthalpy of Atomisation def

Answer 5

ΔHatꝋ is the enthalpy change when 1 mole of gaseous atoms is formed from its elements under standard conditions

Always endothermic (energy required to break bonds in atoms in element into gaseous form)

Na (s) → Na (g) ΔHatꝋ = +108 kJ mol -1

½Cl2 (g) → Cl (g) ΔHatꝋ = +121 kJ mol -1

Question 6

Bond enthalpy def

Answer 6

Energy required to break 1 mole of specific covalent bonds in a gas.

Endothermic, energy required to break bonds hence energy taken in. (When doing bond forming, put - in front of values given)

Cl2 (g) → 2Cl (g) E(Cl-Cl) = +242 kJ mol -1

Difference between this and atomisation is 1 mole of bonds are broken here, whereas in atomisation 1 mole of gaseous atoms are formed.

Question 7

Lattice enthalpy formation def

Answer 7

ΔHlattꝋ Enthalpy change when 1 mol of ionic lattice is formed from its constituent ions in gaseous form.

Na+(g) + Cl-(g) →NaCl (s) ΔHlattꝋ = -776 kJ mol -1

Question 8

Lattice enthalpy dissociation def

Answer 8

ΔHlattꝋ enthalpy change when 1 mole of ionic lattice is separated into its constituent ions in gaseous form.

Question 9

Lattice Enthalpy notes

Answer 9

• Greater negative value of ΔHlattꝋ, the stronger the ionic bonds within the lattice are, and the more stable the ionic compound
• Cannot be found directly in 1 experiment, eg born haber

Na+(g) + Cl-(g) →NaCl (s) ΔHlattꝋ = -776 kJ mol -1

Question 10

Electron Affinity definition

Answer 10

ΔHeaꝋ is the enthalpy change when 1 mole of gaseous atoms gains 1 mole of electrons to form 1 mole of gaseous 1- ions.

Cl (g)+ e– → Cl- (g) ΔHeaꝋ = -364 kJ mol-1

First affinity always exothermic, because energy released when electrons are attracted to atoms.

But second affinity can be endothermic, eg.

– (g) + e– → O2- (g) ΔHeaꝋ = +844 kJ mol-1

Because large fore of repulsion must be overcome between negatively charged ion and second electron, which requires a large input of energy.