Energetics/Thermochemistry

Question 1

Enthalpy changes during formation of ionic compounds

Answer 1

1. Formation of free (g) atoms frm elements, std state
2. Form ions (add/remove e-)
3. Form ionic compound (attraction of ions)

Question 2

Enthalpy change of atomization

Answer 2

• energy absorbed
• one mole of (g) atoms formed frm element
• std conditions
• Metals: Strong metallic bond broken
• Non-metals: Strong covalent bonds broken
• always +ve (endothermic) as energy needed to break bonds

Question 3

Ionization energy

Answer 3

• IE is always +ve (endo)
• energy required to overcome attractive force between +ve charged nucleus and electron being removed
• lower IE value, easier to form cation
• 2nd IE > 1st IE
• electrostatic attraction forces between remaining electrons and +ve nucleus stronger than 1st IE

Question 4

1st IE

Answer 4

• energy needed to remove 1 mol of e-
• from 1 mol of (g) atoms
• form 1 mol of (g) cations
• under std conditions
• e.g. Mg (g) –> Mg+ (g) + e-

Question 5

2nd IE

Answer 5

• energy needed to remove 1 mol of e-
• from 1 mol of free (g) unipositive ions of that element
• e.g. Mg+ (g) –> Mg2+(g) + e-

Question 6

1st Electron affinity (EA) (Non-metal)

Answer 6

• enthalpy change when 1 mol of (g) atoms gain 1 mole of e-
• form 1 mol of (g) anions
• e.g. O(g) + e- –> O- (g)

Question 7

2nd electron affinity (EA)

Answer 7

• energy change when 1 e- added
• to each mol of free (g) uninegative ions of that element
• O- (g) + e- –> O2-

Question 8

Electron affinity

Answer 8

• 1st EA exothermic, 2nd EA endothermic
• 2nd EA: electronic repulsion when 2nd electron added to a negative ion
• more electronegative the element, more exo the 1st EA

Question 9

Lattice enthalpy

Answer 9

• LE of ionic compound is the enthalpy change
• when 1 mol of ionic solid
• separated into its (g) ions
• under std conditions
• cannot be determined directly from experiments
• formation of ionic crystals frm (g) ions cannot be done in lab
• LE calculated from other experimentally determined enthalpy changes using Born-Haber cycle

Question 10

Born-Haber cycle

Answer 10

• enthalpy cycle
• calculate the std enthalpy change of a particular step or stage
• in formation of ionic compound