Energetics II

Question 1

What is lattice energy?

Answer 1

Enthalpy change when 1 mole of a ionic compounds formed from its gaseous ions (under standard conditions)
- exothermic as energy released when ions combine to form ionic lattice so enthalpy change will be NEGATIVE

E.g . Na+ (g) + Cl- (g) —> NaCl (s)

Question 2

What does a large negative value for lattice enthalpy suggest ?

Answer 2

Ionic compounds are much more stable than its gaseous ions
- due to strong electrostatic forces of attraction between oppositely charged ions in solid lattice

The more exothermic the value is , the stronger the ionic bonds within the lattice are

Question 3

Enthalpy change of atomisation?

Answer 3

enthalpy change when 1 mole of gaseous atoms is formed from its element under standard conditions
- always endothermic as energy is taken in to break any bonds between atoms in element, to break into gaseous atoms (POSITIVE VALUE)

Eg. Na(s) —> Na (g)

Question 4

Electron affinity?

Answer 4

energy change when 1 mole of electrons is gained by 1 mole of gaseous atoms of an element to form 1 mole of gaseous ions under standard conditions

E.g Cl(g) + e- —> Cl- (g)
First electron affinity is ALWAYS EXOTHERMIC as energy is released when electrons attracted to atoms

Second electron affinity can be ENDOTHERMIC :
O-(g) + e- —> O2-(g)
Because large force of repulsion must be overcome between negative ion /second electron