3.6 Enthalpy Changes Flashcards
Define lattice enthalpy
Enthalpy change when one mole of an ionic compound is formed from its gaseous ions under standard conditions
What factors impact lattice enthalpy?
- ionic radius
- nuclear charge
- charge density
- electron shielding
What is meant by the biggest?
The most exothermic / most negative
Why can’t lattice enthalpies be calculated directly?
- it is impossible to form exactly one mole of an ionic compound from gaseous ions
What are the steps to forming lattices?
- atomisation
- ionise
- ionise
- lattice enthalpy
What happens to lattice enthalpy as you descend a group?
- increases because ionic radius increases, so charge density decreases
Define Hess’ law
The energy change of any chemical reaction is the same regardless of the route tsken
Define enthalpy change of atomisation
Enthalpy change that occurs when one mole of gaseous atoms is formed under standard conditions
Define enthalpy change of ionisation
Energy required to remove one mole of electrons from one mole of atoms in their gaseous state under standard conditions
Define enthalpy change of electron affinity
The enthalpy change when one mole of gaseous 1- ions are formed from gaseous atoms
Define the enthalpy change of solution
The enthalpy change when one mole of a compound dissolves in water under standard conditions
Define the enthalpy change of hydration
The enthalpy change that occurs when one mole of aqueous ions are formed from gaseous ions under standard conditions
What does a negative enthalpy change of formation mean?
The ionic compound is more stable than the elements from which it is formed
What does a positive enthalpy change of formation mean?
Ionic compound is unstable compared to its elements
Why is the second ionisation more endothermic than the first?
- X+ is smaller than X so there is a greater nuclear attraction, resulting in a greater ionisation energy
Suggest why the second electron affinity of oxygen is positive
Oxide ion and electron are both negative so energy is required to overcome the repulsion
Why is the first ionisation endothermic?
Lots of energy needed to overcome the force of attraction between outer electrons and nucleus
Explain why the first electron affinity for oxygen is exothermic
Electron affinity involves an electron experiencing attraction to the nucleus
Suggest a reason for the difference in lattice enthalpy between calcium
oxide and iron(II) oxide.
- ionic radius of iron is less than that of a calcium ion so has a greater charge density
Describe how, and explain why, the lattice enthalpy of magnesium fluoride
differs from that of calcium chloride.
- MgF2 is more exothermic than CaCl2
- ionic radius Mg2+ less than Ca2+
- stronger attraction in MgF2 than CaCl2
