O abc: Enthalpies of ionic lattices & solutions Flashcards
How is lattice enthalpy represented symbolically?
ΔLEH
Define lattice enthalpy.
The enthalpy change when one mole of ionic solid is formed by the coming together of gaseous anions and cations.
Usually 1 mole of each ion, except in cases like Mg(OH)2
ΔLEH:
- Exothermic or endothermic?
- or -?
- Which way does the arrow point?
- Exothermic (ionic bonds formed)
- Negative
- Gaseous ions to ionic solid (down)
The stronger the lattice, the more exothermic ΔLEH is
What factors increase lattice enthalpy?
Stronger ionic forces due to increased charge density:
- Increased charge
- Decreased ionic radii
State and explain which of MgO and Na2O has the more exothermic lattice enthalpy.
MgO
- Mg2+ has the same ionic radius as, but has a higher charge than, Na+
- So Mg2+ has a higher charge density
- So attracts O2- more strongly
- So MgO has the more exothermic lattice enthalpy
State the bonds broken and made when an ionic substance dissolves in water.
Broken: ionic + hydrogen bonds between water molecules
Made: ion-dipole
Draw diagrams of the hydrated ions in NaCl(aq)
Draw 4 water molecules around each ion
What factor increases the size of a hydrated ion?
Increased charge density:
- Increased charge
- Decreased ionic radii
The higher an ion’s charge density, the more water molecules it attracts.
How is the enthalpy change of hydration represented symbolically?
ΔhydH
ΔhydrationH
Define the enthalpy change of hydration.
The enthalpy change when one mole of gaseous ions form a very dilute solution in water.
Values quoted depend on concentration of solution
ΔhydH:
- Exothermic or endothermic?
- or -?
- Exothermic (gaseous ions form bonds with water)
- Negative
What factors make the value of ΔhydH more exothermic?
Higher charge density:
- Smaller ionic radii
- Greater charge
What is the equivalent of ΔhydH when considering solvents other than water?
Enthalpy of solvation: ΔsolvH
How is enthalpy change of solution represented symbolically?
ΔsolutionH
ΔsolnH
Define the enthalpy change of solution.
The enthalpy change when one mole of solute dissolves in water to form a very dilute solution.