5.2.1 Lattice enthalpy Flashcards
Define enthalpy of formation
The enthalpy change when 1 mole of a compound is formed from it’s constituent elements (in their standard states under standard conditions)
Is the enthalpy of formation exo or endothermic?
Exo/endothermic
What is the enthalpy of formation formula of NaCl (s)?
Na (s) + 1/2 Cl (g) –> NaCl (s)
Define first ionisation enthalpy
The energy needed to remove 1 electron from each atom in a mole of gaseous atoms to produce one mole of gaseous 1+ ions
Define second ionisation enthalpy
The energy needed to remove a second electron from each ion in a mole of gaseous 1+ ions to produce 1 mole of gaseous 2 + ions
Is ionisation enthalpy exo or endothermic?
Endothermic
What is the first ionisation enthalpy of Na (g)?
Na (g) –> Na+ (g) + e-
What is the second ionisation enthalpy of Na + (g)?
Na + (g) –> Na 2+ (g) + e-
Define first electron affinity
The enthalpy change when 1 mole of gaseous atoms gains 1 electron per atom to produce gaseous 1 - ions
Define second electron affinity
The enthalpy change when 1 mole of gaseous 1 - ions gain 1 electron per ion to produce gaseous 2 - ions
Is first electron affinity exo or endothermic?
Exothermic
Is second electron affinity exo or endothermic?
Endothermic
Why is the second electron affinity positive?
An electron is added to an already negative ion so energy is needed to overcome the repulsion
What is the first electron affinity of Cl (g)?
Cl (g) + e - –> Cl - (g)
What is the second electron affinity of O - (g)?
O - (g) + e - –> O 2- (g)
Define enthalpy of atomisation
The enthalpy change when 1 mole of gaseous atoms is produced from an element in its standard state
Is the enthalpy of atomisation exo or endothermic?
Exothermic
What is the enthalpy go atomisation of Na (s)?
Na (s) –> Na (g)
What is the enthalpy go atomisation of 1/2 Cl 2 (g)?
1/2 Cl 2 (g) –> Cl (g)
Define lattice enthalpy
The enthalpy change of formation of 1 mole of a solid ionic compound from its gaseous ions under standard conditions
Is lattice enthalpy exo or endothermic?
Exothermic
What is the lattice enthalpy of K+ (g) and Cl- (g)?
K+ (g) and Cl- (g) –> KCl (s)f
Define enthalpy change of solution
The enthalpy change when 1 mole of an ionic solid compound is completely dissolved in water
Is the enthalpy change of solution exo or endothermic?
Exo/endothermic
What is the enthalpy change of solution for MgCl2 (s)?
MgCl2 (s) + aq –> Mg2+ (aq) + 2Cl- (aq)
What is the enthalpy change of solution for Na+Cl- (s)?
Na+Cl- (s) + aq –> Na+ (aq) + Cl- (aq)
How do you calculate the enthalpy change of solution?
Enthalpy change of solution = hydration enthalpies - lattice enthalpy
How do we determine if a substance is soluble or not?
By comparing the lattice enthalpy with the sum of the hydration energies of the ions
- If the LE is greater, solid doesn’t dissolve
- If the sum of hydration energies is grater, solid will dissolve
What two factors affect charge density?
Ionic radius
Ionic charge
How does charge density increase?
Ionic radius decreases
Ionic charge increases
Define enthalpy change of hydration
The enthalpy change when 1 mole of isolated gaseous ions are dissolved in water to form 1 mole of aqueous ions
Is hydration enthalpy exo or endothermic?
Exothermic
What is the hydration enthalpy of Na + (g)?
Na+ (g) + aq –> Na+ (aq)
What is the hydration enthalpy of Cl - (g)?
Cl- (g) + aq –> Cl- (aq)
What makes the lattice enthalpy more exothermic?
Greater the attraction between ions, the stronger the ionic bonding and greater the charge density
Why is the hydration enthalpy of Na+ lower than that of Mg2+?
Mg2+ has a higher ionic charge and a smaller ionic radius. Mg2+ has a higher charge density than Na+ and therefore, a stronger electrostatic force of attraction on the water molecules. More energy is released so hydration enthalpy of Mg2+ is more exothermic
Describe and explain the factors that affect lattice enthalpy values
Higher ionic charge and smaller ionic radius = a higher charge density. Therefore, there is stronger electrostatic forces of attraction on oppositely charged ion so more energy released. More exothermic LE
How do you work out the lattice enthalpy from a born harber cycle?
Lattice enthalpy = enthalpy of formation - other enthalpies
In a born harber cycle, an upwards arrow shows what energy change?
Endothermic - positive
In a born harber cycle, a downwards arrow shows what energy change?
Exothermic - negative
