13: Energentics II
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What is lattice formation enthalpy?
The standard enthalpy change when one mole of an ionic compound is formed from its constituent ions in the gaseous state
Why does lattice energy have a high negative value (exothermic)?
There is a release of energy when the ionic crystal is formed from free gaseous ions
What happens as the lattice enthalpy increases?
1) the strength of the ionic bond increases 2) the ionic lattice is more stable
What factors affect the strength of the ionic lattice?
Charge and size, in other words charge density
How do you calculate charge density?
Charge / volume = Charge density
The lattice enthalpy and stability increases as….
….charge density of ions increase and distance between ions gets smaller
What is the standard enthalpy change of atomisation?
The enthalpy change when one mole of gaseous atoms is formed from its elements in its standard state under standard conditions.
Why are all enthalpies of atomisation positive (endothermic)?
1) to overcome the attraction of nuclei for the electrons being shared in covalent bond(s) 2) to vaporise solid or liquid elements
What is electron affinity?
The enthalpy change when one mole of electrons is added one mole of atoms or ions in the gaseous state to form -1 ions
Why is first electron affinity always negative whilst second and third electron affinity is always positive?
Because it become difficult to force a negative electron onto an already negative ion, due to electrostatic repulsion
What are the assumptions when calculating theoretical lattice energy?
1) Ions are in contact with each other 2) Ions are perfectly spherical 3) Charge on each ion is evenly distributed
What does the difference between measured lattice enthalpy and theoretical lattice enthalpy suggest?
The smaller the difference, the closer to having a perfect ionic structure The bigger the difference, the more covalent character (distorted ions)
In an ionic bond, when will the covalent character increase?
When the power of cation increases or the size of the anion increases
What does the extent of covalent character depend on?
The polarisation of the anion by the cation
Why does it mean to have a covalent character?
The cation pulls back the electrons lost to the anion and so the bond is less ionic and more covalent
What is lattice dissociation enthalpy?
The enthalpy change when one mole of a solid ionic compound is completely dissociated into its gaseous constituent ions under standard conditions
How are pure ionic compounds formed?
By the complete transfer of electron from a metallic atom to a non-metallic atom
What increases polarising power?
Increase in charge or decrease in size
What is the enthalpy change of solution?
The enthalpy change of solution is the enthalpy change when one mole of an ionic solid is dissolved completely in water to form an infinite dilute solution
What is the enthalpy change of hydration?
The enthalpy change when one mole of ion in gaseous state is hydrated completely by water
Why is the change in hydration enthalpy of an ionic always exothermic?
Because the electrostatic energy of stabilisation is released
What makes the enthalpy value of hydration more negative?
The smaller the ion and the higher its charge, the stronger it attracts water molecules
How do you construct a Born–Haber Cycle?
1) Write the enthalpy of formation equation (solid compound from elements). 2) Convert elements to gaseous atoms (atomisation enthalpy). 3) Remove electrons from metal atoms (ionisation energy). 4) Add electrons to non-metal atoms (electron affinity). 5) Combine gaseous ions to form lattice (lattice enthalpy of formation) on the right
What does a more negative lattice enthalpy mean?
More stronger electrostatic attraction between ions.