Entropy and Free Energy
No description yet. This curated deck is ready for daily study.
Card preview (24)
What does lattice enthalpy measure?
The strength of bonding in a giant ionic lattice.
Why do ionic compounds typically have very high melting/boiling points?
The electrostatic attractions between oppositely-charged ions are very strong and require lots of thermal energy to overcome.
Define lattice enthalpy.
The energy change associated with the formation of one mole of an ionic compound from its’ elements under standard conditions.
Why is lattice enthalpy always negative (exothermic)?
Energy is released when bonds form.
What type of cycle can we use to indirectly determine lattice enthalpy?
Born-Haber cycle.
Define standard enthalpy of formation.
The enthalpy change associated with the formation go one mole of a compound from its’ elements under standard conditions.
Define standard enthalpy of atomisation.
The enthalpy change associated with the formation of one moleof gaseous atoms from their elements.
Why is atomisation always an endothermic process?
Energy is required to overcome bonds within molecules.
Why are ionisation energies endothermic?
Energy is required to overcome electrostatic attractions to remove an electron from an atom.
What is the difference between electron affinity and ionisation energy?
They are opposites - electron affinity measured enthalpy change when atoms are gained, whereas ionisation energy refers to their removal.
Define first electron affinity.
The enthalpy change associated with the addition of one electron to each atom in a mole of gaseous atoms to form 1- ions.
Why is the first electron affinity exothermic, whilst second affinity is endothermic?
For first affinity, the electron experiences a strong electrostatic attraction to the nucleus - so energy is released due to the lower-potential arrangement that forms. However, electron-electron repulsion takes greater influence for second affinity; energy must be input to overcome this.
Describe the two major routes from elements to a lattice in a Born-Haber cycle.
Route 1: formation of gaseous atoms, then ionisation, then lattice enthalpy Route 2: formation from elements
Why can enthalpy of solution be either exo- or endothermic?
Dissolving an ionic compound depends on two processes: breaking up the lattice to form gaseous ions (reverse of lattice energy, always endothermic), then surrounding these with water molecules (exothermic). The balance between these enthalpies describes the overall change.
Define enthalpy of hydration.
The enthalpy change where 1 mol of gaseous ions is dissolved in water to form 1 mol of aqueous ions.
Define enthalpy of solution.
The enthalpy change associated with dissolving 1 mol of an ionic compound in water.
What happens when an ionic compound dissolves in water?
The lattice breaks up releasing aqueous ions which are attracted electrostatically to polar water molecules.
What enthalpy changes are combined to determine enthalpy of solution indirectly?
Enthalpy of hydration and lattice enthalpy.
Describe how solubility is affected by the value of enthalpy of solution.
The more negative the enthalpy of solution, the more readily an ionic compound dissolves. Large +ve values indicate insolubility, where small +ve values may indicate some solubility if there is a sufficient increase an entropy.
What mass do we use when determining enthalpy of solution?
Mass of solute + solvent.
How is lattice enthalpy affected by ionic radius?
Down the group, ionic radius increases The electrostatic attraction between oppositely charged ions becomes weaker as they are held further apart Lattice enthalpy becomes less negative as less work must be done to overcome weaker attractions
How does ionic charge impact lattice enthalpy?
Electrostatic attraction between oppositely charged ions becomes stronger Requiring more energy to overcome So lattice enthalpy is more -ve
What is entropy?
A measure of the dispersal of energy in a system, which is greater the more disordered a system is.
Describe the tend in entropy during all chemical reactions.
There is always a tendency towards greater entropy (greater disorder).