Module 5: Enthalpy and entropy Flashcards
Define lattice enthalpy.
Enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions under standard conditions.
Always exothermic as when ions are combined to form an ionic solid lattice there is an extremely large release of energy.
Na+(g) + Cl-(g) → NaCl (s)
The more exothermic the value is, the stronger the ionic bonds within the lattice are.
Define first ionisation energy.
Energy needed to remove one mole of electrons from one mole of gaseous atoms, forming a mole of gaseous 1+ ions.
Endothermic as energy is needed to overcome the attraction between an electron and nucleus.
Na(g) → Na+(g) + e-
Define standard enthalpy change of atomisation (may not need to know).
Enthalpy change when 1 mole of gaseous atoms is formed from its element under standard conditions. energy needed to break all the bonds in one mole of a substance (element or compound) and convert it into individual gaseous atoms.
Endothermic as energy is required to break any bonds between the atoms to form gaseous atoms.
Na(s) → Na(g)
½Cl2 (g) → Cl (g)
Define bond enthalpy.
Energy required to break 1 mole of a specific covalent bond in the gas phase.
Cl2 (g) → 2Cl (g)
What is first electron affinity?
One electron is added to each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1- ions.
Exothermic as the electron being added is attracted to the nucleus.
Define enthalpy change of solution.
Dissolving of 1 mol of solute in a solvent.
NaCl(s) + aq → Na+(aq) + Cl-(aq)
Enthalpy of solution can be endo or exothermic.
Define enthalpy change of hydration.
Dissolving of 1 mol of gaseous ions in water to form 1 mole of aqueous ions.
Na+(g) + aq → Na+(aq)
Cl-(g) + aq → Cl-(aq)
Factors affecting lattice enthalpy.
As ionic radius increases, attraction between ions decreases, lattice enthalpy less negative, melting point decreases.
CaO has a higher melting point than Na2O as Ca2+ means as ionic charge increases, attraction between ions increase so lattice enthalpy becomes more negative, melting point increases.
Factors affecting enthalpy of hydration.
As ionic radius increases, attraction between ion and water molecules decrease so hydration is less negative.
As ionic charge increases, attraction with water molecules increase so more negative hydration value.
Predicting solubility?
If the sum of hydration enthalpies is higher than lattice enthalpy, the overall enthalpy change will be exothermic and the compound should dissolve.
What is entropy?
It measures how disordered particles are. The more disorder, the higher the entropy.
Limitations of using delta G? (Gibbs)
Although Gibbs indicate feasibility, it doesn’t take into account the rate of reaction or kinetics.
