5.2.2 Enthalpy and entropy

Define lattice enthalpy?

• the enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions under standard conditions

what happens to lattice enthalpy when ionic radius increases?

• lattice enthalpy becomes less negative as the attraction between ions decreases

• melting point decreases

define enthalpy change of atomisation?

• the enthalpy change when 1 mole of gaseous ions are formed from the element in its standard state under standard conditions

define first ionization energy?

• the enthalpy change required to remove 1 electron from each atom in 1 mol of gaseous atoms to form 1 mol of gaseous 1+ ions

• Na(g)→Na+(g) +e-

define first electron affinity?

• the enthalpy change when 1 electron is added to each atom in 1 mole of gaseous atoms to form 1 mol of gaseou 1- ions

• Cl(g) + e- → Cl-(g)

why are first electron affinities exothermic?

• because the electron added is attracted to the positive nucleus

why are second electron affinities endothermic?

• a second ion is being gained by a negative ion so the electron is repelled

• energy must be put in to force

Define standard enthalpy change of solution?

• the enthalpy change when 1 mol of an ionic compound dissolves in a solvent

is enthalpy change of solution endothermic or exothermic?

• can be either

state what occurs when an ionic compound dissolves in water?

• the ionic lattice breaks up forming separate gaseous ions

• the separate gaseous ions interact with polar water molecules to form hydrated aqueous ions

define standard enthalpy change of hydration?

• the enthalpy change when 1 mole of gaseous ions dissolve in water to form 1 mole of aqueous ions

what 2 factors influence enthalpy change of hydration?

• charge

• size of ion

state how a higher ionic charge influences enthalpy change of hydration?

• ions with a higher charge attract water molecules more strongly

• more ebergy is released when a bond is made

• more exothermic enthalpy of hydraton

• th larger the charge the greater the enthalpy of hydration

state how the size of an ion influences enthalpy change of hydration?

• smaller ions have a higher charge than larger ions

• they attract water molecules more strongly

• more exothermic enthalpy of hydration

• smaller ion means greater enthalpy change of hydration

how does increased ionic charge affect lattice enthalpy?

• bigger charge means stronger electrostatic attraction between ions

• more energy needed to overcome these forces

• lattice enthalpy becomes more negative(more exothermic) as attraction between ions increases

• melting point therefore increases

Define enthalpy change of formation?

• the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions

is enthalpy change of formation endo or exo thermic?

• can be either

Define entropy?

• the dispersion of energy within chemicals which make up a chemical system in jk-1 mol-1

what does a greater entropy mean?

• the greater the dispersal of energy and the greater the disorder

state the order of increasing entropies in solids, liquids and gas?

solid, liquid , gas

when can enthalpically unfavourable reactions(endotermic) still occur?

if changes in entropy overcome changes in enthalpy

what is the value of entropy at 0 kelvin?

0

state the entropy when reactants have more gas moles than products?

• negative

state the entropy when products have more gas moles than reactants?

• positive

give the entropy change formula?

what does gibbs free energy tell us?

• if a reaction is feasible or not

define feasibility?

• if a reaction will be able to happen

state the gibbs free energy equation?

state the units of gibbs free energy?

Jmol-1

If Delta G is negative ($< 0$):

• The reaction is spontaneous. It can occur without outside energy

If $\Delta G$ is positive ($> 0$):

The reaction is non-spontaneous. It requires work or energy to be put in.

If $\Delta G = 0$:

the system is at equilibrium.