Thermodynamics

The standard enthalpy change of formation of a compound - is energy transferred when 1 mole of compound is formed - from its elements under standard conditions (298 K and 100 kpa), - all reactants and products in standard states Na (s) + ½Cl2 (g) > NaCl (s) [ΔfH = NEGATIVE kJ mol-1]

The enthalpy of atomisation of an element is - the enthalpy change when 1 mole of gaseous atoms is formed - from the element in its standard state Na (s) > Na(g) [ΔatH = POSITIVE kJ mol-1] or ½ O2 (g) > O (g) [ΔatH = POSITIVE kJ mol-1] .. • The enthalpy change for a solid metal turning to gaseous atoms • also is called Enthalpy of sublimation • numerically same as enthalpy of atomisation [ΔsubH = POSITIVE kj mol-1]

The first ionisation enthalpy is the enthalpy change - required to remove 1 mole of electrons - from 1 mole of gaseous atoms to form - 1 mole of gaseous ions with a +1 charge Mg (g) → Mg+ (g) + e [ΔIE 1H]

The bond dissociation enthalpy is standard molar enthalpy change - when one mole of a covalent bond is - broken into two gaseous atoms (or free radicals) Cl2 (g) > 2Cl (g) [ΔdissH = POSITIVE kJ mol-1] .. For diatomic molecules the ΔdissH of the molcule is - the same as 2x ΔatH of the element Cl2 (g) > 2Cl (g) [ΔdissH= POSITIVE mol-1]

The second ionisation enthalpy is the enthalpy change to - remove 1 mole of electrons from - one mole of gaseous 1+ ions - to produces one mole of gaseous 2+ ions Mg+ (g) → Mg2+(g) +e- (ΔIE 2H)

The first electron affinity is the enthalpy change that occurs - when 1 mole of gaseous atoms gain - 1 mole of electrons to form - 1 mole of gaseous ions with a -1 charge O(g) +e → O- (g) [ΔEA 1H] = NEGATIVE kj mol-1] … • The first electron affinity is exothermic for atoms that • normally form negative ions because ion is more stable than atom, • and there is an attraction between the nucleus and the electron

The second electron affinity is the enthalpy change when - one mole of gaseous 1- ions gains - one electron per ion to produce - gaseous 2- ions. O- (g) +e- → O2 (g) [ΔEA 2H= POSITIVE kJ mol-1] • The second electron affinity for oxygen is endothermic because • it takes energy to overcome repulsive force • between negative ion and the electron.

The enthalpy of lattice formation is the - standard enthalpy change when - 1 mole of an ionic crystal lattice is formed - from its constituent ions in gaseous form. Na+ (g) + Cl- (g) → NaCl (s) [ΔLattH = NEGATIVE kJ mol-1)

The enthalpy of lattice dissociation is - the standard enthalpy change when 1 mole - of an ionic crystal lattice form is separated - into its constituent ions in gaseous form. NaCl (s) → Na+ (g) + Cl- (g) [ΔLattH = POSITIVE kJ mol-1]

Enthalpy change when - one mole of gaseous ions become aqueous ions. X+/-(g) + aq → X+/-(aq) Eg. Li+ ΔhydH = NEGATIVE kj mol-1) Eg. F- ΔhydH = NEGATIVE kj mol-1

The enthalpy of solution is the - standard enthalpy change When one mole - or an lonic solid dissolves in a - large enough amount of water to ensure that - dissolved ions are well separated and dont interact with one another. NaCl (s) + aq → Na+ (aq) + Cl- (aq)

Lattice enthalpy cant be determined directly; we calculate it indirectly - by making use of changes for which datas available - and link them together in an enthalpy cycle > the born haber cycle .. • This Born Haber cycle diagram 1 has been constructed using a lattice enthalpy of formation. • Sometimes questions will give enthalpy of lattice dissociation which • has opposite sign and arrow points in the opposite direction. • This changes the calculation. ΔfH = ΔatH + ΔIEH + ΔatH + ΔeaH + ΔLattH ΔLatt = everything has opposite sign apart from ΔfH

diagram 2 > do questions on ittt

The strength of an enthalpy of lattice formation depends on the following factors: • The sizes of the ions - The larger the ions, the less negative the enthalpies of lattice formation - (i.e. a weaker lattice). As the ions are larger, charges become further apart - and so have a weaker attractive force between them. • The charges on ion - The bigger the charge of ion, the greater attraction between ions so - the stronger lattice enthalpy (more negative values). .. • The lattice enthalpies become less negative down any group. e.g. LiCi, NaCl, KCI, etc

• Theoretical lattice enthalpies assume a perfect ionic model where • the ions are 100% ionic and spherical and the attractions are purely electrostatic. … There is a tendency towards covalent character in ionic substances when • the positive ion is small • the positive ion has multiple charges • the negative ion is large • the negative ion has multiple negative charges. - - When a compound has some covalent character- it tends towards giant covalent - so the lattice is stronger than if it was 100% ionic. So the - Born-Haber value would be larger than the theoretical value. • When the negative ion becomes distorted and more covalent we say it becomes polarised. • The small metal cation is called polarising if it polarises the larger negative ion.

A spontaneous process (e.g. diffusion) will proceed on its own without any external influence. • problem with ΔH - A reaction that is exothermic will result in products that are more - thermodynamically stable than the reactants. - - This is a driving force behind many reactions and causes them to be - spontaneous (occur without any external influence). Some spontaneous reactions, however, are endothermic. - this is explained? We need to consider entropy.

• The Born Haber lattice enthalpy is the real experimental value. • When a compound shows covalent character, the theoretical and the born Haber lattice enthalpies differ. • The more the covalent character the bigger the difference between the values.

DIAGRAM 3 It is useful to draw out the born haber cycles for each potential case. - We need to calculate an enthalpy of formation for each case. - The one with the most exothermic enthalpy of formation will be the one - that forms as it will be the most thermodynamically stable

• Substances with more ways of arranging their atoms and energy • (more disordered) have a higher entropy • Entropy is a description of the number of ways atoms can share quanta of energy. • If number of ways of arranging the energy (W) is high, • then the system is disordered and entropy (S) is high. .. Elements, Simpler compounds, pure substances ..tend to have lower entropies than… Compounds, Complex compounds, Mixtures • Solids have lower entropies than liquids, wch are lower than gases • When a solid increases in temp its entropy increases as particles vibrate more. • here is a bigger jump in entropy w boiling than that with melting. • gases have large entropies as they are much more disordered 0K substances = zero entropy. - no disorder as particles are stationary.

• In general, a significant increase in the entropy will occur if: -there is a change of state from solid or liquid to gas • theres a significant increase in number of molecules between products and reactants. .. NH4CI (s) → HCI (g) + NH3 (g) AS = +ve • change from solid reactant to gaseous products • increase in number of molecules both will increase disorder • Na (s) + ½ Cl2 (g) → NaCl (s) AS = -ve • change from gaseous and solid reactant to solid • decrease in number of molecules both will decrease disorder

Data books list standard entropies per mole for a variety of substances - not possible for a substance to have a standard entropy of below 0 The unit of entropy is J K-1 mol-1 ΔS°= ΣS products - ΣS° reactants .. • Elements in their standard states do not have zero entropy. • Only perfect crystals at absolute zero (0 K) will have zero entropy.

DIAGRAM 4 units • The balance between entropy and enthalpy determines feasibility of a reaction. • given by the relationship : ΔG = ΔH - TΔS • Gibbs free energy is a term that combines effect of enthalpy and entropy into one number. • For any spontaneous change, ΔG will be negative. • A reaction that has increasing entropy (+ve ΔS) and is exothermic (-ve ΔH) • will make ΔG be negative and will always be feasible. .. • if ΔG is negative theres still a possibility, however, that reaction • wont occur or will occur so slowly that effectively it doesn’t happen. • If the reaction has a high activation energy, reaction will not occur.

Reacts feasible when ΔG = 0 diagram 5 > rearrange for T

• As physical phase changes like melting and boiling are equilibria, • the ΔG for such changes is zero. Calculate the temperature methane melts at. DIAGRAM 6