5 chemical energetics (part 1)

endo & exo

endo:

pos Hr, heat absorbed, less energetically stable

exo:

neg Hr, heat released, more energetically stable

sec

ec when molar qts of reactants, as specified by balanced chemical eqn, react to form products under sc

• Q = Hr x n

sec of atomisation

ec when 1 mole of gaseous atoms is formed from its element in its standard state under sc

• endo

• higher Hr, stronger bonds

sec of formation

ec when 1 mole of substance is formed from its constituent elements in their standard states under sc

• exo

• more exo, more stable

sec of combustion

ec when 1 mole of substance is completely burnt in O₂ under sc

• exo

sec of neutralisation

ec when 1 mole of H2O is formed from the reaction of an acid and alkali under sc

• exo

• strong + strong

  • complete ionisation

• strong + weak

  • partial dissociation (endo process)

  • some energy from neutralisation is absorbed for further dissociation of weak acid/alkali, less neg Hr

• weak + weak

  • even less neg Hr

  • more energy absorbed for complete ionisation of acid and alkali

sec of hydration

ec when 1 mole of gaseous ions is dissolved in excess water to form 1 mole of aqueous ions under sc

• exo (involves formation of ion-dipole interactions b/w h2o molecules and ions)

• magnitude depends on charge density

  • high charge density, stronger ion dipole interaction, more neg Hr

sec of solution

ec when 1 moly of substance is completely dissolved to give an infinitely dilute solution under sc, so no further enthalpy change occurs upon adding more solvent

• Hr = -LE + H_hyd

• more neg, more soluble

1. breaking of ionic bonds: estatic attraction b/w oppositely charged ions in solid ionic lattice is overcome to form gaseous ions (endo)

2. hydration of gaseous ions: ion dipole interactions (exo)

lattice energy

ec when 1 mole of solid ionic compound is formed from its constituent gaseous ions under sc

• exo (-ve charged ions come together to form ionic bonds

• more neg, stronger ionic bonds

• large discrepancy in theoretical & actual Hr: bonding is not purely ionic, has some covalent character

bond dissociation energy

(average) enthalpy chane one 1 mole of covalent bonds b/w atoms in gasoues molecules is broken

• endo, more pos, stronger cov bond

• polyatomic molecules have avg BDE

  • after one bond is broken, electronic environment of remaining bond is altered

• diatomic molucule: E(X-X) = 2 H_atm

• Hr = E (bonds broken) - E (bonds formed)

ionisation energy

1st IE is ec when 1 mole of e^- is removed from 1 mole of gaseous atoms

• endo (energy required to overcome forces of attraction b/w nucleus and e^- to be removed)

electron affinity

1st EA is ec when 1 mole gaseous atoms accepts 1 mole of e^-

• more neg, stronger attraction

• 2nd EA onwards normally endo

  • energy required to overcome repulsion b/w anion & e^- as they are both negatively charged

Hess’ Law

enthalpy change of a chemical rxn depends only on initial & final states of system, and is independent of pathway taken

• Hr = mH_f (products) - nH_f (reactants)

• Hr = mH_c (reactants) - nH_c (products)