C17 thermodynamics

• an enthalpy change is a heat change under constant pressure
• standard conditions for measuring an enthalpy change are temperature of 298K and pressure of 100kPa
• enthalpy of formation = enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions, so using standard states
• enthalpy of combustion = enthalpy change when one mole of substance is completely burnt in an excess of oxygen
• enthalpy of atomisation = enthalpy change when one mole of an element is converted into one mole of gaseous atoms under standard conditions
• first ionisation energy = enthalpy change when one mole of gaseous atoms is converted into one mole of gaseous ions each with a 1+ charge
• second ionisation energy = enthalpy change when one mole of gaseous ions each with a 1+ charge is converted into one mole of gaseous ions each with a 2+ charge
• first electron affinity = enthalpy change when one mole of gaseous atoms is converted to one mole of gaseous ions each with a 1- charge, by having one mole of electrons added
• second electron affinity = enthalpy change when one mole of gaseous ions each with a 1- charge is converted to one mole of gaseous ions each with a 2- charge, by having one mole of electrons added
• lattice enthalpy of formation = enthalpy change when one mole of solid ionic compound is formed from its gaseous ions
• lattice enthalpy of dissociation = enthalpy change when one mole of solid ionic compound dissociates into gaseous ions
• mean bond enthalpy = enthalpy change when one mole of gaseous molecules each breaks a covalent bond to form two free radicals, averaged over a range of compounds
• a born-haber cycle is a thermochemical cycle which includes all the enthalpy changes involved in the formation of an ionic compound
• a born-haber cycle starts with the elements in their standard states, which have zero enthalpy
• in a born-haber cycle, the arrows represent a reaction and can be treated like vectors

  if the route taken is the reverse direction of the arrow, reverse the sign of the enthalpy change
• enthalpy of solution and enthalpy of hydration can be used to indirectly measure lattice enthalpies
• enthalpy of solution = enthalpy change when one mole of solute dissolves completely in sufficient solvent to form a solution in which the molecules are far enough apart not to interact
• enthalpy of hydration = enthalpy change when one mole of gaseous ions are converted into aqueous ions or an aqueous solution
• enthalpy of hydration is almost always negative as water has a delta-positive region
• a thermochemical cycle can be formed using enthalpy of solution, enthalpy of hydration and lattice enthalpy

  this cycle has enthalpy of solution at the top, with the gaseous ions at the bottom, so lattice enthalpy goes up to the left, and enthalpy of hydration of both ions goes up to the right

  this means that: lattice enthalpy = both enthalpies of hydration - the enthalpy of solution
• ions are often assumed to be perfectly ionic as values for enthalpies change a lot if the size or charge of the ion changes
• the perfect ionic model assumes that:

  • all ions are perfectly spherical
  • ions display no covalent character
• covalent character occurs in ions when two joined ions have varying sizes or charges, so the charge is unevenly distributed
• the compounds which display the most covalent character so are worst at the perfect ionic model are those with a small, highly charged positive ion and a large, highly charged negative ion

  this is because the small positive ion can approach closely to the large negative ion, and distort its electron cloud, which is large as it is highly charged
• the compounds which display the least covalent character so are best at the perfect ionic model are those with a large, singly charged positive ion and a small, singly charged negative ion

  this is becase the large positive ion cannot approach as closely to the small negative ion, so distorts its electron cloud less, which is smaller anyway as it is singly charged
• a bond between two identical atoms will be 100% covalent with no ionic character
• in chemistry there is a tendency towards disordering, creating chaos

  this means that it is favourable for the reactants to have less disorder than the products
• the randomness of a system is the entropy, represented by S
• if a reaction has products with more disorder than reactants, the entropy is positive
• in general, gases have more entropy than liquids which have more entropy than solids
• to calculate the entropy change for a reaction, add all the entropies of the products and subtract all the entropies of the reactants
• two factors control the feasibility of a reaction:

  • enthalpy change
  • entropy change
• the enthalpy change and the entropy change can be combined into the gibbs free energy, represented by G
• if delta G is positive, the reaction is not feasible
• if delta G is negative, the reaction is feasible
• if delta G is zero, the reaction has just become feasible, so this is the minimum temperature where it becomes feasible
• the equation for gibbs free energy: $\Delta G =$$\Delta H - T \Delta S$

  G = gibbs free energy

  H = enthalpy

  T = temperature

  S = entropy
• gibbs free energy depends on temperature so some reactions may be feasible at one temperature but not at another temperature