1.08 Thermodynamics

• What does Hess' Law state?

  The enthalpy change for a reaction is independent of the route taken.
• Define standard enthalpy of formation.

  The enthalpy change when one mole of a compound is formed from its constituent elements in standard conditions, with all products and reactants in their standard states.

  • Mg(s) + 1/2 O2 (g) → MgO(s)
• Define standard enthalpy of combustion.

  The enthalpy change when one mole of a substance is completely burnt in excess oxygen.

  • CH4(g) + 2O2 → CO2(g) + 2H2O(g)
• Define standard enthalpy of atomisation.

  Enthalpy change when one mole of gaseous atoms is formed from a compound in its standard state in standard conditions.

  • 1/2 I2(g) → I(g)
• Define first ionisation energy.

  Enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to form one mole of gaseous 1+ ions.

  • Li(g) → Li+(g) + e-
• Define first electron affinity.

  Enthalpy change where one mole of gaseous atoms gains one mole of electrons to form one mole of gaseous 1- ions.

  • Cl(g) + e- → Cl-(g)
• Define lattice enthalpy of formation.

  Enthalpy change when one mole of solid ionic lattice is formed from its constituent gaseous ions.

  • Na+(g) + Cl-(g) → NaCl(s)
• Define lattice enthalpy of dissociation.

  Enthalpy change when one mole of solid ionic lattice is dissociated into its gaseous ions.

  • NaCl(s) → Na+(g) + Cl-(g)
• Define enthalpy of hydration.

  Enthalpy change where one mole of gaseous ions become dissolved in water to infinite dilution (water molecules totally surround the ion).

  • Na+(g) → Na+(aq)
• Define enthalpy of solution.

  Enthalpy change when one mole of solute dissolves completely in a solvent to infinite dilution.

  • NaCl(s) → Na+(aq) + Cl-(aq)
• Define mean bond dissociation enthalpy.

  Enthalpy change when one mole of covalent bonds is broken, with all species in the gaseous state.

  • Br2(g) → 2Br(g)
• Draw a labelled born-haber cycle for the formation of sodium chloride lattice.


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• What factors affect the lattice enthalpy of an ionic compound?

  • Size of ions
  • Charge of ions
• How can you increase lattice enthalpy of a compound and why does this increase it?

  Smaller ions.

  • The charge centres of the positive and negative ions will be closer together

  Increased charge.

  • Greater electrostatic forces of attraction between oppositely charged ions.
  • Increasing charge of anion has much smaller effect than increasing charge of cation, as increasing anion charge has the same effect as increasing ionic size.
• What actually happens when a solid is dissolved in terms of interactions of the ions with water molecules?

  Break lattice → gaseous ions; dissolve each gaseous ion in water.

  • The aqueous ions are surrounded by water molecules which have a permanent dipole due to polar O-H bond.
• What is the perfect ionic model?

  Assumes that ions are perfectly spherical and that there is an even charge distribution, meaning bonds are 100% polar.
• Why is the perfect ionic model often not accurate?

  • Ions are not perfectly spherical due to polarisation - small positive ions/large negative ions are involved, so ionic bond gains covalent character (degree of an ionic bond sharing characteristics of a covalent bond e.g sharing of electrons)
  • Some lattices are not regular and the crystal structure can differ
• What kind of bonds will be the most ionic and why?

  Between large positive ions and small negative ions e.g CsF.
• Define the terms spontaneous and feasible.

  If a reaction is spontaneous and feasible, it will take place of its own accord.

  • This does not take account of rate of reaction.
• Is a reaction with a positive or negative enthalpy change more likely to be spontaneous?

  Negative - exothermic
• Define entropy.

  The disorder of a system.

  • Higher value of entropy = more disordered
• What is the second law of thermodynamics?

  Entropy of an isolated system always increases, as it is overwhelmingly more likely for molecules to be disordered than ordered.
• Is a reaction with more positive or negative entropy change more likely to be spontaneous?

  Positive.

  • Reactions always try to increase the amount of disorder.
• How would you calculate the entropy change for a reaction?

  Sum of products' entropy - sum of reactants' entropy.
• Define Gibbs free energy using an equation.

  ΔG = ΔH - TΔS

  • G=gibbs free energy, H=enthalpy change, S=entropy change, T=Temperature
  • G<0 = feasible, G>0= not feasible, G=0 = JUST feasible (indicates the temperature where the reaction becomes feasible)
• How would you calculate the temperature at which the reaction becomes feasible?

  Rearrange to T=(ΔH)/(ΔS) as G=0
• What are the limitations of using G as an indicator of whether a reaction will occur?

  • Only indicates if a reaction is feasible and does not take into account the rate of reaction
  • In reality, many reactions that are feasible at a certain temperature have a rate of reaction that is so slow that effectively no reaction is occurring.