Thermodynamics

• Enthalpy of Atomisation (AHa)

  • Endothermic - energy taken in to break bonds
  • 1 mol of gaseous atoms are formed from an element in its standard state
• Enthalpy of Bond Dissociation (AHdiss)

  • Endothermic - energy required to break electrostatic attraction between positive nuclei and a shared pair of electrons
  • 1 mol of a covalent bond is broken to form 2 moles of gaseous atoms
• Enthalpy change of first ionisation (AHIE1)

  • Endothermic - energy is used to overcome the electrostatic attraction between the nucleus and electron being removed
• Enthalpy change of first electron affinity (AHEA1)

  • Exothermic - energy released from the electrostatic attraction between nucleus and electron being added
• Enthalpy change of second electron affinity (AHEA2)

  • Endothermic - energy required to overcome the repulsion between negatively charged ion and electron being added
• Enthalpy of lattice formation (AHLE)

  • Exothermic - energy released from the electrostatic attraction between oppositely charged ions
  • 1 mole of a solid ionic compound is formed from its gaseous ions
• Enthalpy of lattice dissociation (AHLE)

  • Endothermic - energy is needed to overcome the electrostatic attraction between oppositely charged ions
  • 1 mole of solid ionic compound is broken into gaseous ions
• Lattice formation = -

  Lattice dissociation = +
• What is the difference between enthalpy and enthalpy change ?

  Enthalpy = absolute

  Enthalpy change = difference between initial and final
• How is enthalpy change measured ?

  Measure the temperature using a thermometer
• What is enthalpy change ?

  Heat energy change measured at constant pressure
• What is standard enthalpy change ?

  Heat energy change measured at constant pressure under standard conditions
• What are the standard conditions ?

  • 298K
  • 100kPa
• Why are lattice enthalpies calculated ?

  • Lattice enthalpies indicate the strength of ionic bonds
  • The larger the lattice enthalpy the stronger the ionic bonds therefore the higher the melting and boiling point of the ionic compound
• Why can lattice enthalpies not be measured directly ?

  • Difficult to measure the temperature rise of a solid
• A special form of Hess's law called Born-Haber cycles are used to calculate enthalpy change. Why ?

  • It is impossible to carry out experiments in which gaseous ions are spread out at infinite distances
  • Reactions take place in the gaseous state therefore gas particles move randomly so it is difficult to bring them close together to measure
• Factors affecting lattice enthalpies

  • Ionic charge
  • Ionic radius
• Ionic radius

  • Smaller ions can pack together closely in a lattice and attract each other strongly
  • Larger ions are further apart in their lattice and the force of attraction between them are weaker
• Ionic charge

  • The larger the charge the stronger the attraction
• Suggest whether NaCl or MgCl2 has the larger lattice enthalpy. Explain why.

  • MgCl2 has a larger lattice enthalpy than NaCl
  • Mg2+ ion has a greater charge than Na+
  • Mg2+ has a smaller ionic radius than Na+
  • Electrostatic attraction between Mg2+ and Cl- is stronger than Na+ and Cl-
• What are lattice enthalpy values calculated from perfect ionic model called ?

  Theoretical lattice enthalpies - as it is the lattice enthalpy that is expected
• What are lattice enthalpy values calculated from Born-Haber cycles called ?

  Experimental lattice enthalpies
• What does the 'perfect ionic model' mean ?

  • Ions are point charges
  • Only electrostatic attraction between ions/no covalent character
• • If experimental and theoretical lattice enthalpies are similar then the compound has almost 100% ionic character
• • If the experimental and theoretical lattice enthalpies are different then the compound has covalent character present in addition to ionic bonding
• • Difference between experimental and theoretical is due to covalent bonding
  • Larger the difference the greater the covalent character
  • Bonding in NaBr is almost 100% ionic as the two values are similar
  • Bonding of AgBr is ionic with some covalent character
• What is polarisation ?

  • When the cation will attract the electrons of the anion
  • This distorts the electron density in the anion towards the cation
• What is polarising power ?

  The ability of a cation to attract electrons from the anion towards itself
• Tendency towards covalent character:

  • Positive ion is small
  • Positive ion has multiple positive charges
  • Negative ion is large
  • Negative ion has multiple negative charges
• When a compound has some covalent character:

  • Low solubility in water
  • Melting point and electrical conductivity not as high as expected
• When enthalpy change of solution occurs why can the ions no longer attract each other ?

  Water gets in between them so they can no longer attract to each other
• What is enthalpy change of solution ?

  • Enthalpy change when 1 mole of solid ionic compound dissolves completely in water to form an infinitely dilute solution under standard conditions
  • NaCl(s) --> Na+(aq) + Cl-(aq)
• Why is the enthalpy solution of some metal oxides not found in data books ?

  Metal oxides react with water to form insoluble metal hydroxide
• Why is the bond enthalpy of NaBr not found in data books ?

  NaBr is ionic not covalent
• What is enthalpy of hydration ?

  • Enthalpy change when 1 mol of aqueous ions are formed from 1 mol of gaseous ions
  • Na+ (g) --> Na+(aq)
• What type of energy transfer is ion-dipole attraction ?

  • Exothermic
  • Energy released from ion-dipole attraction
• Why is hydration of Na+ exothermic ?

  • Water is polar
  • Ion-dipole attraction between positive sodium ion and δ− charge on oxygen of water molecule releases energy
• The size of AHhyd depends on:

  • Ionic charge - the larger the charge the stronger the ion-dipole attraction and the more exothermic it is
  • Ionic radius - the smaller the radius of the ion the stronger the ion-dipole attraction and the more exothermic it is
• Why is the enthalpy of hydration of chloride ion more negative than bromide ion ?

  • Cl- is a smaller ion than Br-
  • Cl- ion attracts the δ+ on the H of water molecule
  • Ion-dipole attraction between chloride ions and water is stronger than bromide ions and water
• For a substance to dissolve in water:

  • Enthalpy change of hydration > Enthalpy change of lattice dissociation
  • Positive values = less likely to dissolve
  • Negative values = more likely to dissolve