Thermodynamics

• why might the theoretical value for the enthalpy of lattice dissociation for a compound be different from the one obtained from a Born-Haber cycle?

  the compound may have covalent character, the theoretical lattice enthalpy value assumes that the compound is perfectly ionic
• explain why the theoretical enthalpy of lattice dissociation for AgCl is less than the value for AgF?

  chloride is a larger ion, so the attraction between Ag+ and Cl- is weaker
• why is enthalpy change positive when ice is melting?

  there are hydrogen bonds between the water molecules, energy is required to break them
• what does ⦵ mean?

  standard conditions
• why can a sample of ice stay at a constant temperate of 0°C ?

  the heat given out escapes
• why can pure ice appear pale blue when it is illuminated by white light?

  light is absorbed by the ice, and blue light is reflected
• experimental value comes from the born-haber cycle, theoretical comes from calculations using a perfect ionic model
• • if experimental is slightly higher than theoretical, it has ionic bonding only
  • if experimental is significantly larger than theoretical, then it has covalent character
• there is more disorder when an ionic compound is dissolved in water, this means there entropy will have a positive value
• properties of ions that influence the value of lattice enthalpy using the perfect ionic model?

  • ionic radius
  • charge density
• Born-Haber values are greater than perfect ionic model because they allow for additional covalent interaction
• why does electron affinity of fluorine have a negative value?

  there is attraction between the nucleus and the added electron, energy will be released when the electron is gained
• define the term electron affinity for chlorine?


  the enthalpy change when one mole of chloride ions are formed from chlorine atoms, with the ions and atoms in their gaseous state
• how to obtain an accurate minimum temperature;

  • start a timer when the substance is added into solution
  • record the temperature at regular intervals for about 5 minutes
  • plot a graph of temperature against time
  • extrapolate back to when the time of mixing is equal to 0 and determine the temperature
• atomisation enthalpy is half of the bond dissociation enthalpy